Selection for Litter Size in Mice

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Abstract
Selection for high and low litter size in mice has resulted in a marked response to selection with a difference of 7.6 mice between the high and low lines at generation 15. Realized heritabilities were 0.18 and 0.22 in the high and low lines, respectively, and this asymmetry was not significant. Selection response may have plateaued in the low line at about generation 17 at a level of .apprx. 5.7 mice. In the high line there is no evidence of a selection limit having been reached, and at generation 20 this line averaged about 14 mice. Main determinant of increase in litter size in the high line was an increase in ovulation rate. In the low line, decrease in litter size was about equally due to a decline in ovulation rate and an increase in embryo losses. Using a crossfostering experiment to separate direct genetic effects and maternal effects it was found that there were positive genetic associations between litter size and both direct genetic effects for growth and maternal performance. There was a positive correlated response in testis weight following selection for litter size. Greater response to selection for litter size in this study, compared to most previous studies, is discussed and some possible factors suggested that could account for this. [The encouraging response to selection for litter size in this study with mice has resulted in a change of attitude towards the possibility of improving litter size by selection in pigs and sheep in Norway.].