Abstract
Using a closed colony of Wistar-Imamichi rats, 10, 200 females were impregnated with 214 males. One of the sires was found to repeatedly cause small litter size deliveries. Another male with this characteristics was obtained by backcrossing of his daughter to the original male. Thereafter sib-mating was maintained. In the first and the second generations of sib-mating, 3 males out of 5 and 4 males out of 5 showed this characteristics. These animals were designated as small litter-line males and compared with randomly bred males (control males).
The litter size of small litter-line males was 5.4±3.7 (mean±SD), while that of the control males was 12.4±3.8. The number of implantation sites (6.7±2.9) was significantly smaller than that in the control (14.7±2.0). Fertility of the ova (46.2%) was lower than that in the control (96.9%). His-topathological lesions were observed in the testes and accessory reproductive organs in the small lit-ter-line males. Hypospermatogenesis was evident.
These findings indicate that the litter size of a rat is regulated by the sire. Hypospermatogenesis, possibly a genetic abnormality in small litter-line males, would decrease the number of fertilized ova and consequently, the number of implantations.
