REPRODUCTIVE-PERFORMANCE IN MICE SELECTED FOR DIVERGENCE IN BODY-FAT CONTENT

Correlated responses in female reproductive performance were studied i n lines of mice selected for high (HF) and low (LF) fat content relati ve to a randomly selected control line (RC). Selection was based on an indicator trait of body fat percentage in males (epididymal fat pad w eight as a percentage of body weight at 12 weeks of age). LF mice show ed a positive correlated response (P < 0.05) in number of pups born (N B), whereas HF showed no significant change. The increase in NB could be explained partially by an increase in ovulation rate (adjusted for body weight at mating; P < 0.01), measured as number of corpora lutea (CL) 16 days postcoitus (P < 0.05) or number of ova (OR) shed 2 days p ostcoitus (P < 0.01). Asymmetry was observed for CL (P < 0.05) and OR (P < 0.01); the major correlated response was observed in the LF line. Preimplantation and total embryo survival were higher (P < 0.01) in L F than in HF mice, whereas no significant divergence was found for pos t-implantation embryo survival. Reciprocal crosses of HF and LF lines allowed partitioning of divergence in OR into maternal (P < 0.10) and individual (P < 0.10) effects with LF being larger than HF. No signifi cant heterosis was found for OR or mating weight. Body weight at matin g was larger in HF females than in RC controls, while LF females were larger than controls in one experiment and not different from controls in a second experiment. Therefore, body weight at mating did not expl ain the positive correlated response in ovulation rate observed in the LF line.