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Mating Success and Litter Size Variation within and between Inbred and Hybrid Generations of Rats¹

University of New Hampshire, Durham

Abstract

A determination of mating success within and between inbred CAS and LEW rat strains and their F₁ and F₂ hybrids revealed a highly significant difference among females of the inbred strains due to lack of libido by CAS males. A highly significant difference in male sterility was found between the CAS and LEW strains. CAS female sterility was not significantly different from LEW female sterility, but CAS females had smaller litters. None of the rats in the F₁ generation failed to produce F₂ litters, whereas several failures did occur within the F₂ generation. A long generation interval was a consequence of reduced mating success of males in the CAS strain. The effect of a hybrid maternal genotype was to produce the largest litters which varied the least in number of rats per litter, compared to the inbred and F₂ generation parents. The size and variation of endocrine glands revealed various gross abnormalities among sterile females, but only one unilateral cryptorchid LEW male and a runt F₂ male with juvenile reproductive organs were found. All males autopsied, including those in which there was no mating success, had motile epididymal spermatozoa. The genetic basis for male libido should be considered in the evaluation of inbred males for breeding purposes.

¹ Supported in part by University of New Hampshire, Central Research Fund.

² Department of Zoology.