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Direct and indirect effects of day length on the aphid Megoura viciae Buckton
J. InsectPhysiol.,1967, Vol. 13, pp. 1781 to 1785. Pergamon Press Ltd.
Printed in Great Britain
DIRECT AND INDIRECT EFFECTS OF DAY LENGTH ON THE APHID MEGOUliA VICIAE BUCKTON A. D. LEES Agricultural Research Council Unit of Insect Physiology, Department of Zoology, Cambridge (Received
20 July
1967)
Abstract-Although it is known that the production of the sexual and parthenogenetic female morphs in Megoura viciae can be controlled by the direct action of photoperiod on the parent aphid, von Dehn has recently advanced the view that there is an additional, if subsidiary, photoperiodic effect which is exerted through the medium of the plant. However, an examination of the experimental methods employed by von Dehn suggests that the evidence for the indirect action of photoperiod may be far from conclusive. In particular, it is pointed out that no account has been taken of the very early (prenatal) onset of photoperiodic sensitivity in this insect. Very slight differences in the day-length rCgime during this period of ontogeny greatly influence the response pattern.
ALTHOUGH photoperiodically controlled responses (diapause, polymorphism) in phytophagous arthropods are sometimes influenced by the state of maturity of the host plant, experimental studies have so far failed to establish unequivocally that these responses are closely dependent on photoperiodic processes in the plant. The recent observations of VON DEHN (1967) on photoperiodism in the vetch and bean aphid M?goura viciue are therefore of more than usual interest. The production of the sexual and parthenogenetic daughters in Megoura under typical long- and short-day conditions is controlled by the direct action of illumination on the brain of the parent (LEES, 1964). von Dehn accepts this finding but considers that there is an additional day-length effect, exerted through the plant, which is most strongly exhibited when the aphid is in darkness. This line of thought has been prompted by the possibility that morph determination in rootdwelling aphids-which presumably live in darkness during the subterranean phase of their life cycles-is controlled by a photoperiodic stimulus emanating from the leaves. Indeed, it is claimed that a similar mechanism operates in Megoura, even though this species is entirely aerial in habit. The intrinsic day-length detector in Megoura governs (probably by humoral means) the determination of the young embryos as future virginoparae or oviparae. I have shown (LEES, 1959) that the sequence of offspring reflects the environmental photoperiod experienced by the parent aptera during her development. In order, therefore, to find the direction of the response, it is not always necessary to collect 1781
1782
A. D. LEES
and classify all the offspring of a given female. Nevertheless, an understanding of the time of onset of day-length sensitivity and of the chronology of embryonic determination is obviously essential in order that the experimental treatment (whether darkness or a daily photoperiod) may be applied to the parent aphid at the appropriate moment in her ontogeny. von Dehn has not reinvestigated this matter and has unfortunately overlooked a later paper (LEES, 1963) in which it is shown that photoperiod has a highly significant effect before the mother is even born. Further confusion can perhaps best be avoided by recalling some of the essential details. Megoura produces only parthenogenetic daughters (virginoparae) when reared at moderate temperatures (15°C) and a long (16 or 18 hr) photoperiod. These conditions were used both by von Dehn and myself for maintaining stock cultures which formed the starting point for experiments on day-length response. The procedure used in 1959 was as follows: reproducing virginoparae apterae (the ‘grandparents’) were removed from sparse stock cultures and placed on germinating broad beans (Viciu f&z, in the hook stage and about 1 cm high) at the experimental photoperiod. After one illumination cycle the grandparents were discarded. The larvae born overnight for the most part developed into apterae (the ‘parents’), a number of which were then progeny-tested by transferring them serially to fresh germinating bean plants as soon as each parent had deposited about 20 larvae (the ‘progeny’). It should be noted that in this schedule the experimental photoperiod was maintained constant from one day before birth to the end of the reproductive period of the parent. In the absence of light it was then found that 14 per cent of the parents subsequently produced only sexual females (oviparae), 14 per cent produced exclusively virginoparous progeny, and 72 per cent gave birth to mixed ‘families’. In the latter instance the parents almost invariably started as virginopara-producers but later ‘switched over’ to the production of oviparae. Since parents reared in short-day conditions (12 hr light daily) gave birth exclusively to oviparae and under long-day conditions (16 hr light daily) to virginoparae, this result appeared to indicate that constant darkness was an ‘intermediate’ stimulus and that the sequential determination of the larval progeny began immediately after the birth of the parent. Later work (LEES, 1963) showed that continuous darkness could more appropriately be regarded as exerting a weak short-day effect-a mode of action common in many other insects (see LEES, 1960). The explanation of the result cited above is that the maternal determiner begins to function (and is then actuated by long-day photostimulation) 2 to 4 days before the birth of the parent. It is possible to prove experimentally that late embryos respond in this way to light transmitted through the abdominal wall of the grandparent (LEES, 1964). Yet although the young embryos of the next (‘progeny’) generation are already forming at this time, they do not become competent to respond irrevocably to the maternal determiner until several days later, that is, after the parent has been born. These relationships are such that when a grandparental aphid is taken from long-day conditions, the unborn parents are already biased in the direction of
EFFECTS
OF DAY
LENGTH
ON
THE
APHID
MEGOURA
VICIAE
1783
virginopara-production. However, if the mothers are exposed from the day of birth to ‘strong short-day conditions (say, 12 hr light daily) this stimulus is sufficient to reverse the incipient effect of the maternal determiner promptly, with the result that the first offspring born are oviparae. When a weak short-day stimulus is p:rovided (for example, continuous darkness or photoperiods slightly shorter than the critical length) the maternal determiner is not ‘switched over’ promptly and the mother then produces a short series of virginoparae before ending her reproductive life as an ovipara-producer. If, on the other hand, the grandparent is also kept in darkness for several days before the birth of the mother, the latter almost invariably produces oviparae from the outset. The effect of the prenatal light regime on the form of the response curve is shown in Fig. 1 which is based on my 1963 results. An inspection of Fig. 1 shows
Phofoperiod
(h
light
per
24hl
FIG. 1. Th’e influence of two different prenatal regimes on the form of the photoperiodic response curve in Megoura viciae. In one instance (dotted curve) the same photoperiods were maintained both before and after the birth of the parent; in the othe:r (continuous curve), the different postnatal treatments were all preceded by a long-day (16 hr) prenatal regime. The apterous parents either produce owlparae only (A), virginoparae only (B), or mixed families (C).
that the prenatal exposure to long days is very influential when the postnatal photoperiod is nearly critical or when light is absent altogether. In such instances we are, of course, observing the effects of a switch in the maternal determiner. Nevertheless, it should be emphasized that parent apterae may also produce ‘mixed’ families if the lphotoperiodic conditions are truly intermediate, even if these conditions are maintained unchanged throughout the pre- and postnatal developmental
1784
A. D. LEES
periods of the mother. In this case, however, the aphids tend to produce alternating sequences of oviparae and virginoparae. As Fig. 1 illustrates, the production of mixed families is particularly prevalent when the photoperiod is of critical length, although 16 per cent of the parents reared in permanent darkness also showed this type of behaviour. It will be appreciated that differences in the timing of the experimental treatment, although apparently trivial, will exert a substantial effect on the response pattern. von Dehn was unable to confirm my 1959 results since, in her experiments, parents reared from birth in permanent darkness all became virginoparaproducers at the beginning of the reproductive period. However, the procedure adopted by von Dehn differed slightly but significantly from that outlined above. Thus the grandparents from the long-day stock cultures were allowed to deposit larvae in long days and the latter were only subsequently transferred to the experimental conditions (darkness). The prenatal long-day treatment was therefore extended by 1 day and may even have included the first day of postnatal development. In these circumstances all parents would be expected to begin their reproductive period as virginopara-producers. It should be added that the percentage of oviparae in the progeny is not a satisfactory measure of the response intensity. Since the virginoparae are born first, the proportion of oviparae will depend on the total numbers of progeny collected-which differed in von Dehn’s experiments. The evidence presented by von Dehn in favour of a plant-mediated photoperiodic effect is based on experiments of two kinds : (1) reproduction of the aphids in short days is said to be influenced by the pretreatment of the plants with long days. Here, however, the differences were so small that they could easily be accounted for by slight inconsistencies in the time of application of the short-day treatment. In my own experience parental aphids become oviparaproducers, irrespective of the day-length regime in which the plant has been grown-always provided the short-day exposure is begun 24 days before the birth of the parent; but the production of mixed families becomes increasingly prevalent if the short-day treatment is delayed until the first instar. (2) In experiments of the second type, aphids were enclosed on the basal joint of a bean plant by means of a light-excluding sleeve. More oviparae were produced when the top of the plant was exposed to short days than when it received long days. Even, however, if it is assumed that in this instance the rearing procedure was strictly controlled this technique may not be easy to apply. Broad bean stems (which Megoura probes) themselves conduct light to a significant extent. This can readily be appreciated if the eye is applied to a small peep-hole made in the side of an opaque black paper sleeve, similar to that constructed by von Dehn. Although von Dehn showed that photographic paper was not fogged by a 15 rnin exposure inside the sleeve, this does not eliminate the possibility that the aphids were influenced by light conducted down the stem from the illuminated crown. In Megoura the threshold of perception for blue light of wavelength 470 ml*, when applied for 1 hr during the appropriate part of the dark period, is only 0.2 PW cm-’ (roughly equivalent to illumination of 0.02 ft-c. of white light) (LEES, 1966).
EFFECTS OF DAY LENGTH ON THE APHID MEGOURA
VICIAE
1785
von Dehn’s results suggest that more male aphids are formed in short- than in long-day conditions. This may perhaps be a real clonal difference since in my experiments the numbers of males produced per parent, although highly variable, were not statistically different in long and short days. However, von Dehn’s belief that photoperiod is acting through the plant is again based on the results of a small number of experiments involving light exclusion. Plant factors of one sort or another may well be important in controlling polymorphism in some species of aphids (see, for example, the recent observations of SETHI and SWENSON (1967) on the production of sexuparae in Eriosomapyricola); nevertheless, on the basis of the present evidence, it is difficult to accept von Dehn’s contention th.at the formation of oviparae and males in Megoura is influenced by a photoperiodic stimulus transmitted from the host plant. REFERENCES (1967) uber den Photoperiodismus heterogener Aphiden. Zur Frage der direkten od.er indirekten Wirkung der Tageslange. J. Insect Physiol. 13, 595-612. LEES A. D. (1959) The r81e of photoperiod and temperature in the determination of parthenogenetic and sexual forms in the aphid Megoura viciae Buckton-I. The influence of these factors on apterous virginoparae and their progeny. J. Insect Physiol. 3, 92-l 17. LEES A. D. (1960) Some aspects of animal photoperiodism. Cold Spring Harb. Symp. Quant. Biol. 25, 261-268. LEES A. D. (1963) The role of photoperiod and temperature in the determination of parthenogenetic and sexual forma in the aphid Megoura viciae Buckton-III. Further properties of the maternal switching mechanism in apterous aphids. J. Insect Physiol. 9, 153-164. LEES A. D. (1964) The location of the photoperiodic receptors in the aphid Megoura viciae Buckton. J. exp. Biol. 41, 119-133. LEES A. D. (1966) Photoperiodic timing mechanisms in insects. Nature, Lond. 210, 986989. SETHI S. L. a:nd SWENSON K. G. (1967) Formation of sexuparae in the aphid Eriosoma pyricola, on pear roots. Entomologia exp. appl. 10,97-102.
VON DEHN
M.
Printed in Great Britain
DIRECT AND INDIRECT EFFECTS OF DAY LENGTH ON THE APHID MEGOUliA VICIAE BUCKTON A. D. LEES Agricultural Research Council Unit of Insect Physiology, Department of Zoology, Cambridge (Received
20 July
1967)
Abstract-Although it is known that the production of the sexual and parthenogenetic female morphs in Megoura viciae can be controlled by the direct action of photoperiod on the parent aphid, von Dehn has recently advanced the view that there is an additional, if subsidiary, photoperiodic effect which is exerted through the medium of the plant. However, an examination of the experimental methods employed by von Dehn suggests that the evidence for the indirect action of photoperiod may be far from conclusive. In particular, it is pointed out that no account has been taken of the very early (prenatal) onset of photoperiodic sensitivity in this insect. Very slight differences in the day-length rCgime during this period of ontogeny greatly influence the response pattern.
ALTHOUGH photoperiodically controlled responses (diapause, polymorphism) in phytophagous arthropods are sometimes influenced by the state of maturity of the host plant, experimental studies have so far failed to establish unequivocally that these responses are closely dependent on photoperiodic processes in the plant. The recent observations of VON DEHN (1967) on photoperiodism in the vetch and bean aphid M?goura viciue are therefore of more than usual interest. The production of the sexual and parthenogenetic daughters in Megoura under typical long- and short-day conditions is controlled by the direct action of illumination on the brain of the parent (LEES, 1964). von Dehn accepts this finding but considers that there is an additional day-length effect, exerted through the plant, which is most strongly exhibited when the aphid is in darkness. This line of thought has been prompted by the possibility that morph determination in rootdwelling aphids-which presumably live in darkness during the subterranean phase of their life cycles-is controlled by a photoperiodic stimulus emanating from the leaves. Indeed, it is claimed that a similar mechanism operates in Megoura, even though this species is entirely aerial in habit. The intrinsic day-length detector in Megoura governs (probably by humoral means) the determination of the young embryos as future virginoparae or oviparae. I have shown (LEES, 1959) that the sequence of offspring reflects the environmental photoperiod experienced by the parent aptera during her development. In order, therefore, to find the direction of the response, it is not always necessary to collect 1781
1782
A. D. LEES
and classify all the offspring of a given female. Nevertheless, an understanding of the time of onset of day-length sensitivity and of the chronology of embryonic determination is obviously essential in order that the experimental treatment (whether darkness or a daily photoperiod) may be applied to the parent aphid at the appropriate moment in her ontogeny. von Dehn has not reinvestigated this matter and has unfortunately overlooked a later paper (LEES, 1963) in which it is shown that photoperiod has a highly significant effect before the mother is even born. Further confusion can perhaps best be avoided by recalling some of the essential details. Megoura produces only parthenogenetic daughters (virginoparae) when reared at moderate temperatures (15°C) and a long (16 or 18 hr) photoperiod. These conditions were used both by von Dehn and myself for maintaining stock cultures which formed the starting point for experiments on day-length response. The procedure used in 1959 was as follows: reproducing virginoparae apterae (the ‘grandparents’) were removed from sparse stock cultures and placed on germinating broad beans (Viciu f&z, in the hook stage and about 1 cm high) at the experimental photoperiod. After one illumination cycle the grandparents were discarded. The larvae born overnight for the most part developed into apterae (the ‘parents’), a number of which were then progeny-tested by transferring them serially to fresh germinating bean plants as soon as each parent had deposited about 20 larvae (the ‘progeny’). It should be noted that in this schedule the experimental photoperiod was maintained constant from one day before birth to the end of the reproductive period of the parent. In the absence of light it was then found that 14 per cent of the parents subsequently produced only sexual females (oviparae), 14 per cent produced exclusively virginoparous progeny, and 72 per cent gave birth to mixed ‘families’. In the latter instance the parents almost invariably started as virginopara-producers but later ‘switched over’ to the production of oviparae. Since parents reared in short-day conditions (12 hr light daily) gave birth exclusively to oviparae and under long-day conditions (16 hr light daily) to virginoparae, this result appeared to indicate that constant darkness was an ‘intermediate’ stimulus and that the sequential determination of the larval progeny began immediately after the birth of the parent. Later work (LEES, 1963) showed that continuous darkness could more appropriately be regarded as exerting a weak short-day effect-a mode of action common in many other insects (see LEES, 1960). The explanation of the result cited above is that the maternal determiner begins to function (and is then actuated by long-day photostimulation) 2 to 4 days before the birth of the parent. It is possible to prove experimentally that late embryos respond in this way to light transmitted through the abdominal wall of the grandparent (LEES, 1964). Yet although the young embryos of the next (‘progeny’) generation are already forming at this time, they do not become competent to respond irrevocably to the maternal determiner until several days later, that is, after the parent has been born. These relationships are such that when a grandparental aphid is taken from long-day conditions, the unborn parents are already biased in the direction of
EFFECTS
OF DAY
LENGTH
ON
THE
APHID
MEGOURA
VICIAE
1783
virginopara-production. However, if the mothers are exposed from the day of birth to ‘strong short-day conditions (say, 12 hr light daily) this stimulus is sufficient to reverse the incipient effect of the maternal determiner promptly, with the result that the first offspring born are oviparae. When a weak short-day stimulus is p:rovided (for example, continuous darkness or photoperiods slightly shorter than the critical length) the maternal determiner is not ‘switched over’ promptly and the mother then produces a short series of virginoparae before ending her reproductive life as an ovipara-producer. If, on the other hand, the grandparent is also kept in darkness for several days before the birth of the mother, the latter almost invariably produces oviparae from the outset. The effect of the prenatal light regime on the form of the response curve is shown in Fig. 1 which is based on my 1963 results. An inspection of Fig. 1 shows
Phofoperiod
(h
light
per
24hl
FIG. 1. Th’e influence of two different prenatal regimes on the form of the photoperiodic response curve in Megoura viciae. In one instance (dotted curve) the same photoperiods were maintained both before and after the birth of the parent; in the othe:r (continuous curve), the different postnatal treatments were all preceded by a long-day (16 hr) prenatal regime. The apterous parents either produce owlparae only (A), virginoparae only (B), or mixed families (C).
that the prenatal exposure to long days is very influential when the postnatal photoperiod is nearly critical or when light is absent altogether. In such instances we are, of course, observing the effects of a switch in the maternal determiner. Nevertheless, it should be emphasized that parent apterae may also produce ‘mixed’ families if the lphotoperiodic conditions are truly intermediate, even if these conditions are maintained unchanged throughout the pre- and postnatal developmental
1784
A. D. LEES
periods of the mother. In this case, however, the aphids tend to produce alternating sequences of oviparae and virginoparae. As Fig. 1 illustrates, the production of mixed families is particularly prevalent when the photoperiod is of critical length, although 16 per cent of the parents reared in permanent darkness also showed this type of behaviour. It will be appreciated that differences in the timing of the experimental treatment, although apparently trivial, will exert a substantial effect on the response pattern. von Dehn was unable to confirm my 1959 results since, in her experiments, parents reared from birth in permanent darkness all became virginoparaproducers at the beginning of the reproductive period. However, the procedure adopted by von Dehn differed slightly but significantly from that outlined above. Thus the grandparents from the long-day stock cultures were allowed to deposit larvae in long days and the latter were only subsequently transferred to the experimental conditions (darkness). The prenatal long-day treatment was therefore extended by 1 day and may even have included the first day of postnatal development. In these circumstances all parents would be expected to begin their reproductive period as virginopara-producers. It should be added that the percentage of oviparae in the progeny is not a satisfactory measure of the response intensity. Since the virginoparae are born first, the proportion of oviparae will depend on the total numbers of progeny collected-which differed in von Dehn’s experiments. The evidence presented by von Dehn in favour of a plant-mediated photoperiodic effect is based on experiments of two kinds : (1) reproduction of the aphids in short days is said to be influenced by the pretreatment of the plants with long days. Here, however, the differences were so small that they could easily be accounted for by slight inconsistencies in the time of application of the short-day treatment. In my own experience parental aphids become oviparaproducers, irrespective of the day-length regime in which the plant has been grown-always provided the short-day exposure is begun 24 days before the birth of the parent; but the production of mixed families becomes increasingly prevalent if the short-day treatment is delayed until the first instar. (2) In experiments of the second type, aphids were enclosed on the basal joint of a bean plant by means of a light-excluding sleeve. More oviparae were produced when the top of the plant was exposed to short days than when it received long days. Even, however, if it is assumed that in this instance the rearing procedure was strictly controlled this technique may not be easy to apply. Broad bean stems (which Megoura probes) themselves conduct light to a significant extent. This can readily be appreciated if the eye is applied to a small peep-hole made in the side of an opaque black paper sleeve, similar to that constructed by von Dehn. Although von Dehn showed that photographic paper was not fogged by a 15 rnin exposure inside the sleeve, this does not eliminate the possibility that the aphids were influenced by light conducted down the stem from the illuminated crown. In Megoura the threshold of perception for blue light of wavelength 470 ml*, when applied for 1 hr during the appropriate part of the dark period, is only 0.2 PW cm-’ (roughly equivalent to illumination of 0.02 ft-c. of white light) (LEES, 1966).
EFFECTS OF DAY LENGTH ON THE APHID MEGOURA
VICIAE
1785
von Dehn’s results suggest that more male aphids are formed in short- than in long-day conditions. This may perhaps be a real clonal difference since in my experiments the numbers of males produced per parent, although highly variable, were not statistically different in long and short days. However, von Dehn’s belief that photoperiod is acting through the plant is again based on the results of a small number of experiments involving light exclusion. Plant factors of one sort or another may well be important in controlling polymorphism in some species of aphids (see, for example, the recent observations of SETHI and SWENSON (1967) on the production of sexuparae in Eriosomapyricola); nevertheless, on the basis of the present evidence, it is difficult to accept von Dehn’s contention th.at the formation of oviparae and males in Megoura is influenced by a photoperiodic stimulus transmitted from the host plant. REFERENCES (1967) uber den Photoperiodismus heterogener Aphiden. Zur Frage der direkten od.er indirekten Wirkung der Tageslange. J. Insect Physiol. 13, 595-612. LEES A. D. (1959) The r81e of photoperiod and temperature in the determination of parthenogenetic and sexual forms in the aphid Megoura viciae Buckton-I. The influence of these factors on apterous virginoparae and their progeny. J. Insect Physiol. 3, 92-l 17. LEES A. D. (1960) Some aspects of animal photoperiodism. Cold Spring Harb. Symp. Quant. Biol. 25, 261-268. LEES A. D. (1963) The role of photoperiod and temperature in the determination of parthenogenetic and sexual forma in the aphid Megoura viciae Buckton-III. Further properties of the maternal switching mechanism in apterous aphids. J. Insect Physiol. 9, 153-164. LEES A. D. (1964) The location of the photoperiodic receptors in the aphid Megoura viciae Buckton. J. exp. Biol. 41, 119-133. LEES A. D. (1966) Photoperiodic timing mechanisms in insects. Nature, Lond. 210, 986989. SETHI S. L. a:nd SWENSON K. G. (1967) Formation of sexuparae in the aphid Eriosoma pyricola, on pear roots. Entomologia exp. appl. 10,97-102.
VON DEHN
M.