Selection for Increased Rate or Efficiency of Lean Growth in Rats: Correlated Responses in Reproductive Performance

Abstract

Two lines of rats selected for either rate or efficiency of postweaning lean growth and a control line were evaluated to determine whether any differences in reproductive performance had accompanied the genetic improvement in rate or efficiency of lean growth after 14 generations of selection (followed by six generations of relaxed selection). All progeny were reared in litters of nine until weaning at 21 days of age, and in like-sex or unlike-sex pairs thereafter. Like-sex paired females were placed with unrelated males of similar age for mating at approximately 70 days of age, while unlike-sex paired females remained with the original unrelated male until mated or autopsied at 85 days of age. Selection for either rate or efficiency of lean growth increased age at vaginal opening (38.9 and 38.6 vs 36.9 days for controls; P<.01) and weight at vaginal opening (133.1 and 126.6 vs 119.8 g for controls; P<.01) in like-sex paired females; similar findings were made for unlike-sex paired females. Ovulation rate, number of fetuses and pre- and postimplantation losses were not altered by selection. Age at first spontaneous mating did not differ among lines of male rats. Testes weight at 85 days of age were lower (P< .01) for males selected for efficiency of lean growth. The results indicate that there are alterations in reproductive characteristics as a consequence of selection for either rate or efficiency of lean growth in rats.