Direct responses in males and correlated responses for reproduction in females to selection for testicular size adjusted for body weight in young male lambs

Abstract

Selection based upon testicular diameter adjusted for body weight at 6, 10 and 14 weeks of age was used to produce two lines of sheep, with either high or low testicular size. Ten generations of selection were carried out and the estimate of the realized heritability of the selection criterion was 0.53 +/- 0.01. There were significant positive correlated responses to selection for testicular diameter at 6, 10 and 14 weeks of age, but the correlated responses in body weight at these ages were negative. In mature females, there were significant negative correlated responses to selection in premating body weight in the 1st, 2nd and 3rd breeding season and in the day of the first oestrus in the 2nd breeding season. Litter size per ewe mated had a small positive correlated response to selection in the second breeding season. This latter response appeared to be due to a positive correlated response in fertility, ewes from the High-line having a significantly higher probability of conceiving to a single mating than those from the Low-line. There was no significant correlated response in ovulation rate or litter size per ewe lambing and the genetic correlation between these traits and the selection criterion is likely to be close to zero. This may be due to the adjustment for body weight used, but it is possible that, in any event, body weight in young rams may be a better predictor of female ovulation rate than testicular diameter. These results do not rule out the possibility that testicular size in rams older than those selected would provide a good predictor of genetic merit for female ovulation rate.

Association Between Expression of Reproductive Seasonality and Alleles of the Gene for Mel1a Receptor in the Ewe1

“…This study and that of other authors [19,20] suggest the existence of a common overall factor involved in the control of reproductive seasonality. One possible candidate could be melatonin secretion.…”

Genetic parameters of spontaneous spring ovulatory activity in Mérinos d'Arles sheep

“…In selection experiments, Dzabirski and Notter [7] showed an advanced date of lambing in autumn for ewes with spontaneous ovulatory activity in spring in comparison to ewes responding to ram introduction. When selecting for fall lambing fertility, Notter et al [25] [15] and by Montgomery and Hawker [23] when selecting for out-of-season testis size and wool growth, respectively. A candidate overall factor could be melatonin, the hormonal messenger by which animals perceive night duration and then seasonal variations [20].…”

Selection for testis size and correlated responses of female reproductive traits in golden hamsters

“…Selections for testis size were not effective to increase OR in sheep (Haley et al . ) and pigs (Johnson et al . ).…”

“…Selection for testis weight in mice has effectively improved ovulation rates, but not litter size (Islam et al 1976). Selection for testicular diameter adjusted for body weight in young male lambs has not been associated with a significant correlated response in ovulation rate (Haley et al 1990). It has also been suggested that there is a genetic relationship between testis measurements and female reproductive traits in the pig (Toelle & Robison 1985), but selection for predicted testis weight in pigs did not significantly increase ovulation rates in daughters (Johnson et al 1994).…”