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 litter size in mice, responses in uterine capacity and ovulation rate were 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 ovulation rate and the proportion of ova shed that resulted in fully formed offspring; UT = selection on uterine capacity measured as litter size from females unilaterally ovariectomized at 4 wk of age; and LC = unselected 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 ovulation rate and uterine capacity, respectively. Selection in IX, LS, and UT increased ( P \u3c .01) ovulation rate from unilaterally ovariectomized females but by a greater amount ( P \u3c .01) in IX and LS than in UT. Selection also increased (P \u3c . O l ) uterine capacity of IX, LS, and UT (average response relative to LC = 1.76 pups); response was at least as great in LS and IX as in UT. Direct selection in UT was successful at improving uterine capacity but was no more effective than IX or LS selection. Cases in which ovulation rate limited expression of uterine capacity in UT may have shifted some selection emphasis to ovulation rate and reduced response in uterine capacity

Uterine Mass and Uterine Blood Volume in Mice Selected 21 Generations for Alternative Criteria to Increase Litter Size

Lines of mice, selected for 21 generations using alternative criteria to increase litter size, were evaluated for uterine mass and uterine blood volume to help explain differences in uterine capacity. For this study, mice were sampled from Generation 27, the sixth generation after relaxation of selection. Mice came from all four criteria of selection (LS = selection on number born to unaltered females; M = selection on an index of ovulation rate and ova success; UT = selection on uterine capacity; and LC = unselected control) in each of three replicates (a total of 12 lines). Measurement was at one of two stages, either 3 d or 6 d of gestation. Matings were at 10 wk of age, and a total of 508 mice (17 to 26 per line-day of pregnancy subclass) were measured. The mean of the three selected groups exceeded the control in uterine mass ( P \u3c .001), uterine blood volume ( P \u3c .002), uterine mass/body mass (P \u3c .03), and uterine blood volume/body mass ( P \u3c .04 ) but not in uterine blood volume/uterine mass. Greater uterine mass and concomitantly greater uterine blood volume may have been partly responsible for greater uterine capacity resulting from LS, M, and UT selections

Estimates of Heritabilities and Genetic and Environmental Correlations for Left- and Right-Side Uterine Capacity and Ovulation Rate in Mice

Heritabilities for and genetic and environmental correlations between uterine capacity, ovulation rate, and body mass (BM) were estimated in mice. Uterine capacity was defined as the number of fetuses (LUC or RUC for left or right side) in one uterine horn for unilaterally ovariectomized females. Ovulation rate (corpora lutea, LCL or RCL for left or right ovary) was measured on the remaining single ovary in these same females. Data on 1,931 mice from four selection populations were used. Left ovulation rate and LUC were measured on 958 animals, and RCL and RUC of another 972 animals were recorded. Genetic and environmental variances and covariances were estimated simultaneously using an animal model with a multiple-trait, derivative-free, restricted maximum- likelihood procedure. Averages for heritability and correlation estimates derived from separate analyses of the selection populations are presented below. Heritability of LUC was higher (.33 ± .06) than that of RUC (.19 ± .02). Heritability of LCL and RCL ranged from .17 ± .03 to .27 ± .06, and heritability for BM was .65 ± .05. The genetic correlation between LUC or RUC and LCL or RCL ranged between .43 ± .29 and .68 ± .05, and between LUC and RUC was .92 ± .05. Body mass had a higher genetic correlation with LCL and RCL (.70 ± .12 and .93 ± .02) than with LUC and RUC (.37 ± .05 and .47 ± .12). Environmental correlations between LCL and LUC and RCL and RUC were .32 ± .09 and .36 ± .05, respectively