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Semen Production of White Leghorn Males from Strains Selected for High and Low Fecundity
Semen Production of White Leghorn Males from Strains Selected for High and Low Fecundity Department
D. G. JONES AND W. F. LAMOEEUX of Poultry Husbandry, Cornell University, Ithaca, New
York
(Received for publication August 12, 1941)
S
OME physiological characters, such as egg production in the fowl and milk production in dairy cattle, can only be expressed by the female and few, if any, reliable means except the progeny and sib tests are available for the estimation of the genotype of the male. It is known, however, that the gonads of both males and females are subject to control by endocrine secretions of the body. Whether or not these hormones are one of the means by which genetic differences in fecundity are expressed in the fowl has not been shown directly, but much indirect evidence indicates that they are involved. If this is true, it seems probable that in strains of fowl with most birds capable of high production, and in other strains capable of only low egg production, the males as well as the females should show some difference in their efficiency of reproduction. This study was undertaken, therefore, to determine whether or not the genotypes, which in females induce relatively high or low egg production, are also responsible for differences in the size of testes, age at the onset of spermatogenesis, or the volume of semen produced in males. MATERIAL AND METHODS
The high- and low-fecundity strains of White Leghorns used for this study have been selected for high and for low egg production, respectively, since 1915. The early development of the two strains by
means of mass selection has been described by Marble and Hall (1931) and Hall (1934). Following the'original divergence of the two strains, the average age at first egg and the first year's egg production for the low-fecundity females did not change materially until after 1935. During the same period there was an increase in egg production in the high-fecundity strain and a slight decrease in the average age at first egg, while body weight of both strains increased. Since 1935 the progeny test and sib test have been used, and breeders have been selected not only on the basis of their individual performance, but also with regard to the average performance of their families. Such selection was accompanied by an appreciable decrease in the production of the females of the low-fecundity strain (Table 1). However, during this period the major emphasis in the selection of breeders in the high-fecundity strain was placed upon viability rather than upon egg production. Under these circumstances, less increase in egg production was obtained than might have been expected had egg production been the primary basis of selection. Since the beginning of the progeny test, egg production has been tabulated to 500 days of age, while previous to that time it was determined for the 365 days following the date on which the first egg was laid. Since most of the females of the highfecundity strain continue to lay for a
[173]
174
D. G. JONES AND W.
F. LAMOREUX
TABLE 1.—A comparison of the females in the high- and low-fecundity strains with respect to first year's egg production, age at first egg, and mature body weight Ave. first year egg production
Ave. age at first egg
Ave. body weight (in grams)
Year High fecundity
Low fecundity
High fecundity
Low fecundity
High fecundity
Low fecundity
1913-14 1919-20 1925-26 1931-32
119 161 172 209
99 125 122 96
232 205 199 203
252 225 240 254
1438 1683 1728 1819
1529 1497 1642 1696
1937-38* 1939^0*
174 179
57 67
190 212
300 310
1784 1813
1674 1755
* Egg production to 500 days of age.
period of time after they reach 500 days of age, the production for a year following the date the first egg was laid would be somewhat greater than the production to 500 days of age. On the other hand, most of the low-fecundity females have ceased laying by the time they are 500 days old so that egg production to 500 days of age is not greatly different from egg production for a year following the date the first egg was laid. Therefore, the difference between the two strains in any given year is somewhat less when calculated for the period ending 500 days after hatching than it is for the period ending 365 days after the first egg was laid. Such large and increasing differences in egg production of the two strains, as shown by Table 1, and their long history of genetic selection make these strains particularly suitable for a study, in the males, of the expression of genotypes which are responsible for high and low fecundity in the females. Chicks from these two strains were obtained from hens selected in the following ways. In the high-fecundity strain, only those females were used which had completed their first laying year (365 days) with production records of not less than 200 eggs. Such selection was practiced because there is considerable variation in each strain and it seemed desirable to avoid use of those birds in'the high-fecundity
strain which had relatively poor records of egg production. It was not possible, however, except for birds reported in Table 3 (groups 3 and 4), to use breeders of the low-fecundity strain which had completed their first year of production. Therefore, pullets were selected which had not laid before March 1 of the year after hatching and whose sisters had not laid before January 1 of the same year. Although these birds were untested with respect to their ability to lay, it seemed likely that they were typical of the low-fecundity strain, since the time remaining till the birds would be 500 days of age was inadequate for high records of egg production. Brooding and Rearing Most of the chicks used in this study were kept throughout their life in batteries which had wire floors. Because of inadequate space it was necessary for a short time to brood two hatches on a floor covered with shavings. However, these birds were on wire floors after they were four weeks of age. Since there was known to be a difference between the two strains with respect to mature body weight (Table 1), the chicks of the two strains were brooded separately to avoid such inequalities in growth and development as result from mixing birds of different sizes. The mature males reported in Table 3 (groups 3 and 4) were selected from among prospective
SEMEN PRODUCTION IN SELECTED STRAINS OF LEGHORNS
breeders which were reared on range. The feed used during the brooding period was a standard chick mash. Those birds reared to maturity received both mash and grain. Usually the mash was fed in the form of pellets to encourage greater consumption of feed after the cockerels were 12 to 14 weeks of age. Dubbing Because of the fact that the combs and wattles of fowl grow very large in the absence of direct sunlight, it often becomes difficult for males to eat when placed in the type of cages used. For that reason, a large part of the comb and wattles of those males which were reared to maturity was cut off at 13 weeks of age. Other males were not dubbed. Artificial Illumination Since it was not possible to provide a uniform amount of sunlight to the cages in which birds were placed to be reared to maturity, it was necessary to furnish artificial illumination. These birds received 14 hours of light daily from electric lamps. Males of the high- and low-fecundity strains were alternated in the cages so that any uncontrolled variations in intensity of illumination would not affect one strain more than the other. Collections of Semen These collections were made by means of a technic similar to that described by Burrows and Quinn (1935) with the exception of a modification in the method of holding the males as described by Lamoreux (1940). Histological Studies Histological studies were made of the testes from males 4 to 24 weeks of age. Since it was desired to compare the relative rates of spermatogenesis in birds of the two strains, the cockerels were injected with 20 mg. of colchicine per kilogram of
175
body weight at midnight, seven-and-onehalf to eight hours before they were sacrificed. This interval, midnight to 8:00 a.m., includes the period when meiosis is most active in the fowl (Riley, 1940). Colchicine was used to arrest cell divisions in metaphase, thus facilitating comparison of meiotic activity in birds of the low- and high-fecundity strains. Immediately after death of a bird, the testes were removed and weighed. A transverse section through the larger testis of the pair was then removed and placed in Allen's PFA3 modification of Bouin's fixing solution. The usual methods of washing, dehydrating in alcohol, and embedding in paraffin ("Tissuemat") were used. Sections of each testis so prepared were cut 6 microns thick and stained with hematoxylin and eosin. EXPERIMENTAL RESULTS
Differences in the Body Weight of Two Strains Soon after the experiment was begun, it became apparent that the two strains differed markedly in'their rates of growth. The resulting differences in body weight at different ages during the experimental period are shown in Table 2. Analysis of the variance in body weight shows that the males of the high fecundity strain are significantly heavier than cockerels of the other strain (value for P = <0.01). Because the degree of significance indicated by the F value obtained by analysis of variance is not accurate unless the standard deviations are independent of the mean values obtained (Snedecor, 1940, p. 378), it has been necessary to transform all data in this study to which statistical analyses have been applied, with the exception of Tables 5 and 6. The original values given in the tables have therefore been transformed to logarithms for the calculation of the significance of differences. It was not known whether or not the
176
D. G. JONES AND W. F. LAMOREUX
TABLE 2.—A comparison of the )
and low-fecundity strains with respect to body weight and weight of testes. Each average represents five birds
Ave. weight of body
Ave. weight of both testes
Age
weeks 4 6 8 9 10 12 14 16 20 24
High fecundity
Low fecundity
Diff.*
High fecundity
Low fecundity
Diff.*
grams 207 402 663 787 912 1128 1311 1569 1822 1921
grams 177 283 494 579 664 848 1061 1216 1418 1508
grams 30 119 169 208 248 280 250 353 404 413
grams .0612 .1802 .5493 .4914 2.274 3.526 5.516 5.885 16.72 18.30
grams .0608 .0846 .1938 .6786 1.183 5.456 3.296 6.070 16.85 13.95
grams .0004 .0956 .3555 - .1872 1.091 -1.930 2.220 -.185 -.13 4.35
' Average weight of high-fecundity strain minus that of low-fecundity strain.
difference in body weight between the two strains would be related to the size of testes or to semen production, but it is obvious that if such a relationship does exist between these variables, an error would be introduced unless corrections were made for differences in body weight. It was subsequently found that there was very little correlation (r = 0.121) between body weight and size of testes in birds of the same age in the two strains, thus indicating that analysis of covariance was not necessary. However, there was a significant correlation (r = 0.32) between size of body and semen production at 24 weeks of age, making it necessary to correct for differences in body weight in the two strains (Table 4). In two experiments, high- and low-fecundity-strain males were selected by pairs so that differences in body weight between males of the two strains were not significantly different and analysis of covariance was not required (Table 3, groups 3 and 4).
ovary, one might reasonably expect the testes of the males in the low-fecundity strain either to be somewhat smaller than those of the high-fecundity strain or to be somewhat less efficient in their production of semen. To test the first of these two possibilities, the testes of those males killed for histological study were removed and weighed to determine whether there was a difference between their weights in the two strains. The average weights of the testes from males 4 to 24 weeks of age are given in Table 2. In spite of the difference in body size, shown above, an analysis of variance shows that there was no significant difference in the size of the testes in the two strains. It was also found that there was no significant interaction between strains and ages with respect to either body weight or size of testes, and we may therefore conclude that increments in age influence the rates of growth in both strains to a similar degree.
Size of Testes It seemed logical to assume that, since the selection for high and low fecundity must have affected either the activity of the ovary in these two strains or some other gland which exerts an influence on the
Semen Production The first attempt to obtain semen was made when the males were 12 weeks old. At that time, attempts were made to collect semen on each of five successive days and a comparison was made between the strains
177
SEMEN PRODUCTION IN SELECTED STRAINS OF LEGHORNS
TABLE 3.—Differences in semen production between males of the high- and low-fecundity strains at various ages
Group
1 2 3* 4 :
No. in each strain
Age
12 24 243-271 316-365
weeks weeks days days
High
Low
Length of collection period
21 20 5 6
22 20 5 6
days 4 5 63 11
Ave. daily yield of semen (c.c.) High
Low
.13 .39 .47 .40
.045 .26 .32 .28
Value for P
<0.01 <0.01 <0.01 <0.01
Yields are averages of three collections a week taken on alternate days.
with respect to the number of males producing semen. Nineteen of the 21 highfecundity males produced semen at that time, while but 17 of the 22 low-fecundity males yielded any. This difference is small and the Chi-square test for homogeneity shows that the difference between the proportion of males producing semen in the two strains is not significant. In determining the difference between the volumes of semen produced by the two strains, the average production during the last four days of the five-day collection period was used. Although the volume of semen produced at this age was very low, the males of the high-fecundity strain produced significantly more semen (value for P = < 0.01) than those of the low-fecundity strain (Table 3, group 1). Since many of the males produced very little semen, it seems likely that this difference at 12 weeks of age was really a measure of the difference in age at sexual maturity in the two strains
rather than a measure of the difference in the ability of the two strains to produce semen. Collections of semen were again made from these males at the age of 24 weeks. It is quite apparent from the data presented in Table 3 (group 2) that the high-fecundity males continued to produce considerably more semen than the males of the low-fecundity strain. Also, there was a marked difference between body weights of the two strains. Analysis of variance for body weight of the males shows that at 24 weeks of age the high-fecundity males continue to be significantly heavier than those of the low-fecundity strain. A similar analysis of differences in semen production shows that the males of the high-fecundity strain produce significantly more semen than do those of the low-fecundity strain. The coefficient of correlation (r = 0.32) between body weight and the volume of semen produced was found to be significant
TABLE 4,—Analysis of covariance of body weight and semen production in the high- and low-fecundity strains {original data transformed to logarithms) Errors of estimate Source
Total Strains Within strains
D.F.
39 1 38
Sx2
0.1131 0.0625 0.0506
Sxy
0.1748 0.1369 0.0379
Sy2
0.5847 0.2998 0.2849
sum of squares
D.F.
.3145
38
.2565
37
Difference for testing adjusted means .0580 F = ^ ^ = 8.37, a highly significant difference. r-0.32, value for P = <0.05.
mean square
.0069
.0580*
178
D. G. JONES AND W. F. LAMOREUX
at the 5 percent level. It was necessary, therefore, to make some correction for the differences in body weight in order to determine whether the differences in semen production were merely the result of differences in body size, or whether an actual difference between the strains does exist irrespective of differences in size. Accord50 j 46
-
'
A
HIGH FECUNDITY LOW FECUNDITY
,^
1
1
1
1
1
16
[7
18
19
20
1
1
1
1
1
1
21 2 2 MARCH
1
23
24
25
26
27
CHART 1. The average daily semen production by six males in the low- and six in the highfecundity strain.
ingly, analysis of covariance was employed to correct for the difference in body weight between the two strains (Table 4). The analysis showed that even after correction was made for this difference the males of the high-fecundity strain still yielded a significantly larger volume of semen than those of the low-fecundity strain. Two additional comparisons of semen production in the two strains were obtained by selecting mature males of comparable body size from prospective breeding stock reared on range. In one study, semen was collected from five high- and five low-fecundity males, 243 to 271 days of age, three times each week during January and February. The average yields of the high-fecundity strain consistently exceeded those of the lowfecundity birds (Table 3, group 3) and the average difference was shown by an analysis of variance to be highly significant (value for P = < 0 . 0 1 ) . ' A similar group, composed of six males from each strain, 316 to 365 days of age, was selected and daily collections of semen
were made during a period of 11 days in March. Chart 1 shows the consistently higher daily yield of semen by males of the high-fecundity strain. The average yields of birds from each strain are shown in Table 3 (group 4) and again the difference in yield between the two strains was found to be highly significant as measured by analysis of variance (value for P = < 0.01). Altogether, four comparisons between the strains with respect to yield of semen have been made. At 12 and 24 weeks of age, and in two groups of mature males, the difference between the strains was found to be highly significant. It seems clear that the strain which is genetically incapable of high egg production is also unable to produce semen equal in volume to that produced by the high-fecundity strain. Production of Spermatozoa The difference in semen production between the two strains, even after correction for differences in body weight, is of particular importance and interest in this case because of the fact that there was no significant difference between the weights of testes in the two strains. This situation might be explained in either of two ways. First, the differences in volume of semen may result largely from differences in the proportion of accessory fluids in the semen. Second, the volume of semen may be an accurate measure of the number of spermatozoa produced. It is known from other studies in this laboratory and the work of Munro (1938) that if fowl semen is centrifuged, about 75 percent of its volume may be removed as a supernatent fluid, freed from spermatozoa. Studies have not been made to determine the variation between males or between daily collections of semen from the same male, with respect to the proportion
179
SEMEN PRODUCTION IN SELECTED STRAINS OF LEGHORNS
of accessory fluid in the semen. One might suspect, however, that differences between cockerels in the production of accessory fluid would be somewhat less than those between mammals, since the fowl lacks accessory glands of secretion common to mammals. A study of the second possibility was made by obtaining estimates of the number of spermatozoa in the semen produced by males in the two strains. This was done by a photometric method similar to that described by Comstock and Green (1939) for mammalian semen. Samples of semen were diluted 1:800 with Ringer's solution and placed in standard glass cells. These were placed in photometer and the proportion of light absorbed by different samples was determined. The samples of semen obtained from 12 males in each strain were combined following each of eight consecutive daily collections. The relative amounts of light absorbed by these different samples from the two strains did not differ significantly, as is shown in Table 5. This was confirmed by actual counts of spermatozoa made with a haemocytometer which showed that combined samples of semen from males in the high-fecundity strain contain as many or slightly more spermatozoa per unit of volume than sample yields from the lowfecundity strain. We may therefore conclude that the average volume of semen produced by a group of several males is a reliable measure of the relative numbers of spermatozoa produced. This makes it apparent that the testes of males in the low-fecundity strain, although comparable in size, are somewhat less efficient in the production of spermatozoa than are those of the strain selected for high egg production. Histological Studies Since spermatozoa are produced at an
earlier age and in greater numbers by the males of the high-fecundity strain, while the testes of the two strains do not differ significantly in size, it seems logical to suspect that there might be a difference in the histology of the testes. Age at Onset oj Spermatogenesis Examination of the histological preparations of the testes of males from 4 to 24 TABLE 5.—A comparison of the amount of light absorbed by standard samples of diluted semen from the high- and low-fecundity strains Photometer reading High fecundity
Low fecundity
Difference high-low
microamperes microamperes microamperes 5.3 5.5 -.2 5.6 5.4 .2 6.2 6.1 .1 6.0 5.5 .5 5.8 6.0 -.2 5.8 6.4 -.6 5.2 5.0 .2 5.6 5.5 .1 Ave.
5.7
5.7
.1
Value for t = . l l , a non-significant difference.
weeks of age gave very little indication of meiotic activity before the eighth week, with the exception that one male of the high-fecundity strain showed many meiotic figures at six weeks. In this exceptional male, the testes were nearly twice as large as those of any other male of the same age. Although an occasional meiotic figure could be seen in the other preparations at the age of six weeks, the general appearance of the testes showed that little meiotic activity had occurred. Mature spermatozoa were found at the age of eight weeks in the testis of one highfecundity male. The testis of this male contained numerous meiotic figures, many spermatids, and a few mature spermatozoa. Another male in this same group showed advanced stages of spermatogenesis but no
180
D. G. JONES AND W. F. LAMOREUX
spermatids or mature spermatozoa could be found. All testes of the low-fecundity strain were still quite juvenile at this time. In the group of males killed at 14 weeks of age, mature spermatozoa were found in the tubules of the testes of all males of both strains. These observations show that the period from the time spermatozoa are first seen in the tubules of the testes of the more rapidly developing males until mature spermatozoa can be found in the testes of all males of both strains is relatively short. However, it is apparent that spermatogenesis begins in the low-fecundity strain at a slightly older age than in the high-fecundity strain. It should also be mentioned that the testes of the high-fecundity strain showing mature spermatozoa in the tubules at various ages were considerably more active as measured by the numbers of meiotic divisions, the numbers of sperm present in the tubules, and the proportion of the tubules that were active, than were testes in the low-fecundity strain. General Activity In order to determine whether the testes of the males of the high-fecundity strain differed significantly in development or activity from those of the low-fecundity strain, the histological sections from the testes of males killed at any one age were ranked according to their degree of development or activity as measured by the number of meiotic figures and spermatozoa present in the tubules. This ranking was done without determining which strain was represented by any section until the classification was complete. Values from 10 (indicating most development or activity) to 1 (indicating least development or activity) were assigned to all slides and the values assigned were then tabulated in an order-of-rank table according to strains (Table 6).
An analysis of variance shows that the values assigned to males of the highfecundity strain differed significantly from those assigned to the low-fecundity strain. Since the sections from the testes of males of the high-fecundity strain ranked above those of the other strain, this difference presents histological proof of greater spermatogenic activity in the testes of the high fecundity strain. Photomicrographs of the Testes. To illustrate the differences in the histology of the testes of the two strains, photomicrographs of testes typical of the strains they represented have been prepared for three different ages. Inspection of Table 6 indicates the age of the male and his rank in development or activity of the testes as compared to the other males in the two strains. It will be noted that in every instance the section of the testis chosen to represent either strain was from the male which ranked third. It was discovered that these slides represented the histological picture for either strain somewhat better than slides from either extreme. It is apparent from the illustrations in Figures 1 to 6 that testes of the highfecundity strain (Figs. 2, 4, and 6) tend to have larger tubules which contain greater numbers of metaphase plates than are found in the low-fecundity strain (Figs. 1, 3, and 5). The difference in tubule size is somewhat more apparent at earlier than at later ages, whereas the difference in the activity of the testes is revealed somewhat better by considering the relative abundance of spermatids and mature spermatozoa in males killed at later ages. There was also a noticeable difference in the relative amounts of intertubular tissue as compared with the tubular tissue. This was particularly noticeable at ages of 10 weeks or less when the low-fecundity strain was characterized by a relatively greater proportion of intertubular tissue.
SEMEN PRODUCTION IN SELECTED STRAINS OF LEGHORNS
181
Other differences worthy of note were present only at certain ages. At six weeks the tubules of the high-fecundity strain were much more sharply differentiated from the surrounding tissue than were those of the low-fecundity strain. Figure 2 shows the distinctive difference between tubules and surrounding intertubular tissue in the high-fecundity strain, while Figure 1 illustrates the lack of distinction between the tubular and intertubular tissue. The greatest histological difference between the strains was found at 10 weeks of
age (Figs. 3 and 4). Figure 4 shows the greater size of tubules, the greater proportion of tubular tissue, and the greater meiotic activity in the testis from a male of the high-fecundity strain as compared with a male of the same age in the low-fecundity strain (Fig. 3). It is also evident that at 14 weeks, although testes of males from both strains show considerable activity, the testis of the high-fecundity male (Fig. 6) is much further developed, as indicated by the open lumen and many sperm present, while the testis of the low-fecundity male
TABLE 6.—Order of rank of males in the high- and low-fecundity strains as determined by the histological development of their testes.
TABLE 6.—Continued
Age
High-fecundity strain
weeks 4 4 4 4 4
10 9 8 6 3
7 5 4 2 1
36
19
6 6 6 6 6
8 8 8 8 8
9 9 9 9 9
10 10 10 10 10 10
10 9
•
7 (Fig. 2) 3 2
Low-fecundity strain strain
8 6 5 (Fig.1) 4 1
31
24
10 9 8 7 5
6 4 3 2 1
39
16
8 7 6 5 4
10 9 3 2 1
30
25
10 8 7 (Fig. 4) 6 4 35
9 5 3 (Fig. 3) 2 1 20
Age weeks 12 12 12 12 12
14 14 14 14 14
16 16 16 16 16
20 20 20 20 20
24 24 24 24 24
High-fecundity strain
Low-fecundity strain
10 6 4 2 1
9 8 7 5 3
23
32
10 9 8 (Fig. 6) 6 4
7 5 3 (Fig. 5) 2 1
37
18
10 9 7 4 2
8 6 5 3 1
32
23
10 9 7 6 5
8 4 3 2 1
37
18
10 9 7 3 2
8 6 5 4 1
31
24
182
D. G. JONES AND W. F. LAMOREUX
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PHOTOMICROGRAPHS OF TESTES OF MALES OF THE H I G H - AND LOWFECUNDITY STRAINS (Mag.
2S6X)
Fig. 1. Low fecundity (6 weeks of age). Fig. 2. High fecundity (6 weeks of age). Fig. 3. Low fecundity (10 weeks of age). Fig. 4. High fecundity (10 weeks of age). Fig. 5. Low fecundity (14 weeks of age). Fig. 6. High fecundity (14 weeks of age).
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183
SEMEN PRODUCTION IN SELECTED STRAINS OF LEGHORNS
(Fig. 5) fails to show a lumen in the tubules and few sperm are present. At 20 weeks of age, the most active tubules in testes of both strains are similar in appearance, but spermatogenesis is active in a larger proportion of the tubules in the high-fecundity strain. At each of the 10 ages studied (Table 6), except for the group 12 weeks of age, the high-fecundity strain was found to be superior because of early differentiation of seminiferous tubules, greater proportion of tubular tissue, or greater meiotic activity.
ence in weight between the two strains. This may be the result of chance, and the fact that fewer birds were available from which to select breeders in the low-fecundity strain, or it may be a result of selecting late-maturing birds of the low-fecundity strain, since their egg production would be lower than that of earlier maturing birds. The latter explanation would require that TABLE 7.—Infertility in the breeding pens of the highami low-fecundity strains (1940-41) Strain
Fertility Because of the differences between the strains with respect to histology of the testes and the volume of semen produced, one might expect a corresponding difference in fertility of the males used in the breeding pens. Actually, the number of hens (8-15) used in the regular breeding pens was inadequate to test the fertilizing capacity of males which are highly fertile. However, during 1940 and 1941 the proportion of eggs infertile was much lower among those laid by the high-fecundity strain (Table 7). These data are not critical evidence of a real difference between the strains because of the uncontrolled factors, of which rate of egg production is one (Lamoreux, 1940), that may influence infertility, but in view of other differences they cannot be disregarded. One must conclude, therefore, from the greater meiotic activity of the testes, the greater production of semen, and the probable greater fertility of males in the highfecundity strain, that effective genetic selection for high and for low egg production has been accompanied by relatively high and low reproductive capacity in male fowl. DISCUSSION OF RESULTS
Selection for high- and low-fecundity has apparently resulted in a significant differ-
Sires
Eggs set
Eggs infertile
no.
no.
1940 High-fecundity strain Low-fecundity strain
%
11 3
3853 808
18.7 52.1
1941 High-fecundity strain Low-fecundity strain
15 5
5935 612
19.5 34.2
age at maturity, rate of growth, and mature body size be associated. This obviously is not the case in comparisons between earlyand late-maturing breeds which differ markedly in size of body. However, it was found by Schnetzler (1936) that when he selected for rapid and for slow growth within pure Barred Plymouth Rocks the slowgrowing strain attained sexual maturity (age at first egg) 42 ± 12 (P.E.) days later than the strain that grew more rapidly. Since this difference is significant at the 5 percent level, it indicates that there may be a relationship between age at sexual maturity and rate of growth. Also, the slowly growing birds ceased growth at about the same age as those which grew rapidly, so that they were significantly smaller when mature. If a similar relationship exists in White Leghorns, the difference in body weight between the low- and high-fecundity strains might be explained, since selection for low egg production would have been in part a selection for late maturity and hence for slow growth.
184
D. G. JONES AND W. F. LAMOREUX
It is clear from the results obtained that no difference between the strains in the size of testes has resulted from selection for high- and low-fecundity in the females. However, the differences in meiotic activity, semen production, and fertility show that the testes of the high-fecundity strain are more active or efficient in the production of spermatozoa. It appears, therefore, that although they are two somewhat different processes, semen production and egg production are both subject to control by the same genotype. This suggests that semen production of a male may be a measure of the genotype for fecundity equal to that of egg production in the female.
fecundity strain, it is evident that they must be more active or more efficient in the production of spermatozoa. This difference in activity of the testes of the males of the two strains was confirmed by histological studies which revealed that at 8 to 24 weeks of age there were more metaphase plates and more spermatozoa present in the seminiferous tubules of the testes in males of the high-fecundity strain. After 20 weeks of age, a larger proportion of the tubules were active in high-fecundity males. This study suggests that semen production in the male and egg production in the female are expressions of comparable genotypes for high and low fecundity. REFERENCES
SUMMARY
Two strains of White Leghorns, one genetically capable of high and the other of only low egg production, have been used to determine whether the males of these strains differ with respect to size of body, size of testes, histology of testes, or the volume of semen produced. Males of the high-fecundity strain were found to be significantly heavier than those of the low-fecundity strain. The average weights of testes in the two strains are not significantly different. Males of the high-fecundity strain attain sexual maturity at an earlier age as shown by the order of rank in the development of spermatogenesis, and by the significantly greater volume of semen produced at 12 weeks of age. Furthermore, the highfecundity strain maintains this superiority in the production of semen as shown by their significantly greater yield at 24 weeks, and after 30 weeks of age. Since testes in the high-fecundity strain produce larger amounts of comparable semen than those of similar size in the low-
Burrows, W. H., and J. P. Quinn, 1935. A method of obtaining spermatozoa from the domestic fowl. Poult. Sci. 14:251, 253-254. Comstock, R. E., and W. W. Green, 1939. Methods for semen evaluation. I. Density, respiration, and glycolysis. Proc. Amer. Soc. Animal Prod. 32nd annual meeting. 213-216. Hall, G. O., 1934. Breeding a low producing strain of Single Comb White Leghorns. Poult. Sci. 13:123-127. Lamoreux, W. F., 1940. Spermatozoal antibodies and infertility in the fowl. J. Exp. Zool. 85: 419-430. , 1940. The influence of intensity of egg production upon infertility in the domestic fowl. Jour. Agr. Res. 61:191-206. Marble, D. R., and G. O. Hall, 1931. A statistical analysis of breeding high-line and low-line Leghorns. Cornell Agr. Exp. Sta. Bull. 533. Munro, S. S., 1938. The effect of dilution and density on the fertilizing capacity of fowl sperm suspensions. Canadian Jour. Res., Sec. D, Zool. Sci. 16:281-299. Riley, G. M., 1940. Diurnal variations in spermatogenic activity in the domestic fowl. Poult. Sci. 19:360. Schnetzler, E. E., 1936. Inheritance of rate of growth in Barred Plymouth Rocks. Poult. Sci. 15 :369-376. Snedecor, George W., 1940. Statistical methods. The Iowa State College Press, Ames, Iowa.
D. G. JONES AND W. F. LAMOEEUX of Poultry Husbandry, Cornell University, Ithaca, New
York
(Received for publication August 12, 1941)
S
OME physiological characters, such as egg production in the fowl and milk production in dairy cattle, can only be expressed by the female and few, if any, reliable means except the progeny and sib tests are available for the estimation of the genotype of the male. It is known, however, that the gonads of both males and females are subject to control by endocrine secretions of the body. Whether or not these hormones are one of the means by which genetic differences in fecundity are expressed in the fowl has not been shown directly, but much indirect evidence indicates that they are involved. If this is true, it seems probable that in strains of fowl with most birds capable of high production, and in other strains capable of only low egg production, the males as well as the females should show some difference in their efficiency of reproduction. This study was undertaken, therefore, to determine whether or not the genotypes, which in females induce relatively high or low egg production, are also responsible for differences in the size of testes, age at the onset of spermatogenesis, or the volume of semen produced in males. MATERIAL AND METHODS
The high- and low-fecundity strains of White Leghorns used for this study have been selected for high and for low egg production, respectively, since 1915. The early development of the two strains by
means of mass selection has been described by Marble and Hall (1931) and Hall (1934). Following the'original divergence of the two strains, the average age at first egg and the first year's egg production for the low-fecundity females did not change materially until after 1935. During the same period there was an increase in egg production in the high-fecundity strain and a slight decrease in the average age at first egg, while body weight of both strains increased. Since 1935 the progeny test and sib test have been used, and breeders have been selected not only on the basis of their individual performance, but also with regard to the average performance of their families. Such selection was accompanied by an appreciable decrease in the production of the females of the low-fecundity strain (Table 1). However, during this period the major emphasis in the selection of breeders in the high-fecundity strain was placed upon viability rather than upon egg production. Under these circumstances, less increase in egg production was obtained than might have been expected had egg production been the primary basis of selection. Since the beginning of the progeny test, egg production has been tabulated to 500 days of age, while previous to that time it was determined for the 365 days following the date on which the first egg was laid. Since most of the females of the highfecundity strain continue to lay for a
[173]
174
D. G. JONES AND W.
F. LAMOREUX
TABLE 1.—A comparison of the females in the high- and low-fecundity strains with respect to first year's egg production, age at first egg, and mature body weight Ave. first year egg production
Ave. age at first egg
Ave. body weight (in grams)
Year High fecundity
Low fecundity
High fecundity
Low fecundity
High fecundity
Low fecundity
1913-14 1919-20 1925-26 1931-32
119 161 172 209
99 125 122 96
232 205 199 203
252 225 240 254
1438 1683 1728 1819
1529 1497 1642 1696
1937-38* 1939^0*
174 179
57 67
190 212
300 310
1784 1813
1674 1755
* Egg production to 500 days of age.
period of time after they reach 500 days of age, the production for a year following the date the first egg was laid would be somewhat greater than the production to 500 days of age. On the other hand, most of the low-fecundity females have ceased laying by the time they are 500 days old so that egg production to 500 days of age is not greatly different from egg production for a year following the date the first egg was laid. Therefore, the difference between the two strains in any given year is somewhat less when calculated for the period ending 500 days after hatching than it is for the period ending 365 days after the first egg was laid. Such large and increasing differences in egg production of the two strains, as shown by Table 1, and their long history of genetic selection make these strains particularly suitable for a study, in the males, of the expression of genotypes which are responsible for high and low fecundity in the females. Chicks from these two strains were obtained from hens selected in the following ways. In the high-fecundity strain, only those females were used which had completed their first laying year (365 days) with production records of not less than 200 eggs. Such selection was practiced because there is considerable variation in each strain and it seemed desirable to avoid use of those birds in'the high-fecundity
strain which had relatively poor records of egg production. It was not possible, however, except for birds reported in Table 3 (groups 3 and 4), to use breeders of the low-fecundity strain which had completed their first year of production. Therefore, pullets were selected which had not laid before March 1 of the year after hatching and whose sisters had not laid before January 1 of the same year. Although these birds were untested with respect to their ability to lay, it seemed likely that they were typical of the low-fecundity strain, since the time remaining till the birds would be 500 days of age was inadequate for high records of egg production. Brooding and Rearing Most of the chicks used in this study were kept throughout their life in batteries which had wire floors. Because of inadequate space it was necessary for a short time to brood two hatches on a floor covered with shavings. However, these birds were on wire floors after they were four weeks of age. Since there was known to be a difference between the two strains with respect to mature body weight (Table 1), the chicks of the two strains were brooded separately to avoid such inequalities in growth and development as result from mixing birds of different sizes. The mature males reported in Table 3 (groups 3 and 4) were selected from among prospective
SEMEN PRODUCTION IN SELECTED STRAINS OF LEGHORNS
breeders which were reared on range. The feed used during the brooding period was a standard chick mash. Those birds reared to maturity received both mash and grain. Usually the mash was fed in the form of pellets to encourage greater consumption of feed after the cockerels were 12 to 14 weeks of age. Dubbing Because of the fact that the combs and wattles of fowl grow very large in the absence of direct sunlight, it often becomes difficult for males to eat when placed in the type of cages used. For that reason, a large part of the comb and wattles of those males which were reared to maturity was cut off at 13 weeks of age. Other males were not dubbed. Artificial Illumination Since it was not possible to provide a uniform amount of sunlight to the cages in which birds were placed to be reared to maturity, it was necessary to furnish artificial illumination. These birds received 14 hours of light daily from electric lamps. Males of the high- and low-fecundity strains were alternated in the cages so that any uncontrolled variations in intensity of illumination would not affect one strain more than the other. Collections of Semen These collections were made by means of a technic similar to that described by Burrows and Quinn (1935) with the exception of a modification in the method of holding the males as described by Lamoreux (1940). Histological Studies Histological studies were made of the testes from males 4 to 24 weeks of age. Since it was desired to compare the relative rates of spermatogenesis in birds of the two strains, the cockerels were injected with 20 mg. of colchicine per kilogram of
175
body weight at midnight, seven-and-onehalf to eight hours before they were sacrificed. This interval, midnight to 8:00 a.m., includes the period when meiosis is most active in the fowl (Riley, 1940). Colchicine was used to arrest cell divisions in metaphase, thus facilitating comparison of meiotic activity in birds of the low- and high-fecundity strains. Immediately after death of a bird, the testes were removed and weighed. A transverse section through the larger testis of the pair was then removed and placed in Allen's PFA3 modification of Bouin's fixing solution. The usual methods of washing, dehydrating in alcohol, and embedding in paraffin ("Tissuemat") were used. Sections of each testis so prepared were cut 6 microns thick and stained with hematoxylin and eosin. EXPERIMENTAL RESULTS
Differences in the Body Weight of Two Strains Soon after the experiment was begun, it became apparent that the two strains differed markedly in'their rates of growth. The resulting differences in body weight at different ages during the experimental period are shown in Table 2. Analysis of the variance in body weight shows that the males of the high fecundity strain are significantly heavier than cockerels of the other strain (value for P = <0.01). Because the degree of significance indicated by the F value obtained by analysis of variance is not accurate unless the standard deviations are independent of the mean values obtained (Snedecor, 1940, p. 378), it has been necessary to transform all data in this study to which statistical analyses have been applied, with the exception of Tables 5 and 6. The original values given in the tables have therefore been transformed to logarithms for the calculation of the significance of differences. It was not known whether or not the
176
D. G. JONES AND W. F. LAMOREUX
TABLE 2.—A comparison of the )
and low-fecundity strains with respect to body weight and weight of testes. Each average represents five birds
Ave. weight of body
Ave. weight of both testes
Age
weeks 4 6 8 9 10 12 14 16 20 24
High fecundity
Low fecundity
Diff.*
High fecundity
Low fecundity
Diff.*
grams 207 402 663 787 912 1128 1311 1569 1822 1921
grams 177 283 494 579 664 848 1061 1216 1418 1508
grams 30 119 169 208 248 280 250 353 404 413
grams .0612 .1802 .5493 .4914 2.274 3.526 5.516 5.885 16.72 18.30
grams .0608 .0846 .1938 .6786 1.183 5.456 3.296 6.070 16.85 13.95
grams .0004 .0956 .3555 - .1872 1.091 -1.930 2.220 -.185 -.13 4.35
' Average weight of high-fecundity strain minus that of low-fecundity strain.
difference in body weight between the two strains would be related to the size of testes or to semen production, but it is obvious that if such a relationship does exist between these variables, an error would be introduced unless corrections were made for differences in body weight. It was subsequently found that there was very little correlation (r = 0.121) between body weight and size of testes in birds of the same age in the two strains, thus indicating that analysis of covariance was not necessary. However, there was a significant correlation (r = 0.32) between size of body and semen production at 24 weeks of age, making it necessary to correct for differences in body weight in the two strains (Table 4). In two experiments, high- and low-fecundity-strain males were selected by pairs so that differences in body weight between males of the two strains were not significantly different and analysis of covariance was not required (Table 3, groups 3 and 4).
ovary, one might reasonably expect the testes of the males in the low-fecundity strain either to be somewhat smaller than those of the high-fecundity strain or to be somewhat less efficient in their production of semen. To test the first of these two possibilities, the testes of those males killed for histological study were removed and weighed to determine whether there was a difference between their weights in the two strains. The average weights of the testes from males 4 to 24 weeks of age are given in Table 2. In spite of the difference in body size, shown above, an analysis of variance shows that there was no significant difference in the size of the testes in the two strains. It was also found that there was no significant interaction between strains and ages with respect to either body weight or size of testes, and we may therefore conclude that increments in age influence the rates of growth in both strains to a similar degree.
Size of Testes It seemed logical to assume that, since the selection for high and low fecundity must have affected either the activity of the ovary in these two strains or some other gland which exerts an influence on the
Semen Production The first attempt to obtain semen was made when the males were 12 weeks old. At that time, attempts were made to collect semen on each of five successive days and a comparison was made between the strains
177
SEMEN PRODUCTION IN SELECTED STRAINS OF LEGHORNS
TABLE 3.—Differences in semen production between males of the high- and low-fecundity strains at various ages
Group
1 2 3* 4 :
No. in each strain
Age
12 24 243-271 316-365
weeks weeks days days
High
Low
Length of collection period
21 20 5 6
22 20 5 6
days 4 5 63 11
Ave. daily yield of semen (c.c.) High
Low
.13 .39 .47 .40
.045 .26 .32 .28
Value for P
<0.01 <0.01 <0.01 <0.01
Yields are averages of three collections a week taken on alternate days.
with respect to the number of males producing semen. Nineteen of the 21 highfecundity males produced semen at that time, while but 17 of the 22 low-fecundity males yielded any. This difference is small and the Chi-square test for homogeneity shows that the difference between the proportion of males producing semen in the two strains is not significant. In determining the difference between the volumes of semen produced by the two strains, the average production during the last four days of the five-day collection period was used. Although the volume of semen produced at this age was very low, the males of the high-fecundity strain produced significantly more semen (value for P = < 0.01) than those of the low-fecundity strain (Table 3, group 1). Since many of the males produced very little semen, it seems likely that this difference at 12 weeks of age was really a measure of the difference in age at sexual maturity in the two strains
rather than a measure of the difference in the ability of the two strains to produce semen. Collections of semen were again made from these males at the age of 24 weeks. It is quite apparent from the data presented in Table 3 (group 2) that the high-fecundity males continued to produce considerably more semen than the males of the low-fecundity strain. Also, there was a marked difference between body weights of the two strains. Analysis of variance for body weight of the males shows that at 24 weeks of age the high-fecundity males continue to be significantly heavier than those of the low-fecundity strain. A similar analysis of differences in semen production shows that the males of the high-fecundity strain produce significantly more semen than do those of the low-fecundity strain. The coefficient of correlation (r = 0.32) between body weight and the volume of semen produced was found to be significant
TABLE 4,—Analysis of covariance of body weight and semen production in the high- and low-fecundity strains {original data transformed to logarithms) Errors of estimate Source
Total Strains Within strains
D.F.
39 1 38
Sx2
0.1131 0.0625 0.0506
Sxy
0.1748 0.1369 0.0379
Sy2
0.5847 0.2998 0.2849
sum of squares
D.F.
.3145
38
.2565
37
Difference for testing adjusted means .0580 F = ^ ^ = 8.37, a highly significant difference. r-0.32, value for P = <0.05.
mean square
.0069
.0580*
178
D. G. JONES AND W. F. LAMOREUX
at the 5 percent level. It was necessary, therefore, to make some correction for the differences in body weight in order to determine whether the differences in semen production were merely the result of differences in body size, or whether an actual difference between the strains does exist irrespective of differences in size. Accord50 j 46
-
'
A
HIGH FECUNDITY LOW FECUNDITY
,^
1
1
1
1
1
16
[7
18
19
20
1
1
1
1
1
1
21 2 2 MARCH
1
23
24
25
26
27
CHART 1. The average daily semen production by six males in the low- and six in the highfecundity strain.
ingly, analysis of covariance was employed to correct for the difference in body weight between the two strains (Table 4). The analysis showed that even after correction was made for this difference the males of the high-fecundity strain still yielded a significantly larger volume of semen than those of the low-fecundity strain. Two additional comparisons of semen production in the two strains were obtained by selecting mature males of comparable body size from prospective breeding stock reared on range. In one study, semen was collected from five high- and five low-fecundity males, 243 to 271 days of age, three times each week during January and February. The average yields of the high-fecundity strain consistently exceeded those of the lowfecundity birds (Table 3, group 3) and the average difference was shown by an analysis of variance to be highly significant (value for P = < 0 . 0 1 ) . ' A similar group, composed of six males from each strain, 316 to 365 days of age, was selected and daily collections of semen
were made during a period of 11 days in March. Chart 1 shows the consistently higher daily yield of semen by males of the high-fecundity strain. The average yields of birds from each strain are shown in Table 3 (group 4) and again the difference in yield between the two strains was found to be highly significant as measured by analysis of variance (value for P = < 0.01). Altogether, four comparisons between the strains with respect to yield of semen have been made. At 12 and 24 weeks of age, and in two groups of mature males, the difference between the strains was found to be highly significant. It seems clear that the strain which is genetically incapable of high egg production is also unable to produce semen equal in volume to that produced by the high-fecundity strain. Production of Spermatozoa The difference in semen production between the two strains, even after correction for differences in body weight, is of particular importance and interest in this case because of the fact that there was no significant difference between the weights of testes in the two strains. This situation might be explained in either of two ways. First, the differences in volume of semen may result largely from differences in the proportion of accessory fluids in the semen. Second, the volume of semen may be an accurate measure of the number of spermatozoa produced. It is known from other studies in this laboratory and the work of Munro (1938) that if fowl semen is centrifuged, about 75 percent of its volume may be removed as a supernatent fluid, freed from spermatozoa. Studies have not been made to determine the variation between males or between daily collections of semen from the same male, with respect to the proportion
179
SEMEN PRODUCTION IN SELECTED STRAINS OF LEGHORNS
of accessory fluid in the semen. One might suspect, however, that differences between cockerels in the production of accessory fluid would be somewhat less than those between mammals, since the fowl lacks accessory glands of secretion common to mammals. A study of the second possibility was made by obtaining estimates of the number of spermatozoa in the semen produced by males in the two strains. This was done by a photometric method similar to that described by Comstock and Green (1939) for mammalian semen. Samples of semen were diluted 1:800 with Ringer's solution and placed in standard glass cells. These were placed in photometer and the proportion of light absorbed by different samples was determined. The samples of semen obtained from 12 males in each strain were combined following each of eight consecutive daily collections. The relative amounts of light absorbed by these different samples from the two strains did not differ significantly, as is shown in Table 5. This was confirmed by actual counts of spermatozoa made with a haemocytometer which showed that combined samples of semen from males in the high-fecundity strain contain as many or slightly more spermatozoa per unit of volume than sample yields from the lowfecundity strain. We may therefore conclude that the average volume of semen produced by a group of several males is a reliable measure of the relative numbers of spermatozoa produced. This makes it apparent that the testes of males in the low-fecundity strain, although comparable in size, are somewhat less efficient in the production of spermatozoa than are those of the strain selected for high egg production. Histological Studies Since spermatozoa are produced at an
earlier age and in greater numbers by the males of the high-fecundity strain, while the testes of the two strains do not differ significantly in size, it seems logical to suspect that there might be a difference in the histology of the testes. Age at Onset oj Spermatogenesis Examination of the histological preparations of the testes of males from 4 to 24 TABLE 5.—A comparison of the amount of light absorbed by standard samples of diluted semen from the high- and low-fecundity strains Photometer reading High fecundity
Low fecundity
Difference high-low
microamperes microamperes microamperes 5.3 5.5 -.2 5.6 5.4 .2 6.2 6.1 .1 6.0 5.5 .5 5.8 6.0 -.2 5.8 6.4 -.6 5.2 5.0 .2 5.6 5.5 .1 Ave.
5.7
5.7
.1
Value for t = . l l , a non-significant difference.
weeks of age gave very little indication of meiotic activity before the eighth week, with the exception that one male of the high-fecundity strain showed many meiotic figures at six weeks. In this exceptional male, the testes were nearly twice as large as those of any other male of the same age. Although an occasional meiotic figure could be seen in the other preparations at the age of six weeks, the general appearance of the testes showed that little meiotic activity had occurred. Mature spermatozoa were found at the age of eight weeks in the testis of one highfecundity male. The testis of this male contained numerous meiotic figures, many spermatids, and a few mature spermatozoa. Another male in this same group showed advanced stages of spermatogenesis but no
180
D. G. JONES AND W. F. LAMOREUX
spermatids or mature spermatozoa could be found. All testes of the low-fecundity strain were still quite juvenile at this time. In the group of males killed at 14 weeks of age, mature spermatozoa were found in the tubules of the testes of all males of both strains. These observations show that the period from the time spermatozoa are first seen in the tubules of the testes of the more rapidly developing males until mature spermatozoa can be found in the testes of all males of both strains is relatively short. However, it is apparent that spermatogenesis begins in the low-fecundity strain at a slightly older age than in the high-fecundity strain. It should also be mentioned that the testes of the high-fecundity strain showing mature spermatozoa in the tubules at various ages were considerably more active as measured by the numbers of meiotic divisions, the numbers of sperm present in the tubules, and the proportion of the tubules that were active, than were testes in the low-fecundity strain. General Activity In order to determine whether the testes of the males of the high-fecundity strain differed significantly in development or activity from those of the low-fecundity strain, the histological sections from the testes of males killed at any one age were ranked according to their degree of development or activity as measured by the number of meiotic figures and spermatozoa present in the tubules. This ranking was done without determining which strain was represented by any section until the classification was complete. Values from 10 (indicating most development or activity) to 1 (indicating least development or activity) were assigned to all slides and the values assigned were then tabulated in an order-of-rank table according to strains (Table 6).
An analysis of variance shows that the values assigned to males of the highfecundity strain differed significantly from those assigned to the low-fecundity strain. Since the sections from the testes of males of the high-fecundity strain ranked above those of the other strain, this difference presents histological proof of greater spermatogenic activity in the testes of the high fecundity strain. Photomicrographs of the Testes. To illustrate the differences in the histology of the testes of the two strains, photomicrographs of testes typical of the strains they represented have been prepared for three different ages. Inspection of Table 6 indicates the age of the male and his rank in development or activity of the testes as compared to the other males in the two strains. It will be noted that in every instance the section of the testis chosen to represent either strain was from the male which ranked third. It was discovered that these slides represented the histological picture for either strain somewhat better than slides from either extreme. It is apparent from the illustrations in Figures 1 to 6 that testes of the highfecundity strain (Figs. 2, 4, and 6) tend to have larger tubules which contain greater numbers of metaphase plates than are found in the low-fecundity strain (Figs. 1, 3, and 5). The difference in tubule size is somewhat more apparent at earlier than at later ages, whereas the difference in the activity of the testes is revealed somewhat better by considering the relative abundance of spermatids and mature spermatozoa in males killed at later ages. There was also a noticeable difference in the relative amounts of intertubular tissue as compared with the tubular tissue. This was particularly noticeable at ages of 10 weeks or less when the low-fecundity strain was characterized by a relatively greater proportion of intertubular tissue.
SEMEN PRODUCTION IN SELECTED STRAINS OF LEGHORNS
181
Other differences worthy of note were present only at certain ages. At six weeks the tubules of the high-fecundity strain were much more sharply differentiated from the surrounding tissue than were those of the low-fecundity strain. Figure 2 shows the distinctive difference between tubules and surrounding intertubular tissue in the high-fecundity strain, while Figure 1 illustrates the lack of distinction between the tubular and intertubular tissue. The greatest histological difference between the strains was found at 10 weeks of
age (Figs. 3 and 4). Figure 4 shows the greater size of tubules, the greater proportion of tubular tissue, and the greater meiotic activity in the testis from a male of the high-fecundity strain as compared with a male of the same age in the low-fecundity strain (Fig. 3). It is also evident that at 14 weeks, although testes of males from both strains show considerable activity, the testis of the high-fecundity male (Fig. 6) is much further developed, as indicated by the open lumen and many sperm present, while the testis of the low-fecundity male
TABLE 6.—Order of rank of males in the high- and low-fecundity strains as determined by the histological development of their testes.
TABLE 6.—Continued
Age
High-fecundity strain
weeks 4 4 4 4 4
10 9 8 6 3
7 5 4 2 1
36
19
6 6 6 6 6
8 8 8 8 8
9 9 9 9 9
10 10 10 10 10 10
10 9
•
7 (Fig. 2) 3 2
Low-fecundity strain strain
8 6 5 (Fig.1) 4 1
31
24
10 9 8 7 5
6 4 3 2 1
39
16
8 7 6 5 4
10 9 3 2 1
30
25
10 8 7 (Fig. 4) 6 4 35
9 5 3 (Fig. 3) 2 1 20
Age weeks 12 12 12 12 12
14 14 14 14 14
16 16 16 16 16
20 20 20 20 20
24 24 24 24 24
High-fecundity strain
Low-fecundity strain
10 6 4 2 1
9 8 7 5 3
23
32
10 9 8 (Fig. 6) 6 4
7 5 3 (Fig. 5) 2 1
37
18
10 9 7 4 2
8 6 5 3 1
32
23
10 9 7 6 5
8 4 3 2 1
37
18
10 9 7 3 2
8 6 5 4 1
31
24
182
D. G. JONES AND W. F. LAMOREUX
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PHOTOMICROGRAPHS OF TESTES OF MALES OF THE H I G H - AND LOWFECUNDITY STRAINS (Mag.
2S6X)
Fig. 1. Low fecundity (6 weeks of age). Fig. 2. High fecundity (6 weeks of age). Fig. 3. Low fecundity (10 weeks of age). Fig. 4. High fecundity (10 weeks of age). Fig. 5. Low fecundity (14 weeks of age). Fig. 6. High fecundity (14 weeks of age).
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183
SEMEN PRODUCTION IN SELECTED STRAINS OF LEGHORNS
(Fig. 5) fails to show a lumen in the tubules and few sperm are present. At 20 weeks of age, the most active tubules in testes of both strains are similar in appearance, but spermatogenesis is active in a larger proportion of the tubules in the high-fecundity strain. At each of the 10 ages studied (Table 6), except for the group 12 weeks of age, the high-fecundity strain was found to be superior because of early differentiation of seminiferous tubules, greater proportion of tubular tissue, or greater meiotic activity.
ence in weight between the two strains. This may be the result of chance, and the fact that fewer birds were available from which to select breeders in the low-fecundity strain, or it may be a result of selecting late-maturing birds of the low-fecundity strain, since their egg production would be lower than that of earlier maturing birds. The latter explanation would require that TABLE 7.—Infertility in the breeding pens of the highami low-fecundity strains (1940-41) Strain
Fertility Because of the differences between the strains with respect to histology of the testes and the volume of semen produced, one might expect a corresponding difference in fertility of the males used in the breeding pens. Actually, the number of hens (8-15) used in the regular breeding pens was inadequate to test the fertilizing capacity of males which are highly fertile. However, during 1940 and 1941 the proportion of eggs infertile was much lower among those laid by the high-fecundity strain (Table 7). These data are not critical evidence of a real difference between the strains because of the uncontrolled factors, of which rate of egg production is one (Lamoreux, 1940), that may influence infertility, but in view of other differences they cannot be disregarded. One must conclude, therefore, from the greater meiotic activity of the testes, the greater production of semen, and the probable greater fertility of males in the highfecundity strain, that effective genetic selection for high and for low egg production has been accompanied by relatively high and low reproductive capacity in male fowl. DISCUSSION OF RESULTS
Selection for high- and low-fecundity has apparently resulted in a significant differ-
Sires
Eggs set
Eggs infertile
no.
no.
1940 High-fecundity strain Low-fecundity strain
%
11 3
3853 808
18.7 52.1
1941 High-fecundity strain Low-fecundity strain
15 5
5935 612
19.5 34.2
age at maturity, rate of growth, and mature body size be associated. This obviously is not the case in comparisons between earlyand late-maturing breeds which differ markedly in size of body. However, it was found by Schnetzler (1936) that when he selected for rapid and for slow growth within pure Barred Plymouth Rocks the slowgrowing strain attained sexual maturity (age at first egg) 42 ± 12 (P.E.) days later than the strain that grew more rapidly. Since this difference is significant at the 5 percent level, it indicates that there may be a relationship between age at sexual maturity and rate of growth. Also, the slowly growing birds ceased growth at about the same age as those which grew rapidly, so that they were significantly smaller when mature. If a similar relationship exists in White Leghorns, the difference in body weight between the low- and high-fecundity strains might be explained, since selection for low egg production would have been in part a selection for late maturity and hence for slow growth.
184
D. G. JONES AND W. F. LAMOREUX
It is clear from the results obtained that no difference between the strains in the size of testes has resulted from selection for high- and low-fecundity in the females. However, the differences in meiotic activity, semen production, and fertility show that the testes of the high-fecundity strain are more active or efficient in the production of spermatozoa. It appears, therefore, that although they are two somewhat different processes, semen production and egg production are both subject to control by the same genotype. This suggests that semen production of a male may be a measure of the genotype for fecundity equal to that of egg production in the female.
fecundity strain, it is evident that they must be more active or more efficient in the production of spermatozoa. This difference in activity of the testes of the males of the two strains was confirmed by histological studies which revealed that at 8 to 24 weeks of age there were more metaphase plates and more spermatozoa present in the seminiferous tubules of the testes in males of the high-fecundity strain. After 20 weeks of age, a larger proportion of the tubules were active in high-fecundity males. This study suggests that semen production in the male and egg production in the female are expressions of comparable genotypes for high and low fecundity. REFERENCES
SUMMARY
Two strains of White Leghorns, one genetically capable of high and the other of only low egg production, have been used to determine whether the males of these strains differ with respect to size of body, size of testes, histology of testes, or the volume of semen produced. Males of the high-fecundity strain were found to be significantly heavier than those of the low-fecundity strain. The average weights of testes in the two strains are not significantly different. Males of the high-fecundity strain attain sexual maturity at an earlier age as shown by the order of rank in the development of spermatogenesis, and by the significantly greater volume of semen produced at 12 weeks of age. Furthermore, the highfecundity strain maintains this superiority in the production of semen as shown by their significantly greater yield at 24 weeks, and after 30 weeks of age. Since testes in the high-fecundity strain produce larger amounts of comparable semen than those of similar size in the low-
Burrows, W. H., and J. P. Quinn, 1935. A method of obtaining spermatozoa from the domestic fowl. Poult. Sci. 14:251, 253-254. Comstock, R. E., and W. W. Green, 1939. Methods for semen evaluation. I. Density, respiration, and glycolysis. Proc. Amer. Soc. Animal Prod. 32nd annual meeting. 213-216. Hall, G. O., 1934. Breeding a low producing strain of Single Comb White Leghorns. Poult. Sci. 13:123-127. Lamoreux, W. F., 1940. Spermatozoal antibodies and infertility in the fowl. J. Exp. Zool. 85: 419-430. , 1940. The influence of intensity of egg production upon infertility in the domestic fowl. Jour. Agr. Res. 61:191-206. Marble, D. R., and G. O. Hall, 1931. A statistical analysis of breeding high-line and low-line Leghorns. Cornell Agr. Exp. Sta. Bull. 533. Munro, S. S., 1938. The effect of dilution and density on the fertilizing capacity of fowl sperm suspensions. Canadian Jour. Res., Sec. D, Zool. Sci. 16:281-299. Riley, G. M., 1940. Diurnal variations in spermatogenic activity in the domestic fowl. Poult. Sci. 19:360. Schnetzler, E. E., 1936. Inheritance of rate of growth in Barred Plymouth Rocks. Poult. Sci. 15 :369-376. Snedecor, George W., 1940. Statistical methods. The Iowa State College Press, Ames, Iowa.