UTERINE CAPACITY AND OVULATION RATE IN MICE SELECTED 21 GENERATIONS ON ALTERNATIVE CRITERIA TO INCREASE LITTER SIZE

After 21 generations of selection for alternative criteria to change l itter size in mice, responses in uterine capacity and ovulation rate w ere evaluated. Females from Generations 22 and 23 were sampled from 12 lines, representing three replicates of four selection criteria: LS = direct selection on litter size; M = selection on an index of ovulati on rate and the proportion of ova shed that resulted in fully formed o ffspring; UT = selection on uterine capacity measured as litter size,f rom females unilaterally ovariectomized at 4 wk of age; and LC = unsel ected control. All females in the present evaluation (a total of 1,932 ) were unilaterally ovariectomized (either left or right ovary excised ) at 4 wk, mated at 9 wk, and killed at d 17 of gestation. The number of corpora lutea and number of fetuses were counted to measure ovulati on rate and uterine capacity, respectively. Selection in IX, LS, and U T increased (P < .01) ovulation rate from unilaterally ovariectomized females but by a greater amount (P < .01) in M and LS than in UT. Sele ction also increased (P < .01) uterine capacity of IX, LS, and UT (ave rage response relative to LC = 1.76 pups); response was at least as gr eat in LS and M as in UT. Direct selection in UT was successful at imp roving uterine capacity but was no more effective than M or LS selecti on. Cases in which ovulation rate limited expression of uterine capaci ty in UT may have shifted some selection emphasis to ovulation rate an d reduced response in uterine capacity.