The fertility of mice selected for large or small body size

The fertility of two unrelated strains of mice (strains N and C) which had both been selected for large and small body size has been studied.

The fertility of pairs of mice in the large or small lines of strain C was unimpaired by selection. In strain N, some of the pairs in the large and small lines, but not in the control line, were sterile. Sterility in the large line was due to the low libido of the males, and not to female infertility. Sterility in the small line was probably due to hypo-functioning of the anterior pituitary of some females: the oestrous cycle was delayed or absent, some mice failed to ovulate after mating, and a high proportion of those mating had no implanted embryos at 12 days' gestation. Oestrus and ovulation could be induced in mice of the small line of strain N by exogenous gonadotrophins, and the proportion of mice with implanted embryos was considerably increased by progesterone supplements.

The number of eggs found after natural mating was considerably higher in large mice than in small mice, and was significantly correlated with body weight in the small line of strain C only. The egg-number/body-weight ratio was higher in the lines of strain C than in those of strain N, though the ratios were similar when carcass fat was subtracted from total body weight.

The amount of endogenous follicle-stimulating hormone secreted by the mice of the five lines was estimated by inducing ovulation with various amounts of exogenous gonadotrophins, and comparing the number of eggs found after each dose with the mean number ovulated after natural mating. Estimates of the amount of follicle-stimulating hormone secreted by mice of strain C were higher than those for mice of strain N.

Differences in the rates of growth and in the numbers of eggs ovulated after natural mating indicate a higher level of pituitary activity in strain C than in strain N.

SELECTION INTENSITY AND RESPONSE TO SELECTION FOR YEARLING WEIGHT IN BEEF CATTLE

Ten years of selection for yearling weight in two replicate herds of beef Shorthorns has resulted in genetic increases of 4.8 ± 3.1 and 4.1 ± 3.0 kg/yr in males and 3.3 ± 2.7 and 2.3 ± 1.5 kg/yr in females. These increases were measured as deviations from an unselected control line mean and comprise about 40–45% of the absolute increase in yearling weight that took place during the selection program. The accumulation of selection differential was irregular at first but, in the last 6 yr of the program, advanced regularly at rates of 12.1 ± 0.7 and 11.2 ± 0.9 kg/yr for males and 9.1 ± 0.3 and 7.6 ± 0.4 kg/yr for females. Estimates of realized heritability derived from the two herds were.49 ±.12 and.51 ±.16 for males and.50 ±.11 and.29 ±.08 for females. A separate evaluation, based on line crosses within one herd, tended to confirm the heritability estimates.