- Email: [email protected]
The influence of age of the parents on some characteristics of the offspring of insects bred in the laboratory
7. .rlurcdProd. Res., 1967, Vol. 3, pp. 371-383.
The
Influence
Some
Pergamon Press Ltd. Printed in Great Britain.
of Age of the Parents
on
Characteristics
of Insects
of the Offspring Bred in the Laboratory R. W. HOWE
Pest Infestation Laboratory, Agricultural Research Council, London Road, Slough, Bucks., England (First received 26 April, 1967, and in jnal form 25 Augrut, 1967) Abstract-The literature on the influence of parental age on the size and viability of eggs, longevity and fecundity of offspring and the rate of development of offspring is reviewed. No simple general pattern emerges although a number of examples of clear trends related to parental age have been recorded., The most consistent trend in rate of development is for Tenebrio molitor. In this species the offspring of old parents develop more quickly than those of young parents. A very wide difference of developmental period recorded for Tribolium confwum is considered spurious and is thought to illustrate the difficulty of investigating age effects. Two experiments on Sitophilus granaries are reported. The results of these are complicated by density variation in cultures but the curve relating developmental period to parental age is parabolic with a maximum in middle age. In general it is concluded that the influences of parental age are real but inconsistent so that all controlled experiments must bc designed to take account of them. ISTRODUCTION
CERTAIX attributes of adult insects change obviously as the individual ages. One that clearly has a considerable influence on numbers in the next generation is the rate of oviposition. In most stored products species there is a short pre-oviposition period ; the rate of oviposition then rises rapidly to an early peak which is maintained for a period and then declines slowly. For each species, these events can be represented by a typical oviposition curve that differs only in detail between environmental conditions. Usually there is a post-oviposition period in old age. The length of the oviposition period and the numbers of eggs laid varies considerably between species and the eggs may be laid singly but regularly or in batches spaced at intervals. As a rule the eggs are laid more quickly at higher temperatures up to some optimum but very often the largest number of eggs is laid at a rather lower temperature. Changes in the viability of eggs laid at different ages by the adults of long-lived species cause no surprise, although most stored products insects mate frequently. In recent years, however, there have been a number of papers relating the develop371
372
R. W. HOW=
mental periods of larvae to the age of the parent adults at the time of egg-laying. The larval developmental period is influenced so considerably by the food and physical environment that it is surprising that a parental influence has a detectable effect, whatever the cause. It may be postulated that there are age-related genetic changes, in particular in crossing over patterns, or that some cytoplasmic influence of the mother’s physiology may be carried in the egg, but before such theories are investigated the facts of parental influence in insects must be established. It is very difficult to design experiments to examine the effects of parental age without confounding them with those of other variables of time. &Iany of my own experiments have been so designed that I would expect them to reveal changes of developmental period related to parental age if there were any and the only recent one to show this influence is discussed in this paper. The main purpose of this paper, however, is to examine the published data on the influence of parental age on the offspring in stored products species and a few others. It is not always possible to judge published papers fairly because nowadays through lack of space, experimental methods are usually described very briefly and the author concentrates on those features that he considers most relevant. He may well overlook details that have a considerable influence on his results. It is reasonable, therefore, to speculate about the existence of unsuspected experimental variables when published observations conflict markedly with my own. REVIEW OF LITERATURE Size and viability of eggs
RICHARDS and KOLDERIE (1957) noted that the first batches of eggs laid by freshly emerged females of the milkweed bug Oncopeltusfasciatus (Dallas) were light in weight, that the last eggs were even lighter and that the heaviest eggs were laid DAVID (1962) on the other hand, records that Drosophila at the peak of oviposition. melanogaster Meigen lays large eggs for a short period early in life and then for a similar period lays abnormally small eggs before recovering to lay eggs of normal size. PARSONS(1962) also working with strains of D. melanogaster found that egg size varied with age in the same general way but he did not regard the small eggs as abnormal because the variability of size was minimal when they were laid. In David’s work size variability was maximal when smaller eggs were laid. PARSONS( 1962) and ROBERTSONand SANG (1944) recorded that the viability of the eggs of D. melanogaster fell with the age of the parents except for a short period at the beginning of oviposition when it increased. Robertson and Sang attributed this early trend of increasing viability to the food resen-es of the egg. In eggs laid by young parents this is derived from stores accumulated by the larvae whereas with older parents adult food is converted into egg reserves. Parsons regarded the changes in viability as another expression of the changes in genetic stability. A similar increase and decrease in the proportion hatching in relation to the age of the parent was recorded for Oncobeltus fasciatus by RICHARDS and KOLDERIE (1957) but amongst stored products insects only the decline of hatching as the parents increase in age has been noted, e.g. by LUDWIG and FIORE (1960) for Tenebrio molitor L. Richards and Kolderie were especially interested in the hatching of eggs near
The Influence of Age of the Parents on the Offspring of Insects
373
the lower temperature threshold and RICHARDS (1959) suggested that larger eggs hatched when smaller eggs did not because they contained more food reserves. DAVID (1961) stressed that an increase in developmental mortality linked to parental age is most obvious when the observed animals are weak and CALLAHAN (1962) commented on the greater ability of the offspring of young houseflies to withstand unfavourable conditions. Longevity and fecundity of ofspring
Much of the literature on the influence of parental age is concerned with the longevity and fecundity of the offspring of young and old parents. GOETSCH (1956) claimed that the descendants of young parents of Drosophila mclanogaster lived longer and were more fecund than the offspring of older parents. BUTZ and HAYDEN (1962) made a similar observation and found that the influence of maternal age on the longevity of offspring was much greater than that of paternal age. O’BRIAN (1961) also found that the offspring of young parents of this species had the longest age span, and on breeding for nine generations from young, middle-aged and old parents obtained an increasing productivity of eggs from the young and a decreasing productivity from the other parents. DAVID (1961) obtained a contrary result for in his experiments the offspring of old parents were more fecund but the repeated use of old parents in successive generations did not produce a cumulative effect. Indeed for parents of a fixed age there was a negative correlation in the fecundity of offspring of successive generations. For Musca domestica L. CALLAHAN(1962) found that the longevity of offspring from the first eggs laid and from the last viable batch was shorter than that of offspring laid by middle-aged parents. Only three generations of flies could be obtained when the last viable eggs were used. FLEMINGS and LUDWIG (1964) reported similar results for Pediculus humanus humanus L. with greater longevity for offspring of young parents. Again very few generations could be maintained if the first eggs or those of old parents were selected. According to TRACEY (1958) and LUDWIG and FIORE (1960) the longevity of adult offspring decreased as the parent beetles of T. molitor aged but LUDWIG and FIORE (1961) obtained the opposite result for the offspring of isolated pairs up to 63 days old at 25°C. FIORE (1960) found no consistent pattern for T. obscurus F. ROCKSTEIN(1959) considered that the connexion between longevity and parental age was much more complex. In experiments in which houseflies were mated and eggs collected at various ages, the longevity of female offspring diminisfred as the parents aged but male longevity was not affected. From further experiments where the old individuals were sometimes virgin, he concluded that male longevity was enhanced in the offspring of old mothers and (see CLARK and ROCKSTEIN, 1964) that the effect on female longevity held true for only one generation. Rate of development of eggs
The duration of the egg stage is usually too short for changes to be established but RICHARDS and KOLDERIE (1957) stated that the rate of egg development at 17°C was slow for eggs laid at the beginning and the end of the fecund period at 25°C of Oncopeltus fasciatus. On the other hand, SAXG (1956) found that the eggs of middle-aged parent Drosophila melanogaster needed the longest incubation period.
371
R. \V. HOWE
KIRITAXI (1963) stated that the incubation period became progressively shorter as the season advanced.
at 25°C of JVecara viridula L.
Rate of development of larvae
Almost all the published papers that relate a change in the rate of larval development to the age of the parent laying the egg refer to stored products beetles. SCHNEIDER (1941) in a nutritional study of Tribolium confwum Duval used two stocks of adults, one group aged 1 month and the other 6 months to get eggs. The freshly hatched larvae, 300 from each group, were grown in batches of 100 on 100 g patent flour. Only 11 died from the younger parents against 35 from the older ones but the mean developmental period of the younger group was 37 *5 days as against 34 days for the older group. There was also a clear difference in the frequency curves for the appearance of pupae. Because two stocks were used these differences might perhaps be ascribed to differences in their genetic make-up but the only likely experimental cause of such a large difference in larval periods is a temperature gradient of some l-2% in the incubator. Most of the other examples refer to eggs laid by the same beetles at different times. LARSOS and SIMMONS (1923) noted that for groups of eggs laid by Callosobruchus maculatus (F.) on particular days, those laid by the older parents had the longer developmental periods. STEFFA~ (1945) kept separately the eggs laid each day at 27°C by a batch of adult xabrotes subfasciatus (Boh.) and found that the shortest mean larval developmental period was for eggs laid on the second day of laying. This developmental period increased for eggs laid by older parents up to 6-7 days old, but for still older parents who laid only 2-6 viable eggs per 10 females, the developmental period was shorter giving an overall parabolic curve for period against parental age. HOWE and CURRIE (1964) collected daily the eggs laid by a freshly emerged group of Callosobruchus maculatus and shared them between several conditions. In some conditions the developmental period decreased and in others it increased as the parents aged. HOWE (1950) noted a decrease in the developmental period of Gibbium psylloides (Czemp.) at 20°C between eggs l$id one month apart by the same parents. Here, of course, there is the possibility that the temperature of the constant temperature room rose during the experimental period, but the records for the rooms in which the experiments were done showed no changes that could account for the trend. In the same paper HOWE reports the developmental periods at 23°C for larvae hatching from the eggs laid by six isolated pairs of Ptinus tectus Boield. on 10 successive days and grown singly. The mean developmental period increased from the beginning to the end of this experiment by 11 days from 45 -6 to 56.7 days and the trend was displayed by the offspring of all but one pair. Nevertheless, in the inspection of over 100 subsequent experiments in which larvae of this species were obtained from eggs laid on successive days only 5 examples of this kind of trend were discovered. The most consistent results have been obtained at Fordham University, New York, with Tenebrio molitor, a species that in warehouses usually has one generation only each year. LUDWIG (1956) discovered that the larvae hatched from eggs laid by young parents pupated after an average of 187 days at 25°C whereas those hatched from eggs laid by the same adults 28 days later needed on average 27 days less to pupate. These observations were repeated and extended by TRACEY (1958).
The Influence of .4ge of the Parents on the Offspring of Insects
373
She successively obtained eggs from a group of adults when their ages were close to 1, 5 and 9 weeks and kept the larvae individually on samples of a chick growing mash. On this occasion the larvae grew more quickly than in Ludwig’s experiments and the differences between the batches were smalier. The average developmental periods of the larvae were 154, 150 and 144 days respectively for the 1, 5 and 9 week old parents. Most of the difference arose early in larval life and might have been carried over from the egg. The first larval instar lasted 5, 3 and 2 days respectively and the periods needed to reach an average weight of 20 mg were 12, 9 and 8 weeks. Results of similar experiments at 30°C were similar but less easy to interpret because this temperature is high for ‘I: molitor. At first larvae grow more quickly at 30 than at 25°C but the period to pupation after they reach their maximum weight is usually long and variable. Tracey found that at 30°C for larvae from 5 and 9 week old parents, pupation was scattered over 50 to 100 days. LUDWIG and FIORE (1960) repeated the work with the adult age at laying spaced at 2 week intervals and with 15 and 20°C added to the experimental temperatures but 15°C was too cold. In this experiment at 25°C the larval period was still shorter than in Tracey’s, 134-135 days for parents up to 3 weeks old decreasing fairly regularly to 121 days for parents 9 weeks old. At 20°C the developmental period was 13 days less when the parents were aged 10 weeks than when they were aged 4 weeks. LUDWIG and FIORE (1961) considered the offspring of 4 isolated pairs of adults and presented the results for a typical pair. The mean larval period for offspring of l-week-old parents was 146 days and for offspring of g-week-old parents it was 109 days. This difference again arose during early larval life, the mean weights of larvae at 12 weeks old being respectively 10 and 70 mg. In this experiment the first instar was not shorter in offspring of old parents but the next 8 instars were. FIORE (1960) did similar experiments with the related species T. obscures at 20, 25 and 3O”C, in which eggs were obtained from adults approximately 1, 2, 3 and 6 weeks old. No consistent effect on the developmental period was found. However, the rate of growth of the offspring of older parents was clearly slower and their final weights lower at 20 and 30°C. At 25°C the growth rate was slightly slower but the final weight was not different. Outside the field of stored products KIRITANI (1963) has recorded that the developmental period of the nymph of Nezara uiriduta at a constant temperature of 25°C became progressively shorter for eggs laid in April, May and June. Experimental technique may sometimes determine whether or not differences related to parental age are apparent. In most of the Fordham University experiments with Tenebrio molitor the relative humidity in the incubators was kept high but LUDWIG and FIORE (1960) carried out a series of experiments in which water ivas added to the food and the humidity was not controlled. In these experiments, the results at 25°C were erratic and the age-related trend was slight, while at 20% there was no clear change. Rate of development of larvae and @fiat
Finally some recent work on Tribolium confks~m (RAYCHAUDHURIand BUTZ, 1965a) gave results so different from any others that they must be scrutinized very closely. 1Vhereas the quickest developmental rates in all other papers mentioned are, at
376
R. I\‘. HOWE
most, only of the order of 25 per cent higher than the slowest, these authors note changes of up to three times. The mean larval period varied from 20 to 60 days and the pupal period from 5 to 12 days (Fig. 1). Normally the pupal period is veq stable (HOWE, 1960) and when less than 10 days all the pupae usually appear on the same day or on successive days even when a batch of adults of mixed ages has laid the eggs. I myself have done a considerable number of experiments with 1. confusum, T. mndens and especially with T. castaneum designed to show the influence of parental age, if any existed. Since none have shown any sign of such an influence it is natural to suspect that the results of Raychaudhuri and Butz can be explained by some unsuspected changes of environment. The authors were at pains to avoid the most likely cause of experimental variation, namely the overcrowding of cultures, and grew the larvae individually. Over 50 different parental age crosses were made but only 12 lamae were used to find each mean and the influence of parental age on developmental period was not followed through on the same beetles. Every pair was kept and the eggs it laid over 9 successive IO-day periods were allowed to hatch and the larvae counted to estimate productivity, but only the first batch of larvae from each was used for developmental experiments. The influence of age was investigated by keeping virgin adults in single sex groups for as much as 80 weeks before pairing. This method of preventing mating until the insects are older is known to influence offspring (CLARK and ROCKSTEIN, 1964). Raychaudhuri and Butz prepared a series of 20 matings with beetles of the same age from a 4-day-old pair to an 80-week-old pair and also two series of 16 matings in which 4-week-old individuals of one sex were mated with individuals of the other sex from 8-80 weeks old. The curves representing the variation of developmental period of both larvae and pupae for these three series correspond very closely and since no variable is common to all three, the similarity must presumably be explained by some outside influence. In his thesis, RAYCHAUDHURI(1963) tabulates the mean those for the series in which the developmental periods for all his experiments; parents were of equal age are presented here as Fig. 1. It is clear that the undulations of the two curves are similar in shape but those for the larval period lag behind those for the pupal period. It is also obvious that the offspring of l-week-old parents were pupae at a time when the larvae from 4-week-old parents were very small. In order to judge the hypothesis that the undulations are caused by the environment, therefore, the position of the pupal curve must be adjusted by adding the mean larval period to the parental age. When this is done (Fig. 1) the undulations correspond much more closely and since the pupal period is not sensitive to humidity, it is possible to conclude that the temperature control of the experiment was at fault. The incubator was not described but the only claim made was that the temperature varied between 25 and 28°C. It was not made clear whether the variation was rapid and consistent or whether the mean wandered within this range over long periods. It was not explicitly stated that the temperature distribution in the incubator was uniform. From the values given for the pupal period by HOWE (1960), the experimental temperature at the end of the first month when the pupal period was less than 6 days can be estimated as nearly 30°C. Towards the end of the experiment when the pupal period was 12 days, the temperature must have been about 23°C. For the rest of the experiment the temperature was probably in the range claimed but this would permit a variation of pupal period between 7 and 9
The Influence of Age of the Parents on the Offspring of Insects
377
days and a correspondingly larger variation of the larval period. I conclude that the observed changes of developmental period in this work are more likely to be associated with the experimental temperature than with parental age. If this is not so, we must wonder why such an enormous age effect has not been found before. Small increases of the pupal period have been ascribed to a deficiency in the larval diet (PARK, 1935) and to the addition of too much vitamin B to the larval food (SCHNEIDER, 1943).
FIG. 1. Inffuence of age of parents on the mean developmental periods of larvae (full line, scale on left) and pupae (broken line, scale on right) of Tribolium confuum according to RAYCHAUDIIURI and Bun (1965a). The symbols show the approximate position on the larval time scale of means for the pupae.
Other factors
relating to parental age
PARSONS (1962) recorded that a morphological feature of the offspring, the asymmetry of the number of Sterno-pleural chaetae of Drosophila melanogaster changed with age. The relationship was parabolic with the very young and the old being most variable. There was some disagreement between the results of two experiments at 25°C. DURRANT (1955) g ave similar information in less detail. CHAUVIN (1958) noted that the growth rate of some individuals of GrylLulus domesticus (L.) increased if kept in a group. These group-sensitive individuals were the offspring of old mothers; the father’s age had no influence. ANDERSEK (1963) commented on a daily decrease of the proportion of females reared from eggs laid by Endrosis sarcitrella (L.) and SIMMONDS(1948) discussed the correlation of parental age with the percentage of offspring of two hymenopterous parasites that enter diapause. The developmental period of those Spalangia drosophilea Ashm. that did not diapause when kept at 75°F increased by 2 days for eggs laid over an 8-day period and for those kept at 83°F the curve of age against developmental period was parabolic. The percentage in diapause rose with parental age up to 6 days.
R. W. HOWE
378
Biochemical
observations
Biochemical analyses of adults of various ages and of the offspring of parents of different ages shed little light on the process of ageing or how it might affect the offspring. GOETSCH (1956), however, claimed that the addition of an extract of old parents, but not of young ones, to the diet of Drosophila melanogaster blocked larval development. LUDWIG and JONES (1964) compared the amino acid content of homogenates of adult Tenebrio molitor at five ages from emergence to 12 weeks old. They found a rapid decrease of methionine in the first week followed by a continual decline, rapid decreases for phenylalanine and tyrosine and a more gradual decline of tryptophane, and no consistent change of ten others. They found small amounts of the cystine-cysteine group in adults of all ages although TRACEY et al. (1958) found it in homogenates of larvae only when these were bred from young parents. Ludwig and Jones felt that the changes in adults mentioned above were greater for offspring of old parents than of young parents. RAYCHAUDHURI(1963) found no marked pattern of differences in the amino acid content of homogenates of larvae of Tribolium confwum for parents of different ages but concentrations were higher than for adults. Only fifteen amino acids were found in larvae as against nineteen in adults. LUDWIG and BARSA (1955) noted that there was more cytochrome oxidase in eggs laid by 2-week-old parents than there was in eggs laid by 6-week-old parents but this did not affect oxygen consumption. LUDWIG et al. (1962) found no differences between the oxygen consumption of offspring either as adults or larvae from old and young parents but cytochrome oxidase activity in adults decreased at 6 weeks old. They recorded that the p glycerophosphatase activity in the adult was greater in the offspring of old parents than in those from young parents. RAYCHAUDHURI and BUTZ (1965b) discovered two peaks of acid phosphatase activity as the female Tribolium conff(sum aged, one early and one late in life, but only an early peak for the male. The activity of alkaline phosphatases showed no marked peaks but was consistently much higher for the female than the male. For this reason these authors suggest that this activity is important in egg production. ORIGINAL OBSERVATIONS If the age of a female insect when she lays an egg consistently influences the developmental period, then the variability of this period in experimental work will depend upon whether the adults used for egg laying are of uniform or mixed age. With parents of uniform age, even a complex trend of developmental period with time should be noticed; with parents of mixed ages simple trends should be apparent although complex ones might be hidden. From the foregoing survey of the literature it is clear that trends related to parental age or time of oviposition are not encountered very frequently in stored products insects. Presumably when such a trend is not found the negative result is not recorded, and when one is found it is treated with scepticism. Experimental
method
The observations
described
here are from two routine experiments
on the rates of
The Influence of Age of the Parents on the Offspring of Insects
379
oviposition and adult emergence in experimental cultures of Sitophilus grunarius L. kept on wheat at 25°C and 70 per cent r.h. (HOWE and HOLE, 1967). Groups of 100 weevils were collected within a few hours of emergence from wheat grains and were placed for short periods on successive lots of 100 g of wheat. In the longer experiment, the adults were moved daily for 8 weeks to a fresh wheat sample to give what will be called l-day cultures, except at weekends when the adults remained in the wheat for 3 days. These cultures will be called 3-day cultures. After 8 weeks the weevils were placed on fresh wheat three times a week to give two 3-day cultures and one l-day culture. In the shorter experiment the weevils were moved daily for 7 weeks to a fresh lot of wheat and then discarded. Later as each culture reached 35 days old it was sieved daily for another 35 days to remove the adult offspring as they emerged from the wheat grains. At 25°C S. granarius females have a preoviposition period of about 5 days. The oviposition rate reaches a peak from 10 to 30 days and then starts to decline steadily. By 100 days it is very low and it ceases after 125 days. Results
Plotting the mean of the developmental period for the weevils bred from each culture reveals some signs of time trends. In the shorter experiment (Fig. 2) there was a statistically significant lengthening of the developmental period of 0 -016 days/day of parental age, but this accounts for only 20 per cent of the variation of the developmental period. The first culture of this series yielded only 5 weevils and these had a very low mean of 38 96 days. The first eggs laid by stored products beetles often have either a very short or a very long developmental period, but this observation has not previously been published.
l .
. .
. l
.
.
.
. ..
. I 10
.*
I
Numbers
1
.
.. .
l
. l
I
15’
.. .*
l
.
.
.
I Age
I
I of
of
I
Parents
100
75
FIG.
l
. .
.
.
l.
*
.
Adults
45
125
50
I
150
Emerged
Relationship of mean developmental periods of cultures in short experiment to age in days of parents laying eggs (above) and to density of cultures (below).
2.
380
R. W. HOWE
There was a similar trend in the longer experiment (Fig. 3) up to an age of about 50-60 days but then the developmental period declined. In this experiment the data for l-day and 3-day cultures are best examined separately because most of the l-day cultures were set up during the first 8 weeks whereas most of the 3-day cultures were set up later. Here there was a statistically significant lengthening of the
100
50 of
Age
Adults
FIG.3. Parabolic relationships of mean developmental periods of l-(a)- and 3-( O)-day cultures of longer experiment to age in days of parents laying eggs. TABLE 1. PERCENTAGE OF VARIABILITYOF THE MEAN DEVELOPMENTAL PERIODSOF 100 g CULTURES OF Sitophilwpnarius ACCOUNTEDFORBYAGE AND CULTURE DENSITY Short experiment
Long experiment 1 day
3 day
1 day
Parental age
mostly young
mostly old
all young
Age linear Age parabolic
1 p
25.9 50.6
** ***
Density
d
23.5
*
Density+age linear Density+ age parabolic
27.5 dp 60.0
Oviposition period
* ***
7.5 l/l 64.2 *** 11.6
p
22.3 66.9
1 1
20.2 23.8
** **
2.9
***
I 20.2 &I 26.1
** **
Asterisks denote the 5, 1 and 0.1 per cent levels of statistical significance and l, p and d denote which of the regression coefficients is significant.
The Influence of Age of the Parents on the Offspring of Insects
381
developmental period with age for the l-day cultures but not for the 3-day cultures. When a parabolic term is added to each regression equation to describe the curvature (Fig. 3) the equations account for between a half and two-thirds of the variability of developmental period in the two sets of cultures (Table 1). Oviposition rate and parental age are highly correlated. There is some risk that in these experiments there were sufficient developing stages in some cultures to generate enough heat to raise the internal temperature of the cultures and so to shorten the mean developmental periods. It is possible, therefore, that the trend of lengthening developmental period for the younger parents is related to internal culture temperatures and that the observed relationship with age is a consequence of the correlation of culture density with both age and developmental period. The relationship between density and developmental period for cultures yielding 300-600 adults/3000 wheat kernels is obvious in Fig. 4 but for sparser cultures it is obscured for 3-day cultures by the quicker development of offspring of aged parents. Nevertheless (Table 1) there is a statistically significant relationship accounting for 1
I
I
I
I
I
I
d
I
.
0 0 I
I
1
I 2
1 Number
I
I
I
3
I
I
4
5
[hundreds]
FIG. 4. Plot of mean developmental periods of l-( l)- and 3-(O)-day cultures of long experiment against culture density with line representing the linear regression for l-day cultures.
one quarter of the variability of developmental period for the l-day cultures, predominantly from young parents, although the highest culture yield only just exceeds 200. Similarly in the short experiment restricted to young parents, if the first two very sparse cultures are omitted (but not otherwise-Table l), the regression of developmental period on culture yield is significant although the period has a range of only l-25 days and the yield varies only from 72 to 157 (Fig. 2). Adding a term for density to the parabolic regression for age diminishes unexplained variation to a statistically significant extent only for the l-day cultures
382
R. W. HOWE
of the long experiment (Table 1). For this it converts a positive linear regression coefficient for age into a negative one, but since neither of these linear components is statistically significant (Table 1) it is not worth speculating about the meaning of this change. The parabolic coefficient for age (Table 1) appears to be important. There is an interesting detail in the early stages of the long experiment. Whilst the trend from one week to the next was for the mean developmental period to increase, during each week it decreased day by day (Fig. 3). The daily transfers were interrupted by a 3 day period of rest but it is difficult to understand how the more frequent disturbance of the parents could influence the developmental period of offspring. In the short experiment (Fig. 2) there are signs of a daily decrease of developmental period from days 6 to 13, and 15 to 18 and perhaps from 19 to 22 and 23 to 27, but as a rule these would probably be dismissed as random variation.
Sign$cance
of results
Summing up these observations, we may conclude that the rate of development is related to parental age and that the relationship is parabolic. Experimental complexities make such relationships difficult to establish. In this work the influence of culture density is obvious at over 300 insects per culture, but is not revealed by the mathematical analysis of the 3-day cultures (Table 1). It is revealed in the l-day cultures at densities as low as 70 per culture but since each individual liberates heat at a rapid rate for only about 5 days, the total heat production at any time depends on the spread of developmental stages as well as their number. The range of mean developmental periods in these experiments at 25°C is very small, about 3 days at most, and is unlikely to be of any practical importance to the species. If the influence of parental age persists throughout the temperature range, however, it would be quite large near the lower threshold temperature. Our ignorance of the biology of S. granarius before man provided it with plentiful food makes it impossible to decide whether or not this would have any evolutionary significance. In its original environment this species probably survived the winter in all stages, though principally as an adult. It probably had to survive a summer food scarcity and did this as an adult or in fleshy stems and tubers. It would probably be an advantage for the numerous offspring of young parents to grow as rapidly as possible in the spring and for the offspring of old parents to complete their development in the autumn before the weather beomes too cold. There is less advantage in quick development
during the mid-summer
food shortage.
DISCUSSION
No clear evidence of a systematic trend related to parental age in any characteristic of the offspring has emerged from the experiments and published data discussed in this paper. Nevertheless, a variety of such trends, often opposite in direction for the same characteristic have been noted and it is clear that trends related to parental age may appear in any experiment. Consequently very great care must be taken to ensure by proper design either that bias is avoided or else that trends related to parental age are recorded and measured. Because they are often complex and seldom seem to be consistent age-related trends may often be dismissed as nuisance factors in experiments, but the most
The Influence of Age of the Parents on the Offspring of Insects
383
frequent trend is parabolic and has most influence on the offspring of the younger middle-aged parents, especially in an unfavourable environment. Since this is the most fertile age group, the behaviour of the offspring may be expected to have an evolutionary significance. The genetic control of developmental rates is probably very complex but the results with .Ne~aru viridulu and Tenebrio molitor in constant conditions compel us to admit that the genetic constitution of offspring or its phenological expression may be consistently influenced by the age of the parents. The rates of crossing-over may be high when gametes are formed in very young or very old insects. There may also be changes caused by maternal physiology. The constituents of the egg may alter as the mother ages and the derivatives of these constituents may be passed to later instars or even to later generations. In some instances the age-related changes might persist as a result of selection operating through the physiology and ecology of the species. ROBERTSONand SAXG (1944) attribute changes of egg viability to a change of source of the food reserves of the egg from the initial reserves of the mother to those derived from her feeding as an adult. Adult longevity is an advantage to a species only if it increases fecundity or if it prolongs oviposition so that the eggs laid late in life, or the larvae hatched from them, escape some lethal or unfavourable conditions that harm those laid earlier. Frequently stocks cannot be maintained by breeding continuously from very young or very old parents (CALLAHAN, 1962 ; FLEMINGSand LUDWIG, 1964). This might be explained, as claimed by GOETSCH (1956), by the passage of an inhibitory substance, or it might be explained genetically. Genes favouring adult longevity may be detrimental during the developmental stages (CLARK and ROCKSTEIN, 1964) or the gene combinations from very young parents may be unstable (PARSONS, 1962). Oscillatory influences of age as reported by DAVID (1961) might arise from the oscillations of population size and hence of selection pressure. If the influence of parental age on rates of development is to be consistent, it might be expected to be linked to the annual seasons. This is so for Jveruru viridulu (KIRITANI, 1963) which grows faster at 25°C in June than it does in May, and is also true for Tenebrio molitor. This latter species has only one generation a year in warehouses and the more rapid development of the offspring of older parents ensures that all the new generation of adults appear at about the same time in the spring. This species passes the winter in the larval stage and although high summer and autumn temperatures may stimulate rapid growth to maximum size, they also inhibit pupation. This diapause-like block to pupation also serves to keep the species in the larval stage during the winter, so it seems probable that in its original habitat only the larvae successfully survived the winter. 7: molitor is not a satisfactory species for the investigation of the influence of parental age because it is so variable, especially in the larval developmental period. LECLERQ (1963) bred two strains with widely different developmental periods and pupal weights from a stock initially intermediate for both. HOWE and BURGES(1953) recorded larval periods in constant conditions ranging from 173 to well over 300 days and in an earlier unpublished experiment in the same conditions recorded periods of 126 to 200 days. The Fordham University experiments at 25°C for parents of a given age and for the same experimental procedure gave a similar wide range of developmental
384
R. IV. HOWE
periods but the rate of development always accelerated as the parents grew older. Most of the other stored products insects for which an influence of parental age has been suspected, e.g. species of Bruchidae, Ptinidae, Sitophi1u.sand Tribolium all complete several generations a year. It is difficult to postulate how selection could act consistently in these species since we do not know their original habitats. In a primitive habitat a rapid rate of increase would rapidly deplete a sparse food, so the most favoured individuals should be those best able to find fresh food. In warehouses with ample food, the characteristics on which selection acts have presumably changed considerably and nowadays possibly resistance to chemicals used to control them is the most potent selective force. There seems to be no reason to expect a consistent relationship with parental age of any feature of development in these species. The work with them concerning parental age warns us of experimental hazards but contributes little of fundamental value to the study of their ecology. REFERENCES ANDERSEN,F. S. (1963) Population density of grain insects. Arsberetn. St Skadedyrlab. 1959 and 1960, 75-77. BUTZ, A. and HAYDEN,P. (1962) The effects of age of male and female parents on the life cycle of Drosophila melanogaster. Ann. ent. Sot. Am. 55, 617-618. CALLAHAN,R. F. (1962) Effects of parental age on the life cycle of the housefly, Musca domestica Linnaeus (Diptera, Muscidae). 3t N.Y. ent. Sot. 70, 150-158. CHAUVIN,R. (1958) L’action du groupement sur la croissance des grillons (Cryllulr~ domesticus). 3. Insect Physiol. 2, 235-248. CLARK, A. M. and ROCKSTEIN, M. (1964) Aging in insects. In: Physiology of Zruecta (Ed. by ROCKSTEIN, M.), Vol. 1, pp. 227-281. Academic Press, New York. DAVID,J. (1961) Influence de l’etat physiologique des parents sur les caracteres des descendants. Annl. Gtw. 3, i-78. DAVID,J. (1962) Influence de l’age de la mere sur les dimensiens des oeufs dans une souche vestigial de Drosophila melanogaster Meig. Et u de experimentale du determinisme physiologique de ces variations. Bull. biol. Ft. Belg. 96, 505-528. DURRANT,A. (1955) Effect of time of embryo formation on quantitative characters in Drosophila. Nature, Land. 175,560-56 1. FIORE, C. (1960) Effects of temperature and parental age on the life cycle of the dark mealworm, Tenebrio obscurus Fabricius. 31 jV.Y. ent. Sot. 68, 27-35. FLEMXNGS, M. B. and LUDWIG, D. (1964) Effects of temperature and parental age on the life cycle of the body louse, Pediculus humanus humanus. Ann. ent. Sot. Am. 57, 560-563. GOETSCH, W. (1956) Estudios sotore edad y vejez de 10s insectos y sustancias que intervienen en allas. Eos. Madr. 32, 185-2 13. HOWE, R. W. (1950) Studies on beetles of the family Ptinidae, 4. A note on an anomalous effect of parental age on the speed of development. Entomologist’s mon. Mag. 86, 325-326. HOWE, R. W. (1960) The effects of temperature and humidity on the rate of development and the mortality of Tribolium confusum Duval (Coleoptera, Tenebrionidae). Ann. a#. Biol. 48, 363-376. HOWE, R. W. and BURGES, H. D. (1953) A note on the resistance of Tenebrio molifor (L.) (Col. Tenebrionidae) to tropical temperatures. Entomologist’s mon. Mag. 89, 4-6. HOWE, R. W. and CURRIE, J. E. (1964) S ome laboratory observations on the rates of development, mortality and oviposition of several species of Bruchidae breeding in stored pulses. Bull. ent. Res. 55,437-477. Howe, R. W. and HOLE, B. D. (1967) The yield of cultures of Sitophilus granarius at 25°C and 70 per cent r.h. with some observations on rates of oviposition and development. 3. stored Prod. Res. 2, 13-27. KIRITANI, K. (1963) Oviposition habit and effect of parental age upon the post embryonic development in the southern green stink bug, &zara viridula. Jap. 3. Ecol. 13, 88-96.
The Influence of Age of the Parents on the Offspring of Insects
385
LARSOS, A. 0. and Smwo~s, P. (1923) Notes on the biology of the four-spotted bean weevil, Bruchus quadrimacuiatus Fab. J. agric. Res. 26, 609-616. LECLERQ,J. (1963) Artificial selection for weight and its consequences in Tenebrio molitor L. ,I’ature, Lond. 198, 106-107. LUDWIG, D. (1956) Effects of temperature and parental age on the life cycle of the mealworm, Tenebrio mofitor Linnaeus (Coleoptera, Tenebrionidae). Ann. ent. Sot. Am. 49, 12-15. LUDWIG,D. and BARSA, M. C. (1955) Relationship between the activity of the succinoxidasc system and the rate of oxygen consumption during the embryonic development of the mealworm, Tenebrio molitor Linnaeus. 3. gen. Physiol. 38, 729-734. LUDWIG,D. and FIORE, C. (1960) Further studies on the relationship between parental age and the life cycle of the mealworm, Tenebrio molitor. Ann. ent. Sot. Am. 53, 595-600. LUDWIO, D. and FIORE, C. (1961) Effects of parental age on offspring from isolated pairs of the mealworm, Tenebrio molitor. Ann. ent. Sot. Am. 54, 463-464. LUDWIG,D., FIORE, C. and JoNas, C. R. (1962) Physiological comparisons between offspring of the yellow mealworm, Tenebrio molitor, obtained from young and from old parents. Ann. ent. Sot. Am. 55,439-442. LUDWIG,D. and Jonas, C. R. (1964) Changes in the concentration of certain amino acids in homogenates of the yellow mealworm, Tenebrio molitor, during aging. Ann. en;. Sot. Am. 57, 210-2 13. O’BRIAN, D. M. (1961) Effects of parental age on the life cycle of Drosophila melanogaster. Ann. ent. Sot. Am. 54, 412-416. PARK, T. (1935) Studies in population physioiogy. IV. Some physiological effects of conditioned flour upon Triboliwn confwum Duval and its populations. Physial.
The
Influence
Some
Pergamon Press Ltd. Printed in Great Britain.
of Age of the Parents
on
Characteristics
of Insects
of the Offspring Bred in the Laboratory R. W. HOWE
Pest Infestation Laboratory, Agricultural Research Council, London Road, Slough, Bucks., England (First received 26 April, 1967, and in jnal form 25 Augrut, 1967) Abstract-The literature on the influence of parental age on the size and viability of eggs, longevity and fecundity of offspring and the rate of development of offspring is reviewed. No simple general pattern emerges although a number of examples of clear trends related to parental age have been recorded., The most consistent trend in rate of development is for Tenebrio molitor. In this species the offspring of old parents develop more quickly than those of young parents. A very wide difference of developmental period recorded for Tribolium confwum is considered spurious and is thought to illustrate the difficulty of investigating age effects. Two experiments on Sitophilus granaries are reported. The results of these are complicated by density variation in cultures but the curve relating developmental period to parental age is parabolic with a maximum in middle age. In general it is concluded that the influences of parental age are real but inconsistent so that all controlled experiments must bc designed to take account of them. ISTRODUCTION
CERTAIX attributes of adult insects change obviously as the individual ages. One that clearly has a considerable influence on numbers in the next generation is the rate of oviposition. In most stored products species there is a short pre-oviposition period ; the rate of oviposition then rises rapidly to an early peak which is maintained for a period and then declines slowly. For each species, these events can be represented by a typical oviposition curve that differs only in detail between environmental conditions. Usually there is a post-oviposition period in old age. The length of the oviposition period and the numbers of eggs laid varies considerably between species and the eggs may be laid singly but regularly or in batches spaced at intervals. As a rule the eggs are laid more quickly at higher temperatures up to some optimum but very often the largest number of eggs is laid at a rather lower temperature. Changes in the viability of eggs laid at different ages by the adults of long-lived species cause no surprise, although most stored products insects mate frequently. In recent years, however, there have been a number of papers relating the develop371
372
R. W. HOW=
mental periods of larvae to the age of the parent adults at the time of egg-laying. The larval developmental period is influenced so considerably by the food and physical environment that it is surprising that a parental influence has a detectable effect, whatever the cause. It may be postulated that there are age-related genetic changes, in particular in crossing over patterns, or that some cytoplasmic influence of the mother’s physiology may be carried in the egg, but before such theories are investigated the facts of parental influence in insects must be established. It is very difficult to design experiments to examine the effects of parental age without confounding them with those of other variables of time. &Iany of my own experiments have been so designed that I would expect them to reveal changes of developmental period related to parental age if there were any and the only recent one to show this influence is discussed in this paper. The main purpose of this paper, however, is to examine the published data on the influence of parental age on the offspring in stored products species and a few others. It is not always possible to judge published papers fairly because nowadays through lack of space, experimental methods are usually described very briefly and the author concentrates on those features that he considers most relevant. He may well overlook details that have a considerable influence on his results. It is reasonable, therefore, to speculate about the existence of unsuspected experimental variables when published observations conflict markedly with my own. REVIEW OF LITERATURE Size and viability of eggs
RICHARDS and KOLDERIE (1957) noted that the first batches of eggs laid by freshly emerged females of the milkweed bug Oncopeltusfasciatus (Dallas) were light in weight, that the last eggs were even lighter and that the heaviest eggs were laid DAVID (1962) on the other hand, records that Drosophila at the peak of oviposition. melanogaster Meigen lays large eggs for a short period early in life and then for a similar period lays abnormally small eggs before recovering to lay eggs of normal size. PARSONS(1962) also working with strains of D. melanogaster found that egg size varied with age in the same general way but he did not regard the small eggs as abnormal because the variability of size was minimal when they were laid. In David’s work size variability was maximal when smaller eggs were laid. PARSONS( 1962) and ROBERTSONand SANG (1944) recorded that the viability of the eggs of D. melanogaster fell with the age of the parents except for a short period at the beginning of oviposition when it increased. Robertson and Sang attributed this early trend of increasing viability to the food resen-es of the egg. In eggs laid by young parents this is derived from stores accumulated by the larvae whereas with older parents adult food is converted into egg reserves. Parsons regarded the changes in viability as another expression of the changes in genetic stability. A similar increase and decrease in the proportion hatching in relation to the age of the parent was recorded for Oncobeltus fasciatus by RICHARDS and KOLDERIE (1957) but amongst stored products insects only the decline of hatching as the parents increase in age has been noted, e.g. by LUDWIG and FIORE (1960) for Tenebrio molitor L. Richards and Kolderie were especially interested in the hatching of eggs near
The Influence of Age of the Parents on the Offspring of Insects
373
the lower temperature threshold and RICHARDS (1959) suggested that larger eggs hatched when smaller eggs did not because they contained more food reserves. DAVID (1961) stressed that an increase in developmental mortality linked to parental age is most obvious when the observed animals are weak and CALLAHAN (1962) commented on the greater ability of the offspring of young houseflies to withstand unfavourable conditions. Longevity and fecundity of ofspring
Much of the literature on the influence of parental age is concerned with the longevity and fecundity of the offspring of young and old parents. GOETSCH (1956) claimed that the descendants of young parents of Drosophila mclanogaster lived longer and were more fecund than the offspring of older parents. BUTZ and HAYDEN (1962) made a similar observation and found that the influence of maternal age on the longevity of offspring was much greater than that of paternal age. O’BRIAN (1961) also found that the offspring of young parents of this species had the longest age span, and on breeding for nine generations from young, middle-aged and old parents obtained an increasing productivity of eggs from the young and a decreasing productivity from the other parents. DAVID (1961) obtained a contrary result for in his experiments the offspring of old parents were more fecund but the repeated use of old parents in successive generations did not produce a cumulative effect. Indeed for parents of a fixed age there was a negative correlation in the fecundity of offspring of successive generations. For Musca domestica L. CALLAHAN(1962) found that the longevity of offspring from the first eggs laid and from the last viable batch was shorter than that of offspring laid by middle-aged parents. Only three generations of flies could be obtained when the last viable eggs were used. FLEMINGS and LUDWIG (1964) reported similar results for Pediculus humanus humanus L. with greater longevity for offspring of young parents. Again very few generations could be maintained if the first eggs or those of old parents were selected. According to TRACEY (1958) and LUDWIG and FIORE (1960) the longevity of adult offspring decreased as the parent beetles of T. molitor aged but LUDWIG and FIORE (1961) obtained the opposite result for the offspring of isolated pairs up to 63 days old at 25°C. FIORE (1960) found no consistent pattern for T. obscurus F. ROCKSTEIN(1959) considered that the connexion between longevity and parental age was much more complex. In experiments in which houseflies were mated and eggs collected at various ages, the longevity of female offspring diminisfred as the parents aged but male longevity was not affected. From further experiments where the old individuals were sometimes virgin, he concluded that male longevity was enhanced in the offspring of old mothers and (see CLARK and ROCKSTEIN, 1964) that the effect on female longevity held true for only one generation. Rate of development of eggs
The duration of the egg stage is usually too short for changes to be established but RICHARDS and KOLDERIE (1957) stated that the rate of egg development at 17°C was slow for eggs laid at the beginning and the end of the fecund period at 25°C of Oncopeltus fasciatus. On the other hand, SAXG (1956) found that the eggs of middle-aged parent Drosophila melanogaster needed the longest incubation period.
371
R. \V. HOWE
KIRITAXI (1963) stated that the incubation period became progressively shorter as the season advanced.
at 25°C of JVecara viridula L.
Rate of development of larvae
Almost all the published papers that relate a change in the rate of larval development to the age of the parent laying the egg refer to stored products beetles. SCHNEIDER (1941) in a nutritional study of Tribolium confwum Duval used two stocks of adults, one group aged 1 month and the other 6 months to get eggs. The freshly hatched larvae, 300 from each group, were grown in batches of 100 on 100 g patent flour. Only 11 died from the younger parents against 35 from the older ones but the mean developmental period of the younger group was 37 *5 days as against 34 days for the older group. There was also a clear difference in the frequency curves for the appearance of pupae. Because two stocks were used these differences might perhaps be ascribed to differences in their genetic make-up but the only likely experimental cause of such a large difference in larval periods is a temperature gradient of some l-2% in the incubator. Most of the other examples refer to eggs laid by the same beetles at different times. LARSOS and SIMMONS (1923) noted that for groups of eggs laid by Callosobruchus maculatus (F.) on particular days, those laid by the older parents had the longer developmental periods. STEFFA~ (1945) kept separately the eggs laid each day at 27°C by a batch of adult xabrotes subfasciatus (Boh.) and found that the shortest mean larval developmental period was for eggs laid on the second day of laying. This developmental period increased for eggs laid by older parents up to 6-7 days old, but for still older parents who laid only 2-6 viable eggs per 10 females, the developmental period was shorter giving an overall parabolic curve for period against parental age. HOWE and CURRIE (1964) collected daily the eggs laid by a freshly emerged group of Callosobruchus maculatus and shared them between several conditions. In some conditions the developmental period decreased and in others it increased as the parents aged. HOWE (1950) noted a decrease in the developmental period of Gibbium psylloides (Czemp.) at 20°C between eggs l$id one month apart by the same parents. Here, of course, there is the possibility that the temperature of the constant temperature room rose during the experimental period, but the records for the rooms in which the experiments were done showed no changes that could account for the trend. In the same paper HOWE reports the developmental periods at 23°C for larvae hatching from the eggs laid by six isolated pairs of Ptinus tectus Boield. on 10 successive days and grown singly. The mean developmental period increased from the beginning to the end of this experiment by 11 days from 45 -6 to 56.7 days and the trend was displayed by the offspring of all but one pair. Nevertheless, in the inspection of over 100 subsequent experiments in which larvae of this species were obtained from eggs laid on successive days only 5 examples of this kind of trend were discovered. The most consistent results have been obtained at Fordham University, New York, with Tenebrio molitor, a species that in warehouses usually has one generation only each year. LUDWIG (1956) discovered that the larvae hatched from eggs laid by young parents pupated after an average of 187 days at 25°C whereas those hatched from eggs laid by the same adults 28 days later needed on average 27 days less to pupate. These observations were repeated and extended by TRACEY (1958).
The Influence of .4ge of the Parents on the Offspring of Insects
373
She successively obtained eggs from a group of adults when their ages were close to 1, 5 and 9 weeks and kept the larvae individually on samples of a chick growing mash. On this occasion the larvae grew more quickly than in Ludwig’s experiments and the differences between the batches were smalier. The average developmental periods of the larvae were 154, 150 and 144 days respectively for the 1, 5 and 9 week old parents. Most of the difference arose early in larval life and might have been carried over from the egg. The first larval instar lasted 5, 3 and 2 days respectively and the periods needed to reach an average weight of 20 mg were 12, 9 and 8 weeks. Results of similar experiments at 30°C were similar but less easy to interpret because this temperature is high for ‘I: molitor. At first larvae grow more quickly at 30 than at 25°C but the period to pupation after they reach their maximum weight is usually long and variable. Tracey found that at 30°C for larvae from 5 and 9 week old parents, pupation was scattered over 50 to 100 days. LUDWIG and FIORE (1960) repeated the work with the adult age at laying spaced at 2 week intervals and with 15 and 20°C added to the experimental temperatures but 15°C was too cold. In this experiment at 25°C the larval period was still shorter than in Tracey’s, 134-135 days for parents up to 3 weeks old decreasing fairly regularly to 121 days for parents 9 weeks old. At 20°C the developmental period was 13 days less when the parents were aged 10 weeks than when they were aged 4 weeks. LUDWIG and FIORE (1961) considered the offspring of 4 isolated pairs of adults and presented the results for a typical pair. The mean larval period for offspring of l-week-old parents was 146 days and for offspring of g-week-old parents it was 109 days. This difference again arose during early larval life, the mean weights of larvae at 12 weeks old being respectively 10 and 70 mg. In this experiment the first instar was not shorter in offspring of old parents but the next 8 instars were. FIORE (1960) did similar experiments with the related species T. obscures at 20, 25 and 3O”C, in which eggs were obtained from adults approximately 1, 2, 3 and 6 weeks old. No consistent effect on the developmental period was found. However, the rate of growth of the offspring of older parents was clearly slower and their final weights lower at 20 and 30°C. At 25°C the growth rate was slightly slower but the final weight was not different. Outside the field of stored products KIRITANI (1963) has recorded that the developmental period of the nymph of Nezara uiriduta at a constant temperature of 25°C became progressively shorter for eggs laid in April, May and June. Experimental technique may sometimes determine whether or not differences related to parental age are apparent. In most of the Fordham University experiments with Tenebrio molitor the relative humidity in the incubators was kept high but LUDWIG and FIORE (1960) carried out a series of experiments in which water ivas added to the food and the humidity was not controlled. In these experiments, the results at 25°C were erratic and the age-related trend was slight, while at 20% there was no clear change. Rate of development of larvae and @fiat
Finally some recent work on Tribolium confks~m (RAYCHAUDHURIand BUTZ, 1965a) gave results so different from any others that they must be scrutinized very closely. 1Vhereas the quickest developmental rates in all other papers mentioned are, at
376
R. I\‘. HOWE
most, only of the order of 25 per cent higher than the slowest, these authors note changes of up to three times. The mean larval period varied from 20 to 60 days and the pupal period from 5 to 12 days (Fig. 1). Normally the pupal period is veq stable (HOWE, 1960) and when less than 10 days all the pupae usually appear on the same day or on successive days even when a batch of adults of mixed ages has laid the eggs. I myself have done a considerable number of experiments with 1. confusum, T. mndens and especially with T. castaneum designed to show the influence of parental age, if any existed. Since none have shown any sign of such an influence it is natural to suspect that the results of Raychaudhuri and Butz can be explained by some unsuspected changes of environment. The authors were at pains to avoid the most likely cause of experimental variation, namely the overcrowding of cultures, and grew the larvae individually. Over 50 different parental age crosses were made but only 12 lamae were used to find each mean and the influence of parental age on developmental period was not followed through on the same beetles. Every pair was kept and the eggs it laid over 9 successive IO-day periods were allowed to hatch and the larvae counted to estimate productivity, but only the first batch of larvae from each was used for developmental experiments. The influence of age was investigated by keeping virgin adults in single sex groups for as much as 80 weeks before pairing. This method of preventing mating until the insects are older is known to influence offspring (CLARK and ROCKSTEIN, 1964). Raychaudhuri and Butz prepared a series of 20 matings with beetles of the same age from a 4-day-old pair to an 80-week-old pair and also two series of 16 matings in which 4-week-old individuals of one sex were mated with individuals of the other sex from 8-80 weeks old. The curves representing the variation of developmental period of both larvae and pupae for these three series correspond very closely and since no variable is common to all three, the similarity must presumably be explained by some outside influence. In his thesis, RAYCHAUDHURI(1963) tabulates the mean those for the series in which the developmental periods for all his experiments; parents were of equal age are presented here as Fig. 1. It is clear that the undulations of the two curves are similar in shape but those for the larval period lag behind those for the pupal period. It is also obvious that the offspring of l-week-old parents were pupae at a time when the larvae from 4-week-old parents were very small. In order to judge the hypothesis that the undulations are caused by the environment, therefore, the position of the pupal curve must be adjusted by adding the mean larval period to the parental age. When this is done (Fig. 1) the undulations correspond much more closely and since the pupal period is not sensitive to humidity, it is possible to conclude that the temperature control of the experiment was at fault. The incubator was not described but the only claim made was that the temperature varied between 25 and 28°C. It was not made clear whether the variation was rapid and consistent or whether the mean wandered within this range over long periods. It was not explicitly stated that the temperature distribution in the incubator was uniform. From the values given for the pupal period by HOWE (1960), the experimental temperature at the end of the first month when the pupal period was less than 6 days can be estimated as nearly 30°C. Towards the end of the experiment when the pupal period was 12 days, the temperature must have been about 23°C. For the rest of the experiment the temperature was probably in the range claimed but this would permit a variation of pupal period between 7 and 9
The Influence of Age of the Parents on the Offspring of Insects
377
days and a correspondingly larger variation of the larval period. I conclude that the observed changes of developmental period in this work are more likely to be associated with the experimental temperature than with parental age. If this is not so, we must wonder why such an enormous age effect has not been found before. Small increases of the pupal period have been ascribed to a deficiency in the larval diet (PARK, 1935) and to the addition of too much vitamin B to the larval food (SCHNEIDER, 1943).
FIG. 1. Inffuence of age of parents on the mean developmental periods of larvae (full line, scale on left) and pupae (broken line, scale on right) of Tribolium confuum according to RAYCHAUDIIURI and Bun (1965a). The symbols show the approximate position on the larval time scale of means for the pupae.
Other factors
relating to parental age
PARSONS (1962) recorded that a morphological feature of the offspring, the asymmetry of the number of Sterno-pleural chaetae of Drosophila melanogaster changed with age. The relationship was parabolic with the very young and the old being most variable. There was some disagreement between the results of two experiments at 25°C. DURRANT (1955) g ave similar information in less detail. CHAUVIN (1958) noted that the growth rate of some individuals of GrylLulus domesticus (L.) increased if kept in a group. These group-sensitive individuals were the offspring of old mothers; the father’s age had no influence. ANDERSEK (1963) commented on a daily decrease of the proportion of females reared from eggs laid by Endrosis sarcitrella (L.) and SIMMONDS(1948) discussed the correlation of parental age with the percentage of offspring of two hymenopterous parasites that enter diapause. The developmental period of those Spalangia drosophilea Ashm. that did not diapause when kept at 75°F increased by 2 days for eggs laid over an 8-day period and for those kept at 83°F the curve of age against developmental period was parabolic. The percentage in diapause rose with parental age up to 6 days.
R. W. HOWE
378
Biochemical
observations
Biochemical analyses of adults of various ages and of the offspring of parents of different ages shed little light on the process of ageing or how it might affect the offspring. GOETSCH (1956), however, claimed that the addition of an extract of old parents, but not of young ones, to the diet of Drosophila melanogaster blocked larval development. LUDWIG and JONES (1964) compared the amino acid content of homogenates of adult Tenebrio molitor at five ages from emergence to 12 weeks old. They found a rapid decrease of methionine in the first week followed by a continual decline, rapid decreases for phenylalanine and tyrosine and a more gradual decline of tryptophane, and no consistent change of ten others. They found small amounts of the cystine-cysteine group in adults of all ages although TRACEY et al. (1958) found it in homogenates of larvae only when these were bred from young parents. Ludwig and Jones felt that the changes in adults mentioned above were greater for offspring of old parents than of young parents. RAYCHAUDHURI(1963) found no marked pattern of differences in the amino acid content of homogenates of larvae of Tribolium confwum for parents of different ages but concentrations were higher than for adults. Only fifteen amino acids were found in larvae as against nineteen in adults. LUDWIG and BARSA (1955) noted that there was more cytochrome oxidase in eggs laid by 2-week-old parents than there was in eggs laid by 6-week-old parents but this did not affect oxygen consumption. LUDWIG et al. (1962) found no differences between the oxygen consumption of offspring either as adults or larvae from old and young parents but cytochrome oxidase activity in adults decreased at 6 weeks old. They recorded that the p glycerophosphatase activity in the adult was greater in the offspring of old parents than in those from young parents. RAYCHAUDHURI and BUTZ (1965b) discovered two peaks of acid phosphatase activity as the female Tribolium conff(sum aged, one early and one late in life, but only an early peak for the male. The activity of alkaline phosphatases showed no marked peaks but was consistently much higher for the female than the male. For this reason these authors suggest that this activity is important in egg production. ORIGINAL OBSERVATIONS If the age of a female insect when she lays an egg consistently influences the developmental period, then the variability of this period in experimental work will depend upon whether the adults used for egg laying are of uniform or mixed age. With parents of uniform age, even a complex trend of developmental period with time should be noticed; with parents of mixed ages simple trends should be apparent although complex ones might be hidden. From the foregoing survey of the literature it is clear that trends related to parental age or time of oviposition are not encountered very frequently in stored products insects. Presumably when such a trend is not found the negative result is not recorded, and when one is found it is treated with scepticism. Experimental
method
The observations
described
here are from two routine experiments
on the rates of
The Influence of Age of the Parents on the Offspring of Insects
379
oviposition and adult emergence in experimental cultures of Sitophilus grunarius L. kept on wheat at 25°C and 70 per cent r.h. (HOWE and HOLE, 1967). Groups of 100 weevils were collected within a few hours of emergence from wheat grains and were placed for short periods on successive lots of 100 g of wheat. In the longer experiment, the adults were moved daily for 8 weeks to a fresh wheat sample to give what will be called l-day cultures, except at weekends when the adults remained in the wheat for 3 days. These cultures will be called 3-day cultures. After 8 weeks the weevils were placed on fresh wheat three times a week to give two 3-day cultures and one l-day culture. In the shorter experiment the weevils were moved daily for 7 weeks to a fresh lot of wheat and then discarded. Later as each culture reached 35 days old it was sieved daily for another 35 days to remove the adult offspring as they emerged from the wheat grains. At 25°C S. granarius females have a preoviposition period of about 5 days. The oviposition rate reaches a peak from 10 to 30 days and then starts to decline steadily. By 100 days it is very low and it ceases after 125 days. Results
Plotting the mean of the developmental period for the weevils bred from each culture reveals some signs of time trends. In the shorter experiment (Fig. 2) there was a statistically significant lengthening of the developmental period of 0 -016 days/day of parental age, but this accounts for only 20 per cent of the variation of the developmental period. The first culture of this series yielded only 5 weevils and these had a very low mean of 38 96 days. The first eggs laid by stored products beetles often have either a very short or a very long developmental period, but this observation has not previously been published.
l .
. .
. l
.
.
.
. ..
. I 10
.*
I
Numbers
1
.
.. .
l
. l
I
15’
.. .*
l
.
.
.
I Age
I
I of
of
I
Parents
100
75
FIG.
l
. .
.
.
l.
*
.
Adults
45
125
50
I
150
Emerged
Relationship of mean developmental periods of cultures in short experiment to age in days of parents laying eggs (above) and to density of cultures (below).
2.
380
R. W. HOWE
There was a similar trend in the longer experiment (Fig. 3) up to an age of about 50-60 days but then the developmental period declined. In this experiment the data for l-day and 3-day cultures are best examined separately because most of the l-day cultures were set up during the first 8 weeks whereas most of the 3-day cultures were set up later. Here there was a statistically significant lengthening of the
100
50 of
Age
Adults
FIG.3. Parabolic relationships of mean developmental periods of l-(a)- and 3-( O)-day cultures of longer experiment to age in days of parents laying eggs. TABLE 1. PERCENTAGE OF VARIABILITYOF THE MEAN DEVELOPMENTAL PERIODSOF 100 g CULTURES OF Sitophilwpnarius ACCOUNTEDFORBYAGE AND CULTURE DENSITY Short experiment
Long experiment 1 day
3 day
1 day
Parental age
mostly young
mostly old
all young
Age linear Age parabolic
1 p
25.9 50.6
** ***
Density
d
23.5
*
Density+age linear Density+ age parabolic
27.5 dp 60.0
Oviposition period
* ***
7.5 l/l 64.2 *** 11.6
p
22.3 66.9
1 1
20.2 23.8
** **
2.9
***
I 20.2 &I 26.1
** **
Asterisks denote the 5, 1 and 0.1 per cent levels of statistical significance and l, p and d denote which of the regression coefficients is significant.
The Influence of Age of the Parents on the Offspring of Insects
381
developmental period with age for the l-day cultures but not for the 3-day cultures. When a parabolic term is added to each regression equation to describe the curvature (Fig. 3) the equations account for between a half and two-thirds of the variability of developmental period in the two sets of cultures (Table 1). Oviposition rate and parental age are highly correlated. There is some risk that in these experiments there were sufficient developing stages in some cultures to generate enough heat to raise the internal temperature of the cultures and so to shorten the mean developmental periods. It is possible, therefore, that the trend of lengthening developmental period for the younger parents is related to internal culture temperatures and that the observed relationship with age is a consequence of the correlation of culture density with both age and developmental period. The relationship between density and developmental period for cultures yielding 300-600 adults/3000 wheat kernels is obvious in Fig. 4 but for sparser cultures it is obscured for 3-day cultures by the quicker development of offspring of aged parents. Nevertheless (Table 1) there is a statistically significant relationship accounting for 1
I
I
I
I
I
I
d
I
.
0 0 I
I
1
I 2
1 Number
I
I
I
3
I
I
4
5
[hundreds]
FIG. 4. Plot of mean developmental periods of l-( l)- and 3-(O)-day cultures of long experiment against culture density with line representing the linear regression for l-day cultures.
one quarter of the variability of developmental period for the l-day cultures, predominantly from young parents, although the highest culture yield only just exceeds 200. Similarly in the short experiment restricted to young parents, if the first two very sparse cultures are omitted (but not otherwise-Table l), the regression of developmental period on culture yield is significant although the period has a range of only l-25 days and the yield varies only from 72 to 157 (Fig. 2). Adding a term for density to the parabolic regression for age diminishes unexplained variation to a statistically significant extent only for the l-day cultures
382
R. W. HOWE
of the long experiment (Table 1). For this it converts a positive linear regression coefficient for age into a negative one, but since neither of these linear components is statistically significant (Table 1) it is not worth speculating about the meaning of this change. The parabolic coefficient for age (Table 1) appears to be important. There is an interesting detail in the early stages of the long experiment. Whilst the trend from one week to the next was for the mean developmental period to increase, during each week it decreased day by day (Fig. 3). The daily transfers were interrupted by a 3 day period of rest but it is difficult to understand how the more frequent disturbance of the parents could influence the developmental period of offspring. In the short experiment (Fig. 2) there are signs of a daily decrease of developmental period from days 6 to 13, and 15 to 18 and perhaps from 19 to 22 and 23 to 27, but as a rule these would probably be dismissed as random variation.
Sign$cance
of results
Summing up these observations, we may conclude that the rate of development is related to parental age and that the relationship is parabolic. Experimental complexities make such relationships difficult to establish. In this work the influence of culture density is obvious at over 300 insects per culture, but is not revealed by the mathematical analysis of the 3-day cultures (Table 1). It is revealed in the l-day cultures at densities as low as 70 per culture but since each individual liberates heat at a rapid rate for only about 5 days, the total heat production at any time depends on the spread of developmental stages as well as their number. The range of mean developmental periods in these experiments at 25°C is very small, about 3 days at most, and is unlikely to be of any practical importance to the species. If the influence of parental age persists throughout the temperature range, however, it would be quite large near the lower threshold temperature. Our ignorance of the biology of S. granarius before man provided it with plentiful food makes it impossible to decide whether or not this would have any evolutionary significance. In its original environment this species probably survived the winter in all stages, though principally as an adult. It probably had to survive a summer food scarcity and did this as an adult or in fleshy stems and tubers. It would probably be an advantage for the numerous offspring of young parents to grow as rapidly as possible in the spring and for the offspring of old parents to complete their development in the autumn before the weather beomes too cold. There is less advantage in quick development
during the mid-summer
food shortage.
DISCUSSION
No clear evidence of a systematic trend related to parental age in any characteristic of the offspring has emerged from the experiments and published data discussed in this paper. Nevertheless, a variety of such trends, often opposite in direction for the same characteristic have been noted and it is clear that trends related to parental age may appear in any experiment. Consequently very great care must be taken to ensure by proper design either that bias is avoided or else that trends related to parental age are recorded and measured. Because they are often complex and seldom seem to be consistent age-related trends may often be dismissed as nuisance factors in experiments, but the most
The Influence of Age of the Parents on the Offspring of Insects
383
frequent trend is parabolic and has most influence on the offspring of the younger middle-aged parents, especially in an unfavourable environment. Since this is the most fertile age group, the behaviour of the offspring may be expected to have an evolutionary significance. The genetic control of developmental rates is probably very complex but the results with .Ne~aru viridulu and Tenebrio molitor in constant conditions compel us to admit that the genetic constitution of offspring or its phenological expression may be consistently influenced by the age of the parents. The rates of crossing-over may be high when gametes are formed in very young or very old insects. There may also be changes caused by maternal physiology. The constituents of the egg may alter as the mother ages and the derivatives of these constituents may be passed to later instars or even to later generations. In some instances the age-related changes might persist as a result of selection operating through the physiology and ecology of the species. ROBERTSONand SAXG (1944) attribute changes of egg viability to a change of source of the food reserves of the egg from the initial reserves of the mother to those derived from her feeding as an adult. Adult longevity is an advantage to a species only if it increases fecundity or if it prolongs oviposition so that the eggs laid late in life, or the larvae hatched from them, escape some lethal or unfavourable conditions that harm those laid earlier. Frequently stocks cannot be maintained by breeding continuously from very young or very old parents (CALLAHAN, 1962 ; FLEMINGSand LUDWIG, 1964). This might be explained, as claimed by GOETSCH (1956), by the passage of an inhibitory substance, or it might be explained genetically. Genes favouring adult longevity may be detrimental during the developmental stages (CLARK and ROCKSTEIN, 1964) or the gene combinations from very young parents may be unstable (PARSONS, 1962). Oscillatory influences of age as reported by DAVID (1961) might arise from the oscillations of population size and hence of selection pressure. If the influence of parental age on rates of development is to be consistent, it might be expected to be linked to the annual seasons. This is so for Jveruru viridulu (KIRITANI, 1963) which grows faster at 25°C in June than it does in May, and is also true for Tenebrio molitor. This latter species has only one generation a year in warehouses and the more rapid development of the offspring of older parents ensures that all the new generation of adults appear at about the same time in the spring. This species passes the winter in the larval stage and although high summer and autumn temperatures may stimulate rapid growth to maximum size, they also inhibit pupation. This diapause-like block to pupation also serves to keep the species in the larval stage during the winter, so it seems probable that in its original habitat only the larvae successfully survived the winter. 7: molitor is not a satisfactory species for the investigation of the influence of parental age because it is so variable, especially in the larval developmental period. LECLERQ (1963) bred two strains with widely different developmental periods and pupal weights from a stock initially intermediate for both. HOWE and BURGES(1953) recorded larval periods in constant conditions ranging from 173 to well over 300 days and in an earlier unpublished experiment in the same conditions recorded periods of 126 to 200 days. The Fordham University experiments at 25°C for parents of a given age and for the same experimental procedure gave a similar wide range of developmental
384
R. IV. HOWE
periods but the rate of development always accelerated as the parents grew older. Most of the other stored products insects for which an influence of parental age has been suspected, e.g. species of Bruchidae, Ptinidae, Sitophi1u.sand Tribolium all complete several generations a year. It is difficult to postulate how selection could act consistently in these species since we do not know their original habitats. In a primitive habitat a rapid rate of increase would rapidly deplete a sparse food, so the most favoured individuals should be those best able to find fresh food. In warehouses with ample food, the characteristics on which selection acts have presumably changed considerably and nowadays possibly resistance to chemicals used to control them is the most potent selective force. There seems to be no reason to expect a consistent relationship with parental age of any feature of development in these species. The work with them concerning parental age warns us of experimental hazards but contributes little of fundamental value to the study of their ecology. REFERENCES ANDERSEN,F. S. (1963) Population density of grain insects. Arsberetn. St Skadedyrlab. 1959 and 1960, 75-77. BUTZ, A. and HAYDEN,P. (1962) The effects of age of male and female parents on the life cycle of Drosophila melanogaster. Ann. ent. Sot. Am. 55, 617-618. CALLAHAN,R. F. (1962) Effects of parental age on the life cycle of the housefly, Musca domestica Linnaeus (Diptera, Muscidae). 3t N.Y. ent. Sot. 70, 150-158. CHAUVIN,R. (1958) L’action du groupement sur la croissance des grillons (Cryllulr~ domesticus). 3. Insect Physiol. 2, 235-248. CLARK, A. M. and ROCKSTEIN, M. (1964) Aging in insects. In: Physiology of Zruecta (Ed. by ROCKSTEIN, M.), Vol. 1, pp. 227-281. Academic Press, New York. DAVID,J. (1961) Influence de l’etat physiologique des parents sur les caracteres des descendants. Annl. Gtw. 3, i-78. DAVID,J. (1962) Influence de l’age de la mere sur les dimensiens des oeufs dans une souche vestigial de Drosophila melanogaster Meig. Et u de experimentale du determinisme physiologique de ces variations. Bull. biol. Ft. Belg. 96, 505-528. DURRANT,A. (1955) Effect of time of embryo formation on quantitative characters in Drosophila. Nature, Land. 175,560-56 1. FIORE, C. (1960) Effects of temperature and parental age on the life cycle of the dark mealworm, Tenebrio obscurus Fabricius. 31 jV.Y. ent. Sot. 68, 27-35. FLEMXNGS, M. B. and LUDWIG, D. (1964) Effects of temperature and parental age on the life cycle of the body louse, Pediculus humanus humanus. Ann. ent. Sot. Am. 57, 560-563. GOETSCH, W. (1956) Estudios sotore edad y vejez de 10s insectos y sustancias que intervienen en allas. Eos. Madr. 32, 185-2 13. HOWE, R. W. (1950) Studies on beetles of the family Ptinidae, 4. A note on an anomalous effect of parental age on the speed of development. Entomologist’s mon. Mag. 86, 325-326. HOWE, R. W. (1960) The effects of temperature and humidity on the rate of development and the mortality of Tribolium confusum Duval (Coleoptera, Tenebrionidae). Ann. a#. Biol. 48, 363-376. HOWE, R. W. and BURGES, H. D. (1953) A note on the resistance of Tenebrio molifor (L.) (Col. Tenebrionidae) to tropical temperatures. Entomologist’s mon. Mag. 89, 4-6. HOWE, R. W. and CURRIE, J. E. (1964) S ome laboratory observations on the rates of development, mortality and oviposition of several species of Bruchidae breeding in stored pulses. Bull. ent. Res. 55,437-477. Howe, R. W. and HOLE, B. D. (1967) The yield of cultures of Sitophilus granarius at 25°C and 70 per cent r.h. with some observations on rates of oviposition and development. 3. stored Prod. Res. 2, 13-27. KIRITANI, K. (1963) Oviposition habit and effect of parental age upon the post embryonic development in the southern green stink bug, &zara viridula. Jap. 3. Ecol. 13, 88-96.
The Influence of Age of the Parents on the Offspring of Insects
385
LARSOS, A. 0. and Smwo~s, P. (1923) Notes on the biology of the four-spotted bean weevil, Bruchus quadrimacuiatus Fab. J. agric. Res. 26, 609-616. LECLERQ,J. (1963) Artificial selection for weight and its consequences in Tenebrio molitor L. ,I’ature, Lond. 198, 106-107. LUDWIG, D. (1956) Effects of temperature and parental age on the life cycle of the mealworm, Tenebrio mofitor Linnaeus (Coleoptera, Tenebrionidae). Ann. ent. Sot. Am. 49, 12-15. LUDWIG,D. and BARSA, M. C. (1955) Relationship between the activity of the succinoxidasc system and the rate of oxygen consumption during the embryonic development of the mealworm, Tenebrio molitor Linnaeus. 3. gen. Physiol. 38, 729-734. LUDWIG,D. and FIORE, C. (1960) Further studies on the relationship between parental age and the life cycle of the mealworm, Tenebrio molitor. Ann. ent. Sot. Am. 53, 595-600. LUDWIO, D. and FIORE, C. (1961) Effects of parental age on offspring from isolated pairs of the mealworm, Tenebrio molitor. Ann. ent. Sot. Am. 54, 463-464. LUDWIG,D., FIORE, C. and JoNas, C. R. (1962) Physiological comparisons between offspring of the yellow mealworm, Tenebrio molitor, obtained from young and from old parents. Ann. ent. Sot. Am. 55,439-442. LUDWIG,D. and Jonas, C. R. (1964) Changes in the concentration of certain amino acids in homogenates of the yellow mealworm, Tenebrio molitor, during aging. Ann. en;. Sot. Am. 57, 210-2 13. O’BRIAN, D. M. (1961) Effects of parental age on the life cycle of Drosophila melanogaster. Ann. ent. Sot. Am. 54, 412-416. PARK, T. (1935) Studies in population physioiogy. IV. Some physiological effects of conditioned flour upon Triboliwn confwum Duval and its populations. Physial.