Alternative methods of selection for litter size in mice: I. Characterization of base population and development of methods.

Abstract

Studies on a base population of mice were used to establish an index of components of litter size and a physiological model for measuring uterine capacity to be used subsequently in a selection experiment evaluating alternative methods for practicing selection to increase litter size. Heritability estimates of litter size, ovulation rate and ova success (fraction of ova resulting in fully formed pups) were .18, .33 and .15, respectively. No significant genetic or phenotypic correlation was found between overall ovulation rate and ova success. Phenotypic means and genetic variances were higher for characteristics measured on the right than on the left side of the reproductive tract. Linear and quadratic selection indexes, derived for a quadratic definition of breeding value, were compared. The linear index was predicted to be .99 as efficient as the quadratic one. Due to simplicity, the linear index (I = 1.21 x ovulation rate + 9.05 x ova success), scaled to have variance the same as litter size, was chosen for use. Ovulation rate in unilaterally ovariectomized females was .95 of that in females with both ovaries. No hypertrophy of the ipsilateral uterine horn in unilaterally ovariectomized females was found before implantation of embryos. Thus, unilateral ovariectomy appears to provide a physiological state to measure uterine capacity (as litter size) in the mouse.

Genetic variability of age and weight at puberty, ovulation rate and embryo survival in gilts and relations with production traits

“…Conversely, the value obtained for weight at puberty is larger than most literature estimates (Young et al, 1978;Hutchens et al, 1981 (Neal et al, 1989) and with estimates obtained in mice and rabbits (Clutter et al, 1990;Blasco et al, 1993b (Young et al, 1977), 50 days of gestation (Neal et al, 1989) or at birth (Young et al, 1978) is observed. This may be due to differences in the populations studied, but may also indicate that uterine competition tends to increase throughout gestation.…”

Alternative methods of selection for litter size in mice: III. Response to 21 generations of selection

“…Estimations of base population parameters were reported by Clutter et al (1990). The population was partitioned into three replicates and then into four lines within each replicate; line in this paper will refer to a replicatecriterion population.…”

Uterine capacity and ovulation rate in mice selected 21 generations on alternative criteria to increase litter size

“…They concluded that selection for litter size in these gilts would be essentially independent of ovulation rate and, thus, would improve genetic merit for uterine capacity. Clutter et al ( 1990) reported phenotypic correlations between numbers of corpora lutea and fetuses of .57 on the left and .70 on the right side in intact females from the base generation of the present population. Clearly, unilateral ovariectomy has resulted in a greater independence between ovulation rate and number of fetuses, but the correlation is still greater than zero.…”

“…1994. 72:577-583 explored by Clutter et al (1990), Gion et al (19901, and Kirby and Nielsen (1993). The purpose of the present study was to evaluate responses in uterine capacity and ovulation rate of mice due to selection for either uterine capacity, litter size, or an index of ovulation rate and ova success.…”