Genetic and Maternal Environmental Factors Influencing Litter Size and Reproductive Efficiency in Mice

Abstract

Lines of mice which have been genetically differentiated in litter size at birth and 6-week body weight by single-trait or index selection were randomly assigned to be reared in postnatal litter sizes (PLS) of N8, N12 or N16. The lines were selected as follows: large litter size (L+), large 6-week body weight (W+), selection index to decrease litter size and increase 6-week body weight (L-W+), selection index to increase litter size and decrease 6-week weight (L+W-) and random selection (K). Line and PLS differences in female body weight at weaning and 6 weeks were significant (P less than .05), with W+ greater than L+ greater than L-W+ greater than K greater than L+W- and N8 greater than N12 greaater than N16. Although there was a significant interaction between line and PLS, ranking of the lines was the same at each PLS. Females of each line by PLS group were exposed to males of the same group. Line and PLS differences (P less than .05) in female body weight at copulatory plug, adjusted for age, exhibited the same rankings as the earlier weights. Litter size at birth (dead plus alive), number born alive and number born alive per female exposed each revealed similar differences (P less than .05) in the ranking of lines and PLS. No significant interactions were found. Means for litter size were as follows for each line: L+ (17.4) greater than [W+ (14.1), L+W- (14.3)] greater than K (12.0) greater than L-W+ (10.8), and for each PLS: [N8 (14.0), N12 (13.9)] greater than N16 (13.2). The decline in litter size as PLS increased did not differ significantly from linearity. Adjusting litter size for body weight at copulatory plug eliminated the significant effect of PLS [N8 (13.9), N12 (13.7), N16 (13.4)]. The established genetic differences in litter size among lines due to selection were considerably larger than the maternal environmental effect due to postnatal litter size. Because of the negligible interaction between line and PLS, valid inferences concerning genetic differences among lines in litter size may be made at any postnatal litter size between eight and 16.

Alternative methods of selection for litter size in mice: III. Response to 21 generations of selection

“…The realized differentials for UT were considerably smaller than those exhibited in the IX and LS criteria (6.2, 12.8, and 10.8 g, respectively) but larger than unintentional selection in LC (.5 g). The higher values in the IX and LS criteria are in agreement with the findings of Eisen and Durrant (1980); their study indicated that improved female reproductive performance, especially ovulation rate, was correlated with increased body size.…”

“…The resulting changes after 21 generations of selection were larger than those noted at Generation 13. Eisen and Durrant (1980) found that for each additional gram of BW at mating, litter size increased by .42 pups. In the present study, total number born was regressed on female mating weight, and regression coefficients for M, UT, LS, and LC, replicates pooled, were .37 f .13 ( P < .11), .24 _+ .04 ( P < .05),…”

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“…They range from o to 0.62, depending on which group one examines; our maximum values are similar in magnitude to those found by Anderson (1975) for Microtus townsendii. However, Anderson's results were based on full-sib estimates for the most part, and the consistent maternal effects demonstrated here and in other quantitative-genetic studies of domestic rodents (Eisen and Durrant, 1980;Falconer, 1981) make it almost certain that her estimates were high. For our data, the full-sib heritability is twice the sum of the sire and dam components.…”

Index Selection for Components of Litter Size in Swine: Response to Five Generations of Selection

“…Improvements in litter size in mice have occurred from direct selection (Falconer, 1955(Falconer, , 1965(Falconer, , 1971Bradford, 1968Bradford, , 1979Joakimsen and Baker, 1977;Bakker et al 1978;Eisen and Durrant, 1980) and from selection for embryo (fetal) survival rate (Bradford, 1969(Bradford, , 1979. Responses from direct selection have been attributed largely to increased ovulation rate and stable or increasing survival rate (Bakker et al, 1978;Bradford, 1979).…”