A simulation model including ovulation rate, potential embryonic viability, and uterine capacity to explain litter size in mice: II. Responses to alternative criteria of selection.

Direct selection for litter size was compared with selection for ovulation rate, ova success, or uterine capacity and for indexes of ovulation rate with ova success or uterine capacity. Selection was simulated for 10 generations in a mouse population based on a model integrating ovulation rate, potential embryonic viability, and uterine capacity. Two indexes including ovulation rate (OR) and ova success (OS) were I = .291 x OR + 2.19 x OS and I = .165 x OR + .736 x OS. Heritabilities for ovulation rate and ova success, assumed in the simulation and to derive the indexes, were .25 and .06, respectively. Both indexes resulted in the same response in litter size, 12.9% greater than response to direct selection for litter size. Two indexes including OR and uterine capacity (TUC = true total uterine capacity; UC = uterine capacity measured as number born for a female with right ovary excised) were I = .881 x OR + .223 x TUC and I = .876 x OR + .568 x UC. Heritabilities assumed for uterine capacity were .09 (TUC) and .065 (UC). The first index assumed true parameters for uterine capacity (TUC) and resulted in a response in litter size that was 23.9% greater than direct selection. The second index was calculated using parameters estimated under a unilateral-ovariectomy model and resulted in response that was 14.7% greater than direct selection. Selection for OR, TUC, UC, or OS resulted in responses that were 4.5, 48.5, 38.7, or 74.8%, respectively, less than that from direct selection for litter size.