- Email: [email protected]
Investigation of a selection limit for litter size in mice
Livestock Production Science, 20 (1988) 161-172
161
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
I n v e s t i g a t i o n o f a S e l e c t i o n L i m i t for L i t t e r S i z e in Mice R.C. B U I S
Department o/Animal Breeding, Agricultural University, P.O. Box 338, 6700 AH Wageningen (The Netherlands) (Accepted 14 March 1988)
ABSTRACT Buis, R.C., 1988. Investigation of a selection limit for litter size in mice. Livest. Prod. Sci., 20: 161-172. A long-term selection experiment for large litter size in mice was conducted, starting with Line L at average litter size of 8.3 ± 2.2. At Generation 30 with litter size of 14.0 ± 2.7, a selection plateau appeared. The nature and causes of the plateau were investigated. From Generation 48 of Line L, divergent selection for litter size was applied upward (Line L H ) and downward (Line LL). After 12 generations, average litter size in LH was 13.9 ± 2.6 and in LL 10.2 _+4.7. Realized heritabilities were zero in L H and 0.20 ± 0.04 in LL. The response to reverse selection was attributed to the continued segregation of recessive alleles at the plateau. The role of increased prenatal mortality as a cause of the plateau was investigated. Prenatal mortality (M) in Line L increased with number of implantations (I) as M = 0.11 + 0.0003 × 13. This exponential increase could eventually balance an increasing number of implantations and thus contribute to a plateau for litter size. The relevance of this study for the understanding of genetic effects on litter size in pigs is indicated.
INTRODUCTION
Reproduction is an important and complex trait in livestock. In pigs, litter size a~ birth and at weaning is a main factor in establishing the efficiency of a production system. Knowledge of the genetic nature of those traits contributes to an optimal use in breeding systems. Their low heritability leads to slow progress in selection experiments for litter size (Ollivier, 1982). Experiments with litter size in mice as a model for litter size in pigs can reveal the genetic nature of this trait. Eisen (1980) reviewed results of long-term selection experiments with mice for growth and litter size, with respect to their usefulness for livestock production. Until that time only 3 long-term selection-limit studies for litter size in mice had been published. Bakker et al. (1978) established a selection line (L) for high number of young at birth per litter (litter size) in mice. The selection criterion was first
0301-6226/88/$03.50
© 1988 Elsevier Science Publishers B.V.
162 parity litter size (alive plus dead), and litters were not standardized. During the first 30 generations of selection, average litter size ( + s.d. ) per generation increased from 8.3 + 2.2 to 14.0 + 2.7, and average coefficient of inbreeding reached 37%. Realized heritability ( + s.e. ) was 0.11 + 0.01 ( P < 0.01 ), and during the period of selection no significant decrease in effectiveness of selection was observed (Bakker et al., 1978). Average percentage of fertile matings was rather constant at 92 _+7%. Selection in Line L was continued after Generation 30, but no significant further progress in litter size was observed. In Generation 60, average litter size ( _+s.d.) was 14.8 +_3.2 with an average coefficient of inbreeding of 56%. Response to selection on litter size is presented in Fig. 1. From Generation 31 to 60, the cumulative selection differential was 95.3 young and cumulative selection response only 0.6. Also, the standard deviation of litter size did not decrease and there was no significant realized heritability. Average fertility of females was 93 _+4% in this period. From these unpublished data we concluded that at Generation 30 a selection limit of 14 to 15 young had been reached for litter size in Line L. Litter size of a control line (C) is presented also in Fig. 1. Litter size in Line C, bred by a system of maximum avoidance of inbreeding, decreased significantly. At Generation 45, when average coefficient of inbreeding was c. 35%, litter size was insufficient to continue regular breeding and the line became extinct. Average fertility of the females, however, was still 93% during the last 5 generations. Its value as a long-term control line, therefore, is subject to doubt. The selection limit of Line L was roughly the same as the limit for litter size in another line ( N ) f r o m a different origin, selected for high litter size without standardization (van den Nieuwenhuizen et al., 1982 ). From Generation 20 to 50, no ~jgnificant increase in litter size was observed in Line N, the average LITTER S I Z E
14
12 =
- LINE
L
10
8
6 L
I 10
I 20
~ 30
40
I 50 60 GENERATION
Fig. 1. Litter size in the selection line L and in the control line C: averageof mean and 1/2 standard deviation per group of 5 generations.
163
( + s.d.) being 14.6 + 3.0. The results in Lines L and N suggest that there is a selection limit for litter size in these lines of mice between 14 and 15 young. The aim of the current experiment was to investigate the presence and nature of the apparent plateau for litter size between 14 and 15 young, using several approaches. First, the genetic cause of the limit was investigated by means of a divergent selection experiment for litter size during 12 generations. A possible negative maternal effect upon litter size, which might contribute to the plateau, was investigated by standardization of litters. Secondly, the possibility of a physiological barrier with respect to prenatal mortality was investigated, based on the hypothesis that increased prenatal mortality prevented an increased litter size despite an increased number of implantations. MATERIALS AND METHODS
For the divergent selection experiment, two lines of mice were established from Generation 48 of the L line. The new base generation consisted of 45 litters, with average litter size of 13.1 _+4.0. These litters were standardized to 10 young within 24 h of birth. Standardization was continued in both divergent lines during selection to avoid environmental maternal effects on litter size which may complicate the interpretation of results (Eisen, 1970). The line selected for high litter size was designated LH and that for low litter size LL. Selection was performed for 12 generations, by randomly choosing 2 females per litter of the 24 largest (LH) or smallest (LL) litters and 3 males per litter of the 8 largest (LH) or smallest (LL) litters. Thus each generation consisted of 48 females and 24 males mated at random 2:1 with avoidance of full-sib or half-sib mating. During the same period, selection for high litter size was continued in the L line with the same procedure but without standardization. No control line was used. Experience showed (see Introduction ) that over 12 generations litter size decreased systematically in a control line, which made comparison with the selection line complicated. There was no opportunity to establish a control line with a sufficiently large effective number as to prevent this decrease. Realized heritabilities in the selection lines LL and LH were estimated as 2 >
164
of the selection plateau. Heritabilities were estimated via d a u g h t e r - d a m and via granddaughter-granddam regression as h 2 -- 2 X b and h 2-- 4 X b, respectively. Heritabilities were also estimated from variance components of full sibs and of half sibs in a nested design with equal subclass numbers (Becker, 1964). The mating scheme provided litters of dams, arranged in groups of 2 full sibs within 4 half sibs. Owing to selection in the parents, a design with this structure was only possible in part of the material. Prenatal mortality was analysed in 160 mice derived from Generation 50 of Line L. At a representative time, Day 13 of pregnancy, mice were dissected and numbers of dead embryos and of foetuses were counted. At this day both very early and rather late mortality could be observed (Bakker et al., 1978). Mice were caged in Makrolon cages, type Hulskamp-Komeko MAK 180, and had free access to water and pellets from 12 days of age onwards. Room temperature was kept at 23 °C and relative humidity at a m i n i m u m of 60%. There was a 12-12 h light-dark regime. RESULTS
Average litter sizes and standard deviations within generations are given in Fig. 2 for Lines LH and LL. Calculated over 12 generations, average litter size (+s.d., corrected for generation means) in Line LH was 14.4+2.9 and the regression coefficient of litter size on generation number was b = - 0.03 _+0.05 (P > 0.05 ). Thus no further response was observed in this line. Average fertility of females over the 12 generations was 91 + 6%. Comparable results for generations 49-61 in Line L were: average litter size ( + s.d.) was 13.7 + 3.3 and b = 0.01 + 0.05 ( P > 0.05 ). The difference of 0.7 young in favour of Line LH can be attributed to the standardization of litters in this line and thus to a reduction of a part of the negative environmental effect of large litters. Also, average weight at 8 weeks in Line LH ( 28.3 _+4.0 g) was higher than in Line L (26.2 + 3.6 LITTER SIZE
16-
14
12 10 o~o
~"
LINE LL
;
;
;
1'2
GENERATION
Fig. 2. Litter size per generation in the divergent selection lines LL and LH: mean and 1/2 standard deviation.
165 g). This difference is also likely to be caused by standardization. Female fertility was 92 ___3%. In the downward selected Line LL, the regression coefficient ( + s.e. ) of litter size on generation number was b = - 0.30 + 0.06 ( P < 0.001 ), suggesting a significant selection response. This response was most obvious after 5 generations of reversed selection. Standard deviation of litter size, corrected for generation means, was 3.8. This higher value in comparison to the s.d.s for litter size in Lines L H and L was caused by a relatively large number of small litters, mainly in the earlier generations of reverse selection. Especially in Generations 3 and 4, small litters caused a large deviation (Fig. 2 ). Average weight of the mice was 28.0 _ 4.4 g and thus very much comparable to the weight in the L H line, which was also standardized. Average female fertility over the 12 generations was 91 __+9%. The relation between cumulative selection differential (Sc) and cumulative selection response (Re) in the divergent lines is shown in Fig. 3. Realized heritability in Line L H was - 0 . 0 2 and thus set to zero. In this case no standard error could be calculated according to Hill (1972). Realized heritability + s.e. in Line LL was 0.20 _+0.04. The estimates of h 2 from family data are given in Table 1. Heritabilities, based on ( g r a n d ) d a u g h t e r - ( g r a n d ) d a m regression are not biased by selection in the parents' generation (Falconer, 1983). However, estimates by daughterdam ( D / D ) regression may be biased downwards by an opposite genetic and environmental maternal effect: females from large litters producing smaller litters compared to their positive genetic capacity owing to their negative maternal environment. In such a case the granddaughter-granddam ( G D / G D ) regression gives a more reliable estimate of heritability (van der Steen, 1985 ). Heritability estimates from half-sib ( H S ) and full-sib (FS) analysis can be LITTER S I Z E : R c
15
14
12
12
11 L I N E LH o.._...o L IN E LL
lO
-3'o
-Jo
o
CUMULATIVE SELECTION
II0 2tO
30
D I F F E R E N T I A L : Sc
Fig. 3. Cumulative selection response (litter size, Re) in relation to cumulative selection differential (So) per generation in divergent selection lines LL and LH.
166 TABLE 1 Heritability estimates for litter size in selected lines based on familydata A. Dam-daughter and granddam-granddaughterregression Line
Generations
Numberof p a i r s of animals
Heritability-+ s.e. from Daughter/dam
L L LH LL
6-14 49-61 0-12 0-12
Gr.daughter/gr.dam
724 576 565 564
0.64___0.16 -0.08_+ 0.16 -0.04_+0:16 0.18 -!-_0.10
Numberof animals used1
Heritability_+ s.e. from
-
0.04 -+0.40 0.33 ± 0.13 0.31_+0.11 0.12_+0.16
B. Half- and full-sibanalysis Line
Generations
Half sibs L L LH LL
6-14 49-61 0-12 0-12
320 (44%) 200 (35%) 184 (32%) 220 (39%)
0.25 _+0.25 0.23 +0.62 0.14 ± 0.26 0.07 ± 0.52
Full sibs 0.40___0.13 0.20+0.33 0.07 ± 0.32 0.00_+0.36
1Fraction of total material used in analysisbetweenparentheses. biased downwards by reduction of covariance between sibs owing to selection in the parents' generation (Ponzoni and James, 1978; Falconer, 1983). In the full-sib analysis, in turn, an upward bias may occur owing to a common maternal environment of litter mates and dominance variance. In the phase of increasing litter size in Line L, a high h2= 0.64 was observed by the D / D analysis. The contrasting zero value in the G D / G D analysis was unexpected and cannot be explained by selection or maternal effect. The difference between h 2 ( H S ) = 0 . 2 5 (not significant) and h 2 ( F S ) = 0 . 4 0 agrees with the effect of common maternal environment and dominance variance between full sibs. T h u s in the phase of increasing litter size, h 2 estimates based on family data are in the same direction as realized heritability. In the phases of the selection plateau of Line L in generations 49-61 and in Line LH, the D / D h 2 estimates were essentially zero, but the G D / G D h2s were 0.33 and 0.31, respectively. There was no difference between both lines irrespective of standardization in Line LH. Differences between G D / G D and D / D estimates were expected based on the maternal effect as described above. All h 2 estimates from sib analysis were not significant. This can be caused by the downward bias in the sib analyses, as described above, but also by the large standard errors caused by the low numbers of animals used for this analysis.
167 Thus, in the phase of constant litter size there was still an indication of additive genetic variance indicated by the GD/GD analysis. In the downward selected Line LL, the D/D h 2 was 0.18. The GD/GD h 2 was not significant, which was as unexpected as in the phase of increasing litter size. Again, the sib analysis produced no significant heritability estimates. The conclusion is that estimates of heritability based on family data, although not completely in agreement, indicate the presence and persistence of at least some additive genetic variance at the selection limit. The second approach to investigate the selection limit involved testing the hypothesis that prenatal mortality was linked with number of implantations (as a measure of ovulation rate) in such a manner that the number of living foetuses (as an upper limit of litter size) was limited by increased relative mortality. In 153 mice, derived from Line L and dissected at Day 13 of gestation, the following average ( + s.d.) numbers of traits were observed: living foetuses 15.3 + 4.2; dead embryos 1.7 + 1.7; dead foetuses 0.1 + 0.3. The average number of implantations was 17.1 + 5.1, and number of prenatal dead individuals was 1.8 + 1.7. In a similar number (n = 149 ) of living mice, derived from the same generation, average litter size was 13.8+ 3.7 of which 13.3_ 3.7 young were alive. The difference of 1.5 between number of living foetuses and litter size is caused by mortality between Day 13 of gestation and shortly after parturition. Two curvilinear regressions were fitted to describe the increase of mortality as a function of number of implantations:
D=0.70-0.175×I+0.013×I2; D=0.11+0.0003×I3;
R2=0.507
R2 = 0.504
(1) (2)
where D = number of dead embryos and foetuses, and I= number of implantations. Model (1) implies a minimal mortality of 0.11 embryos at 7 implantations. It describes a bimodal distribution of mortality, plotted against number of implantations. Mortality below 7 implantations may well have another cause than the exponentially increasing mortality above that number, but its explanation is beyond the scope of this study. Mortality above 7 implantations can be explained by increased 'crowding' at a higher implantation number. Model (2) explained as much variance as model (1) and was considered a suitable description of the response. The models are valid within the range of 1-23 implantations, included by the data. Based on Model 2, the following relation between number of implantations and litter size was found: LT= I-0.0003 × 13-1.5, where LT= total litter size and I= number of implantations; 1.5 is mortality between Day 13 and counting of litter size. Mortality increased sharply with higher numbers of implantations, so that with 23 implantations only 2 3 - 3 . 6 5 - 1 . 5 = 17.85 living young are expected. If the model were valid beyond the range of observations, a max-
168
imum average litter size of 21 would be possible (with 33 implantations). This figure may well be lower because mortality after Day 13 is expected to increase as well at higher numbers of implantations. Furthermore, the genetic association between mortality and number of implantations may be higher than the phenotypic association. Although within the given range no maximum litter size occurs, owing to an equilibrium between number of implantations and mortality, prenatal mortality can, at least together with other causes, contribute to the plateau for litter size. DISCUSSION
In his review on long-term selection experiments, Eisen (1980) mentioned two selection lines for litter size which plateaued between Generations 30 and 35. By way of contrast, he also referred to the fertile line of Bakker et al. (1978), used in the present paper, which had not yet reached a plateau at Generation 29. Now it appears that this line also reached a plateau at Generation 30 and thus did not constitute an exception. The apparent selection plateau in Line L occurred in a situation without standardization of litter size, and the line was continued until Generation 60 (the last one used in this paper). Simultaneously with the last 12 generations, upward selection was performed in Line LH with standardization of litter size to 10 young. Thus the existence of a genotype by environment interaction with respect to the plateau was investigated. Over 12 generations, mice in Line LH produced 0.7 young per litter more than in Line L. There was no further increase in litter size in this period, so upward selection evoked no further response. We concluded that there was some genotype-by-environment interaction with respect to the plateau. The level of the plateau could be caused partially by a negative maternal environmental effect in females born in large litters (in the unstandardized situation) upon their own litter size. Two remarks have to be made. First, by standardization within 24 h after birth we could not remove any pre- and perinatal effect which may still contribute to the level of the plateau. Secondly, the effect may well be nonlinear but exponential with increasing litter size, so its contribution to a plateau may be more serious than we can estimate here. Between Generations 30 and 60 of Line L in our experiment, litter size was constant despite a slow and continuous increase of the average inbreeding coefficient from 37 to 56%. The L line seemed to be insensitive to the effect of inbreeding, or inbreeding depression was balanced by the continuing positive effect of selection. Also, the fertility rate of the females was constant and high (above 90% ), not suffering from inbreeding depression. Selection for low litter size yielded a realized heritability of 0.20, indicating that at the selection plateau for litter size there was still additive genetic variation. The large asymmetry in response in our results made it meaningless to
169 estimate the realized heritability from a comparison of the 'divergent' Lines LH and LL. Estimation of h 2 for litter size by use of family data in various phases of Line L and in the Lines LL and LH provided an indication of additive genetic variation in all lines and phases. This conclusion is mainly based on the regression approach, which was least biased. From the sib analysis, biased in two directions, in Line L the presence of dominance variation in the phase of increasing litter size can be supposed, although not discernable from environmental fullsib variance. In other studies on litter size and selection limits (e.g. Falconer, 1971; Eklund and Bradford, 1977) no heritability estimates in selected lines based on family data are reported. Based on the positive response to reverse selection and the family data analysis, we conclude that in the apparently plateaued line there is still additive genetic variability, perhaps accompanied by dominance or epistatic genetic variability. Comparison of our results with those of other authors indicates the following. Falconer (1971) tried to overcome the selection limit for litter size in a strain of mice. He formed several inbred strains under continued selection. The strains were crossed which resulted in 1.5 young per litter more than at the limit. Falconer concluded that there was still residual genetic variation after reaching the plateau. Because in one inbred strain litter size did not decrease after inbreeding, he excluded overdomlnance as a major cause of the limit. He postulated that the variation should be caused by the continued segregation of recessive genes with a negative effect on litter size. Whilst the positive response before reaching the limit should mainly be based on additive genetic variation for ovulation rate, recessive genes should mainly act on embryonic mortality. Eklund and Bradford (1977) explored the nature of a selection limit for litter size by reverse selection {among other approaches). They observed a positive response, as in our results, mainly after some 5 generations. Their results of relaxed selection and selection with inbreeding were similar to those in the plateaued L strain, indicating that overdominance could not play a major role in the establishment of the limit. As Falconer (1971), Eklund and Bradford (1977) also concluded that recessive genes at low frequencies could explain the residual variation at the plateau, expressed by a decrease in litter size after reversed selection. Because in our experiment a control or a relaxed selection line was omitted, we could not rule out the possibility of overdominance as a cause of the plateau. Eisen (1980) concluded from the foregoing studies that litter size has a low heritability, compared with adult body weight, for example. Selection limits for litter size should, therefore, be caused by depletion of additive genetic variance. Genetic variance, existing at the limit, should be based on segregation of recessive genes at low frequencies, responsible for small litter sizes. Overdominance should not be considered as a major cause of a selection limit. With
170
respect to growth and body size, Barria and Bradford (1981) analysed a selection limit for those traits. Also here, reverse selection was successful. In this case they suggested a negative association between the selected trait, body size and fitness traits, rather than exhaustion of additive genetic variation, as a cause of the plateau. The pattern of decrease in Line LL resembled that obtained by Eklund and Bradford (1977) in their reverse selection line. In both experiments, only a small response was obtained in the first 5 generations of reverse selection, followed by a stronger response in the next 6-7 generations. This pattern is compatible with segregation of recessive genes for low litter size (or high embryonic mortality) at low frequency at the plateau, their frequency only increasing by reversed selection after some generations. In case of overdominance as a cause of the plateau, a larger variation in that phase may be expected, leading to a more immediate response to backward selection. Thus, apart from some remaining additive genetic variation, genetic variation at the plateau is probably present in the form of simple recessive alleles at low frequencies not removable by further upward selection. The fertility rate of females was high and rather constant before and after reaching the plateau for litter size, and did not decrease in Line LL. Thus this trait appeared not to be associated with litter size. The results on prenatal mortality alone did not provide a sufficient explanation for the selection limit. In pigs, however, Blichfeldt and Almlid (1982) observed a curvilinear relationship between ovulation rate (0) and number of live embryos (L), described by the model: L=-12.1+2.68×0-0.074×02. Their model explained 21% of the variation occurring in number of live embryos and predicted a maximum of 12.2 embryos at 18.1 ovulations. The numbers of embryos decreased at higher numbers of ovulations. In such a case, embryonic mortality could play a major role in the establishment of a selection limit for litter size. These data originated from pigs that were not selected for fertility. In the plateaued L line, 50% of the variation in prenatal mortality was explained by number of implantations (as a measure for ovulation rate). Increased prenatal mortality thus may at least contribute to the plateau reached for litter size. It would have been interesting to investigate prenatal mortality also in the reversed selection Line LL after the response was reached. Summarizing the results, we conclude that the plateau for litter size was not only caused by reduction of genetic variation. The family analysis pointed to at least some remaining additive variance and possibly some variance caused by dominant alleles. The lack of response to further upward selection can be explained by an increasing negative maternal environmental effect and increasing mortality owing to higher numbers of implantations. The positive response to reverse selection can be explained by the continuous segregation of recessive alleles with negative effect on litter size (maybe by higher embryonic mortality) at the plateau.
171 T h e r e l e v a n c e of t h e r e s u l t s for pig b r e e d i n g is a b e t t e r u n d e r s t a n d i n g o f t h e genetic n a t u r e of litter size in r e l a t i o n to m a t e r n a l e n v i r o n m e n t a l effects a n d p r e n a t a l m o r t a l i t y . I n pigs, p r e n a t a l losses are c o n s i d e r a b l e a n d c a n a m o u n t to 3 0 - 4 0 % of o v a s h e d (Scofield, 1972). I t is, in general, u n k n o w n h o w far a pig p o p u l a t i o n is r e m o v e d f r o m a selection l i m i t for litter size, b u t n a t u r a l selection m a y h a v e p l a y e d a role in foregoing g e n e r a t i o n s . Also, in pigs d o w n w a r d s e l e c t i o n c o m b i n e d w i t h i n v e s t i g a t i o n o f p r e n a t a l m o r t a l i t y m i g h t elucidate m o r e of t h e genetic a n d e n v i r o n m e n t a l effects on litter size. ACKNOWLEDGEMENT T h e a u t h o r g r a t e f u l l y m e n t i o n s t h e skilful c a r e for t h e a n i m a l s a n d t h e acc u r a t e collection o f d a t a b y M i s s M . A . W . P e t e r s of t h e L a b o r a t o r y A n i m a l Centre of the Agricultural University.
REFERENCES Bakker, H., Wallinga, J.H. and Politiek, R.D., 1978. Reproduction and body weight of mice after long-term selection for large litter size. J. Anim. Sci., 46: 1572-1580. Barria, N. and Bradford, G.E., 1981. Long-term selection for rapid gain in mice. I. Genetic analysis at the limit of response. J. Anim. Sci., 52: 729-738. Becker, W.A., 1964. Manual of quantitative genetics. 1st edn. Washington State University Press, 152 pp. Blichfeld, T. and Almlid, T., 1982. The relationship between ovulation rate and embryonic survival in gilts. Theriogenology, 18: 615-620. Eisen, E.J., 1970. Maternal effects on litter size in mice. Can. J. Genet. Cytol., 12: 209-216. Eisen, E.J., 1980. Conclusions from long-term selection experiments with mice. Z. Tierz. Zuechtungsbiol., 97: 305-319. Eklund, J. and Bradford, G.E., 1977. Genetic analysis of a strain of mice plateaued for litter size. Genetics, 85: 529-542. Falconer, D.S., 1960. The genetics of litter size in mice. Proc. Cold Spring Harbor Symp. Quant. Biol., 20: 178-196. Falconer, D.S., 1971. Improvement of litter size in a strain of mice at a selection limit. Genet. Res., Camb., 17: 215-235. Falconer, D.S., 1983. Introduction to quantitative genetics. Longman, London and New York, 340 pp. Hill, W.G., 1972. Estimation of realized heritabilities from selection experiments. II. Selection in one direction. Biometrics, 28" 767-780. Ollivier, L., 1982. Selection for prolificacy in the pig. Pig News Information, 3: 383-388. Ponzoni, R.W. and James, J.W., 1978. Possible biases in heritability estimates from intraclass correlations. Theor. Appl. Genet., 53: 25-27. Scofield, A.M., 1972. Embryonic mortality. In: D.J.A. Cole (Editor), Pig Production, Butterworths, London, pp. 367-383. Van den Nieuwenhuizen, J., Bakker, H. and Buis, R.C., 1982. Genetic differences in reproduction and growth rate between two lines of mice selected for litter size. Z. Tierz. Zuechtungsbiol., 99: 292-307.
172 Van der Steen, H.A.M., 1985. The implication of maternal effects for genetic improvement of litter size in pigs. Livest. Prod. Sci., 13: 159-168. RESUME Buis, R.C., 1988. Recherches sur un plateau de sdlectionpour la mille de la port~e chez la souris. Livest. Prod. Sci.,20:161-172 (en anglais). Une experience de s$1ection ~ long terme en faveur de la mille de la portde a dt~ effectude sur la souris.Elle a d~but~ avec la lign~e L ayant 8,3 +__2,2 c o m m e mille moyenne de la portde. U n plateau se situant ~ 14,0 _+2,7 est apparu au bout de 30 gdndrations. La nature et les causes de ce plateau ont ~td recherchdes. Une s$1ection divergente sur la mille de la port~e (lignde haute: LH; lignde basse: LL) a ~t~ appliqu~e ~ partir de la 48 ~m~ g~ndration de la lignde L. Apr~s 12 gdndrations, la taillemoyenne de la pottle ~mit de 13,9 +_2,6 pour L H et de 10,2 +_4,7 pour LL. Les hdritabilitds r~alisdes ~mient de z~ro pour L H et de 0,20 + 0,04 pour LL. La r~ponse obtenue par s~lection rdverse a St~ attribuSe au maintien de la s~gr~gation d'allblesr~cessifspendant le plateau. Le r61e possible jou~ par un accroissement de la mortalitd pr~natale dans l'existencedu plateau a 6t~ dtudi~. La mortalit~ pr~natale (M) augmente dans la lignde L avec le nombre d'implantations (1) suivant la relation M = 0.11 + 0.0003 X/3. Cette augmentation de type exponentiel serait susceptible de compenser un accroissement du nombre d'implantations et contribuerait ainsi ~ l'existence d'un plateau pour la taillede la pottle. Les possibilit~sd'application de cette dtude pour comprendre les effets (g~ndtiques) sur la mille de la portde chez le porc sont indiqudes. KURZFASSUNG
Buis, R.C., 1988. Untersuchung einer Selektionsgrenze fiir WurfgrSfie bei M~iusen. Livest. Prod. Sci., 20:161-172 (auf englisch). Mit der Linie L, die die mittlere WurfgrSfie von 8.3 ___2.2 hatte, wurde ein langfristigesSelektionsexperiment fiirgr~fiereWfirfe durchgefiihrt.In der 30. Generation schien bei einer WurfgrSfie von 14.0 _+2.7 ein Selektionsplateau erreicht zu sein. Ab Generation 48 wurde die Linie L in eine auf- (LH) und eine abselektierte (LL) unterteilt.Nach 12 Generationen divergenter Selektion erreichten die Linien L H 13.9 _+2.6 und L L 10.2 _ 4.7 in der WurfgrSfie. Die realisiertenHeritabilit~itenwaren null in der LH- und 0.20 +_0.04 in der LL-Linie. Der Erfolg der Riickselektion wurde der anhaltenden Segregation rezessiverAlleleim Plateau zugeschrieben. Es wurde die RoUe erh~ihterpraenataler Mortali~t als Grund fiirdas Plateau untersucht. Die praenatale Mortali~t (M) in Linie L stieg mit der Anzahl von Implantationen (I) wie M = 0.11 + 0.0003 X/3. Dieser experimentelle Anstieg k~innte schliefilichdie steigende Anzahl Implantationen ausbalancieren und so zum Plateau in der WurfgrSfie beitragen.Die Bedeutung dieserBefunde ftirdas Verstiindnis (genetischer) Effekte auf die Wurfgr~f~e bei Schweinen wird aufgezeigt.
161
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
I n v e s t i g a t i o n o f a S e l e c t i o n L i m i t for L i t t e r S i z e in Mice R.C. B U I S
Department o/Animal Breeding, Agricultural University, P.O. Box 338, 6700 AH Wageningen (The Netherlands) (Accepted 14 March 1988)
ABSTRACT Buis, R.C., 1988. Investigation of a selection limit for litter size in mice. Livest. Prod. Sci., 20: 161-172. A long-term selection experiment for large litter size in mice was conducted, starting with Line L at average litter size of 8.3 ± 2.2. At Generation 30 with litter size of 14.0 ± 2.7, a selection plateau appeared. The nature and causes of the plateau were investigated. From Generation 48 of Line L, divergent selection for litter size was applied upward (Line L H ) and downward (Line LL). After 12 generations, average litter size in LH was 13.9 ± 2.6 and in LL 10.2 _+4.7. Realized heritabilities were zero in L H and 0.20 ± 0.04 in LL. The response to reverse selection was attributed to the continued segregation of recessive alleles at the plateau. The role of increased prenatal mortality as a cause of the plateau was investigated. Prenatal mortality (M) in Line L increased with number of implantations (I) as M = 0.11 + 0.0003 × 13. This exponential increase could eventually balance an increasing number of implantations and thus contribute to a plateau for litter size. The relevance of this study for the understanding of genetic effects on litter size in pigs is indicated.
INTRODUCTION
Reproduction is an important and complex trait in livestock. In pigs, litter size a~ birth and at weaning is a main factor in establishing the efficiency of a production system. Knowledge of the genetic nature of those traits contributes to an optimal use in breeding systems. Their low heritability leads to slow progress in selection experiments for litter size (Ollivier, 1982). Experiments with litter size in mice as a model for litter size in pigs can reveal the genetic nature of this trait. Eisen (1980) reviewed results of long-term selection experiments with mice for growth and litter size, with respect to their usefulness for livestock production. Until that time only 3 long-term selection-limit studies for litter size in mice had been published. Bakker et al. (1978) established a selection line (L) for high number of young at birth per litter (litter size) in mice. The selection criterion was first
0301-6226/88/$03.50
© 1988 Elsevier Science Publishers B.V.
162 parity litter size (alive plus dead), and litters were not standardized. During the first 30 generations of selection, average litter size ( + s.d. ) per generation increased from 8.3 + 2.2 to 14.0 + 2.7, and average coefficient of inbreeding reached 37%. Realized heritability ( + s.e. ) was 0.11 + 0.01 ( P < 0.01 ), and during the period of selection no significant decrease in effectiveness of selection was observed (Bakker et al., 1978). Average percentage of fertile matings was rather constant at 92 _+7%. Selection in Line L was continued after Generation 30, but no significant further progress in litter size was observed. In Generation 60, average litter size ( _+s.d.) was 14.8 +_3.2 with an average coefficient of inbreeding of 56%. Response to selection on litter size is presented in Fig. 1. From Generation 31 to 60, the cumulative selection differential was 95.3 young and cumulative selection response only 0.6. Also, the standard deviation of litter size did not decrease and there was no significant realized heritability. Average fertility of females was 93 _+4% in this period. From these unpublished data we concluded that at Generation 30 a selection limit of 14 to 15 young had been reached for litter size in Line L. Litter size of a control line (C) is presented also in Fig. 1. Litter size in Line C, bred by a system of maximum avoidance of inbreeding, decreased significantly. At Generation 45, when average coefficient of inbreeding was c. 35%, litter size was insufficient to continue regular breeding and the line became extinct. Average fertility of the females, however, was still 93% during the last 5 generations. Its value as a long-term control line, therefore, is subject to doubt. The selection limit of Line L was roughly the same as the limit for litter size in another line ( N ) f r o m a different origin, selected for high litter size without standardization (van den Nieuwenhuizen et al., 1982 ). From Generation 20 to 50, no ~jgnificant increase in litter size was observed in Line N, the average LITTER S I Z E
14
12 =
- LINE
L
10
8
6 L
I 10
I 20
~ 30
40
I 50 60 GENERATION
Fig. 1. Litter size in the selection line L and in the control line C: averageof mean and 1/2 standard deviation per group of 5 generations.
163
( + s.d.) being 14.6 + 3.0. The results in Lines L and N suggest that there is a selection limit for litter size in these lines of mice between 14 and 15 young. The aim of the current experiment was to investigate the presence and nature of the apparent plateau for litter size between 14 and 15 young, using several approaches. First, the genetic cause of the limit was investigated by means of a divergent selection experiment for litter size during 12 generations. A possible negative maternal effect upon litter size, which might contribute to the plateau, was investigated by standardization of litters. Secondly, the possibility of a physiological barrier with respect to prenatal mortality was investigated, based on the hypothesis that increased prenatal mortality prevented an increased litter size despite an increased number of implantations. MATERIALS AND METHODS
For the divergent selection experiment, two lines of mice were established from Generation 48 of the L line. The new base generation consisted of 45 litters, with average litter size of 13.1 _+4.0. These litters were standardized to 10 young within 24 h of birth. Standardization was continued in both divergent lines during selection to avoid environmental maternal effects on litter size which may complicate the interpretation of results (Eisen, 1970). The line selected for high litter size was designated LH and that for low litter size LL. Selection was performed for 12 generations, by randomly choosing 2 females per litter of the 24 largest (LH) or smallest (LL) litters and 3 males per litter of the 8 largest (LH) or smallest (LL) litters. Thus each generation consisted of 48 females and 24 males mated at random 2:1 with avoidance of full-sib or half-sib mating. During the same period, selection for high litter size was continued in the L line with the same procedure but without standardization. No control line was used. Experience showed (see Introduction ) that over 12 generations litter size decreased systematically in a control line, which made comparison with the selection line complicated. There was no opportunity to establish a control line with a sufficiently large effective number as to prevent this decrease. Realized heritabilities in the selection lines LL and LH were estimated as 2 >
164
of the selection plateau. Heritabilities were estimated via d a u g h t e r - d a m and via granddaughter-granddam regression as h 2 -- 2 X b and h 2-- 4 X b, respectively. Heritabilities were also estimated from variance components of full sibs and of half sibs in a nested design with equal subclass numbers (Becker, 1964). The mating scheme provided litters of dams, arranged in groups of 2 full sibs within 4 half sibs. Owing to selection in the parents, a design with this structure was only possible in part of the material. Prenatal mortality was analysed in 160 mice derived from Generation 50 of Line L. At a representative time, Day 13 of pregnancy, mice were dissected and numbers of dead embryos and of foetuses were counted. At this day both very early and rather late mortality could be observed (Bakker et al., 1978). Mice were caged in Makrolon cages, type Hulskamp-Komeko MAK 180, and had free access to water and pellets from 12 days of age onwards. Room temperature was kept at 23 °C and relative humidity at a m i n i m u m of 60%. There was a 12-12 h light-dark regime. RESULTS
Average litter sizes and standard deviations within generations are given in Fig. 2 for Lines LH and LL. Calculated over 12 generations, average litter size (+s.d., corrected for generation means) in Line LH was 14.4+2.9 and the regression coefficient of litter size on generation number was b = - 0.03 _+0.05 (P > 0.05 ). Thus no further response was observed in this line. Average fertility of females over the 12 generations was 91 + 6%. Comparable results for generations 49-61 in Line L were: average litter size ( + s.d.) was 13.7 + 3.3 and b = 0.01 + 0.05 ( P > 0.05 ). The difference of 0.7 young in favour of Line LH can be attributed to the standardization of litters in this line and thus to a reduction of a part of the negative environmental effect of large litters. Also, average weight at 8 weeks in Line LH ( 28.3 _+4.0 g) was higher than in Line L (26.2 + 3.6 LITTER SIZE
16-
14
12 10 o~o
~"
LINE LL
;
;
;
1'2
GENERATION
Fig. 2. Litter size per generation in the divergent selection lines LL and LH: mean and 1/2 standard deviation.
165 g). This difference is also likely to be caused by standardization. Female fertility was 92 ___3%. In the downward selected Line LL, the regression coefficient ( + s.e. ) of litter size on generation number was b = - 0.30 + 0.06 ( P < 0.001 ), suggesting a significant selection response. This response was most obvious after 5 generations of reversed selection. Standard deviation of litter size, corrected for generation means, was 3.8. This higher value in comparison to the s.d.s for litter size in Lines L H and L was caused by a relatively large number of small litters, mainly in the earlier generations of reverse selection. Especially in Generations 3 and 4, small litters caused a large deviation (Fig. 2 ). Average weight of the mice was 28.0 _ 4.4 g and thus very much comparable to the weight in the L H line, which was also standardized. Average female fertility over the 12 generations was 91 __+9%. The relation between cumulative selection differential (Sc) and cumulative selection response (Re) in the divergent lines is shown in Fig. 3. Realized heritability in Line L H was - 0 . 0 2 and thus set to zero. In this case no standard error could be calculated according to Hill (1972). Realized heritability + s.e. in Line LL was 0.20 _+0.04. The estimates of h 2 from family data are given in Table 1. Heritabilities, based on ( g r a n d ) d a u g h t e r - ( g r a n d ) d a m regression are not biased by selection in the parents' generation (Falconer, 1983). However, estimates by daughterdam ( D / D ) regression may be biased downwards by an opposite genetic and environmental maternal effect: females from large litters producing smaller litters compared to their positive genetic capacity owing to their negative maternal environment. In such a case the granddaughter-granddam ( G D / G D ) regression gives a more reliable estimate of heritability (van der Steen, 1985 ). Heritability estimates from half-sib ( H S ) and full-sib (FS) analysis can be LITTER S I Z E : R c
15
14
12
12
11 L I N E LH o.._...o L IN E LL
lO
-3'o
-Jo
o
CUMULATIVE SELECTION
II0 2tO
30
D I F F E R E N T I A L : Sc
Fig. 3. Cumulative selection response (litter size, Re) in relation to cumulative selection differential (So) per generation in divergent selection lines LL and LH.
166 TABLE 1 Heritability estimates for litter size in selected lines based on familydata A. Dam-daughter and granddam-granddaughterregression Line
Generations
Numberof p a i r s of animals
Heritability-+ s.e. from Daughter/dam
L L LH LL
6-14 49-61 0-12 0-12
Gr.daughter/gr.dam
724 576 565 564
0.64___0.16 -0.08_+ 0.16 -0.04_+0:16 0.18 -!-_0.10
Numberof animals used1
Heritability_+ s.e. from
-
0.04 -+0.40 0.33 ± 0.13 0.31_+0.11 0.12_+0.16
B. Half- and full-sibanalysis Line
Generations
Half sibs L L LH LL
6-14 49-61 0-12 0-12
320 (44%) 200 (35%) 184 (32%) 220 (39%)
0.25 _+0.25 0.23 +0.62 0.14 ± 0.26 0.07 ± 0.52
Full sibs 0.40___0.13 0.20+0.33 0.07 ± 0.32 0.00_+0.36
1Fraction of total material used in analysisbetweenparentheses. biased downwards by reduction of covariance between sibs owing to selection in the parents' generation (Ponzoni and James, 1978; Falconer, 1983). In the full-sib analysis, in turn, an upward bias may occur owing to a common maternal environment of litter mates and dominance variance. In the phase of increasing litter size in Line L, a high h2= 0.64 was observed by the D / D analysis. The contrasting zero value in the G D / G D analysis was unexpected and cannot be explained by selection or maternal effect. The difference between h 2 ( H S ) = 0 . 2 5 (not significant) and h 2 ( F S ) = 0 . 4 0 agrees with the effect of common maternal environment and dominance variance between full sibs. T h u s in the phase of increasing litter size, h 2 estimates based on family data are in the same direction as realized heritability. In the phases of the selection plateau of Line L in generations 49-61 and in Line LH, the D / D h 2 estimates were essentially zero, but the G D / G D h2s were 0.33 and 0.31, respectively. There was no difference between both lines irrespective of standardization in Line LH. Differences between G D / G D and D / D estimates were expected based on the maternal effect as described above. All h 2 estimates from sib analysis were not significant. This can be caused by the downward bias in the sib analyses, as described above, but also by the large standard errors caused by the low numbers of animals used for this analysis.
167 Thus, in the phase of constant litter size there was still an indication of additive genetic variance indicated by the GD/GD analysis. In the downward selected Line LL, the D/D h 2 was 0.18. The GD/GD h 2 was not significant, which was as unexpected as in the phase of increasing litter size. Again, the sib analysis produced no significant heritability estimates. The conclusion is that estimates of heritability based on family data, although not completely in agreement, indicate the presence and persistence of at least some additive genetic variance at the selection limit. The second approach to investigate the selection limit involved testing the hypothesis that prenatal mortality was linked with number of implantations (as a measure of ovulation rate) in such a manner that the number of living foetuses (as an upper limit of litter size) was limited by increased relative mortality. In 153 mice, derived from Line L and dissected at Day 13 of gestation, the following average ( + s.d.) numbers of traits were observed: living foetuses 15.3 + 4.2; dead embryos 1.7 + 1.7; dead foetuses 0.1 + 0.3. The average number of implantations was 17.1 + 5.1, and number of prenatal dead individuals was 1.8 + 1.7. In a similar number (n = 149 ) of living mice, derived from the same generation, average litter size was 13.8+ 3.7 of which 13.3_ 3.7 young were alive. The difference of 1.5 between number of living foetuses and litter size is caused by mortality between Day 13 of gestation and shortly after parturition. Two curvilinear regressions were fitted to describe the increase of mortality as a function of number of implantations:
D=0.70-0.175×I+0.013×I2; D=0.11+0.0003×I3;
R2=0.507
R2 = 0.504
(1) (2)
where D = number of dead embryos and foetuses, and I= number of implantations. Model (1) implies a minimal mortality of 0.11 embryos at 7 implantations. It describes a bimodal distribution of mortality, plotted against number of implantations. Mortality below 7 implantations may well have another cause than the exponentially increasing mortality above that number, but its explanation is beyond the scope of this study. Mortality above 7 implantations can be explained by increased 'crowding' at a higher implantation number. Model (2) explained as much variance as model (1) and was considered a suitable description of the response. The models are valid within the range of 1-23 implantations, included by the data. Based on Model 2, the following relation between number of implantations and litter size was found: LT= I-0.0003 × 13-1.5, where LT= total litter size and I= number of implantations; 1.5 is mortality between Day 13 and counting of litter size. Mortality increased sharply with higher numbers of implantations, so that with 23 implantations only 2 3 - 3 . 6 5 - 1 . 5 = 17.85 living young are expected. If the model were valid beyond the range of observations, a max-
168
imum average litter size of 21 would be possible (with 33 implantations). This figure may well be lower because mortality after Day 13 is expected to increase as well at higher numbers of implantations. Furthermore, the genetic association between mortality and number of implantations may be higher than the phenotypic association. Although within the given range no maximum litter size occurs, owing to an equilibrium between number of implantations and mortality, prenatal mortality can, at least together with other causes, contribute to the plateau for litter size. DISCUSSION
In his review on long-term selection experiments, Eisen (1980) mentioned two selection lines for litter size which plateaued between Generations 30 and 35. By way of contrast, he also referred to the fertile line of Bakker et al. (1978), used in the present paper, which had not yet reached a plateau at Generation 29. Now it appears that this line also reached a plateau at Generation 30 and thus did not constitute an exception. The apparent selection plateau in Line L occurred in a situation without standardization of litter size, and the line was continued until Generation 60 (the last one used in this paper). Simultaneously with the last 12 generations, upward selection was performed in Line LH with standardization of litter size to 10 young. Thus the existence of a genotype by environment interaction with respect to the plateau was investigated. Over 12 generations, mice in Line LH produced 0.7 young per litter more than in Line L. There was no further increase in litter size in this period, so upward selection evoked no further response. We concluded that there was some genotype-by-environment interaction with respect to the plateau. The level of the plateau could be caused partially by a negative maternal environmental effect in females born in large litters (in the unstandardized situation) upon their own litter size. Two remarks have to be made. First, by standardization within 24 h after birth we could not remove any pre- and perinatal effect which may still contribute to the level of the plateau. Secondly, the effect may well be nonlinear but exponential with increasing litter size, so its contribution to a plateau may be more serious than we can estimate here. Between Generations 30 and 60 of Line L in our experiment, litter size was constant despite a slow and continuous increase of the average inbreeding coefficient from 37 to 56%. The L line seemed to be insensitive to the effect of inbreeding, or inbreeding depression was balanced by the continuing positive effect of selection. Also, the fertility rate of the females was constant and high (above 90% ), not suffering from inbreeding depression. Selection for low litter size yielded a realized heritability of 0.20, indicating that at the selection plateau for litter size there was still additive genetic variation. The large asymmetry in response in our results made it meaningless to
169 estimate the realized heritability from a comparison of the 'divergent' Lines LH and LL. Estimation of h 2 for litter size by use of family data in various phases of Line L and in the Lines LL and LH provided an indication of additive genetic variation in all lines and phases. This conclusion is mainly based on the regression approach, which was least biased. From the sib analysis, biased in two directions, in Line L the presence of dominance variation in the phase of increasing litter size can be supposed, although not discernable from environmental fullsib variance. In other studies on litter size and selection limits (e.g. Falconer, 1971; Eklund and Bradford, 1977) no heritability estimates in selected lines based on family data are reported. Based on the positive response to reverse selection and the family data analysis, we conclude that in the apparently plateaued line there is still additive genetic variability, perhaps accompanied by dominance or epistatic genetic variability. Comparison of our results with those of other authors indicates the following. Falconer (1971) tried to overcome the selection limit for litter size in a strain of mice. He formed several inbred strains under continued selection. The strains were crossed which resulted in 1.5 young per litter more than at the limit. Falconer concluded that there was still residual genetic variation after reaching the plateau. Because in one inbred strain litter size did not decrease after inbreeding, he excluded overdomlnance as a major cause of the limit. He postulated that the variation should be caused by the continued segregation of recessive genes with a negative effect on litter size. Whilst the positive response before reaching the limit should mainly be based on additive genetic variation for ovulation rate, recessive genes should mainly act on embryonic mortality. Eklund and Bradford (1977) explored the nature of a selection limit for litter size by reverse selection {among other approaches). They observed a positive response, as in our results, mainly after some 5 generations. Their results of relaxed selection and selection with inbreeding were similar to those in the plateaued L strain, indicating that overdominance could not play a major role in the establishment of the limit. As Falconer (1971), Eklund and Bradford (1977) also concluded that recessive genes at low frequencies could explain the residual variation at the plateau, expressed by a decrease in litter size after reversed selection. Because in our experiment a control or a relaxed selection line was omitted, we could not rule out the possibility of overdominance as a cause of the plateau. Eisen (1980) concluded from the foregoing studies that litter size has a low heritability, compared with adult body weight, for example. Selection limits for litter size should, therefore, be caused by depletion of additive genetic variance. Genetic variance, existing at the limit, should be based on segregation of recessive genes at low frequencies, responsible for small litter sizes. Overdominance should not be considered as a major cause of a selection limit. With
170
respect to growth and body size, Barria and Bradford (1981) analysed a selection limit for those traits. Also here, reverse selection was successful. In this case they suggested a negative association between the selected trait, body size and fitness traits, rather than exhaustion of additive genetic variation, as a cause of the plateau. The pattern of decrease in Line LL resembled that obtained by Eklund and Bradford (1977) in their reverse selection line. In both experiments, only a small response was obtained in the first 5 generations of reverse selection, followed by a stronger response in the next 6-7 generations. This pattern is compatible with segregation of recessive genes for low litter size (or high embryonic mortality) at low frequency at the plateau, their frequency only increasing by reversed selection after some generations. In case of overdominance as a cause of the plateau, a larger variation in that phase may be expected, leading to a more immediate response to backward selection. Thus, apart from some remaining additive genetic variation, genetic variation at the plateau is probably present in the form of simple recessive alleles at low frequencies not removable by further upward selection. The fertility rate of females was high and rather constant before and after reaching the plateau for litter size, and did not decrease in Line LL. Thus this trait appeared not to be associated with litter size. The results on prenatal mortality alone did not provide a sufficient explanation for the selection limit. In pigs, however, Blichfeldt and Almlid (1982) observed a curvilinear relationship between ovulation rate (0) and number of live embryos (L), described by the model: L=-12.1+2.68×0-0.074×02. Their model explained 21% of the variation occurring in number of live embryos and predicted a maximum of 12.2 embryos at 18.1 ovulations. The numbers of embryos decreased at higher numbers of ovulations. In such a case, embryonic mortality could play a major role in the establishment of a selection limit for litter size. These data originated from pigs that were not selected for fertility. In the plateaued L line, 50% of the variation in prenatal mortality was explained by number of implantations (as a measure for ovulation rate). Increased prenatal mortality thus may at least contribute to the plateau reached for litter size. It would have been interesting to investigate prenatal mortality also in the reversed selection Line LL after the response was reached. Summarizing the results, we conclude that the plateau for litter size was not only caused by reduction of genetic variation. The family analysis pointed to at least some remaining additive variance and possibly some variance caused by dominant alleles. The lack of response to further upward selection can be explained by an increasing negative maternal environmental effect and increasing mortality owing to higher numbers of implantations. The positive response to reverse selection can be explained by the continuous segregation of recessive alleles with negative effect on litter size (maybe by higher embryonic mortality) at the plateau.
171 T h e r e l e v a n c e of t h e r e s u l t s for pig b r e e d i n g is a b e t t e r u n d e r s t a n d i n g o f t h e genetic n a t u r e of litter size in r e l a t i o n to m a t e r n a l e n v i r o n m e n t a l effects a n d p r e n a t a l m o r t a l i t y . I n pigs, p r e n a t a l losses are c o n s i d e r a b l e a n d c a n a m o u n t to 3 0 - 4 0 % of o v a s h e d (Scofield, 1972). I t is, in general, u n k n o w n h o w far a pig p o p u l a t i o n is r e m o v e d f r o m a selection l i m i t for litter size, b u t n a t u r a l selection m a y h a v e p l a y e d a role in foregoing g e n e r a t i o n s . Also, in pigs d o w n w a r d s e l e c t i o n c o m b i n e d w i t h i n v e s t i g a t i o n o f p r e n a t a l m o r t a l i t y m i g h t elucidate m o r e of t h e genetic a n d e n v i r o n m e n t a l effects on litter size. ACKNOWLEDGEMENT T h e a u t h o r g r a t e f u l l y m e n t i o n s t h e skilful c a r e for t h e a n i m a l s a n d t h e acc u r a t e collection o f d a t a b y M i s s M . A . W . P e t e r s of t h e L a b o r a t o r y A n i m a l Centre of the Agricultural University.
REFERENCES Bakker, H., Wallinga, J.H. and Politiek, R.D., 1978. Reproduction and body weight of mice after long-term selection for large litter size. J. Anim. Sci., 46: 1572-1580. Barria, N. and Bradford, G.E., 1981. Long-term selection for rapid gain in mice. I. Genetic analysis at the limit of response. J. Anim. Sci., 52: 729-738. Becker, W.A., 1964. Manual of quantitative genetics. 1st edn. Washington State University Press, 152 pp. Blichfeld, T. and Almlid, T., 1982. The relationship between ovulation rate and embryonic survival in gilts. Theriogenology, 18: 615-620. Eisen, E.J., 1970. Maternal effects on litter size in mice. Can. J. Genet. Cytol., 12: 209-216. Eisen, E.J., 1980. Conclusions from long-term selection experiments with mice. Z. Tierz. Zuechtungsbiol., 97: 305-319. Eklund, J. and Bradford, G.E., 1977. Genetic analysis of a strain of mice plateaued for litter size. Genetics, 85: 529-542. Falconer, D.S., 1960. The genetics of litter size in mice. Proc. Cold Spring Harbor Symp. Quant. Biol., 20: 178-196. Falconer, D.S., 1971. Improvement of litter size in a strain of mice at a selection limit. Genet. Res., Camb., 17: 215-235. Falconer, D.S., 1983. Introduction to quantitative genetics. Longman, London and New York, 340 pp. Hill, W.G., 1972. Estimation of realized heritabilities from selection experiments. II. Selection in one direction. Biometrics, 28" 767-780. Ollivier, L., 1982. Selection for prolificacy in the pig. Pig News Information, 3: 383-388. Ponzoni, R.W. and James, J.W., 1978. Possible biases in heritability estimates from intraclass correlations. Theor. Appl. Genet., 53: 25-27. Scofield, A.M., 1972. Embryonic mortality. In: D.J.A. Cole (Editor), Pig Production, Butterworths, London, pp. 367-383. Van den Nieuwenhuizen, J., Bakker, H. and Buis, R.C., 1982. Genetic differences in reproduction and growth rate between two lines of mice selected for litter size. Z. Tierz. Zuechtungsbiol., 99: 292-307.
172 Van der Steen, H.A.M., 1985. The implication of maternal effects for genetic improvement of litter size in pigs. Livest. Prod. Sci., 13: 159-168. RESUME Buis, R.C., 1988. Recherches sur un plateau de sdlectionpour la mille de la port~e chez la souris. Livest. Prod. Sci.,20:161-172 (en anglais). Une experience de s$1ection ~ long terme en faveur de la mille de la portde a dt~ effectude sur la souris.Elle a d~but~ avec la lign~e L ayant 8,3 +__2,2 c o m m e mille moyenne de la portde. U n plateau se situant ~ 14,0 _+2,7 est apparu au bout de 30 gdndrations. La nature et les causes de ce plateau ont ~td recherchdes. Une s$1ection divergente sur la mille de la port~e (lignde haute: LH; lignde basse: LL) a ~t~ appliqu~e ~ partir de la 48 ~m~ g~ndration de la lignde L. Apr~s 12 gdndrations, la taillemoyenne de la pottle ~mit de 13,9 +_2,6 pour L H et de 10,2 +_4,7 pour LL. Les hdritabilitds r~alisdes ~mient de z~ro pour L H et de 0,20 + 0,04 pour LL. La r~ponse obtenue par s~lection rdverse a St~ attribuSe au maintien de la s~gr~gation d'allblesr~cessifspendant le plateau. Le r61e possible jou~ par un accroissement de la mortalitd pr~natale dans l'existencedu plateau a 6t~ dtudi~. La mortalit~ pr~natale (M) augmente dans la lignde L avec le nombre d'implantations (1) suivant la relation M = 0.11 + 0.0003 X/3. Cette augmentation de type exponentiel serait susceptible de compenser un accroissement du nombre d'implantations et contribuerait ainsi ~ l'existence d'un plateau pour la taillede la pottle. Les possibilit~sd'application de cette dtude pour comprendre les effets (g~ndtiques) sur la mille de la portde chez le porc sont indiqudes. KURZFASSUNG
Buis, R.C., 1988. Untersuchung einer Selektionsgrenze fiir WurfgrSfie bei M~iusen. Livest. Prod. Sci., 20:161-172 (auf englisch). Mit der Linie L, die die mittlere WurfgrSfie von 8.3 ___2.2 hatte, wurde ein langfristigesSelektionsexperiment fiirgr~fiereWfirfe durchgefiihrt.In der 30. Generation schien bei einer WurfgrSfie von 14.0 _+2.7 ein Selektionsplateau erreicht zu sein. Ab Generation 48 wurde die Linie L in eine auf- (LH) und eine abselektierte (LL) unterteilt.Nach 12 Generationen divergenter Selektion erreichten die Linien L H 13.9 _+2.6 und L L 10.2 _ 4.7 in der WurfgrSfie. Die realisiertenHeritabilit~itenwaren null in der LH- und 0.20 +_0.04 in der LL-Linie. Der Erfolg der Riickselektion wurde der anhaltenden Segregation rezessiverAlleleim Plateau zugeschrieben. Es wurde die RoUe erh~ihterpraenataler Mortali~t als Grund fiirdas Plateau untersucht. Die praenatale Mortali~t (M) in Linie L stieg mit der Anzahl von Implantationen (I) wie M = 0.11 + 0.0003 X/3. Dieser experimentelle Anstieg k~innte schliefilichdie steigende Anzahl Implantationen ausbalancieren und so zum Plateau in der WurfgrSfie beitragen.Die Bedeutung dieserBefunde ftirdas Verstiindnis (genetischer) Effekte auf die Wurfgr~f~e bei Schweinen wird aufgezeigt.