EMBRYONAL SURVIVAL TO 6 DAYS IN MICE SELECTED ON DIFFERENT CRITERIA FOR LITTER SIZE

Embryonal survival was compared in mice resulting from four criteria o f selection: LS = selection on number born; IX = selection on an index of ovulation rate and ova success; UT = selection on number born to u nilaterally ovariectomized females; and LC = unselected control. Selec tion occurred for 21 generations with three replicates of the four cri teria; thereafter, relaxed selection was practiced. The evaluation was performed using mice of two replicates at Generation 35 and one repli cate at Generation 36. Data on a total of 289 female mice were recorde d. Females; at an average age of 9 wk, were mated to males of the same line. Six days after mating, each female was killed, ovaries were exc ised, corpora lutea were counted and equated to number of ova shed, an d the numbers of implantation sites in each uterine horn were recorded . Least squares means were .84, .91, .85, and .82 for left embryonal s urvival (left implantations/left oval and .91, .90, .86, and .87 for r ight embryonal survival for LS, IX, UT, and LC, respectively. The righ t side had greater ovulation rate (P < .001) and number of implantatio ns (P <.001). For embryonal survival, the criterion x side interaction was possibly important (P <.09). Selection for litter size by differe nt criteria increased ovulation rate (P < .003) and embryonal survival (P <.05) to 6 d. However, responses in embryonal survival were not gr eater after UT selection compared with LS or IX selection.