Higher Litter Size Research Articles

Ovulation in ewes selected for fecundity: effect of synthetic Gn-RH injected on the day of oestrus.

When ewes are injected with synthetic Gn-RH on the day of oestrus, the magnitude of the LH release is greater than that following injection at other stages of the oestrous cycle (Reeves, Arimura & Schally, 1971; Hooley, Baxter, Chamley, Cumming & Findlay, 1974; Thimonier, Terqui & Pelletier, 1974). It is not known, however, whether additional LH on the day of oestrus has any effect on the number of ovulations, especially in ewes with the genetic potential for high ovulation rates. This question was investigated in four flocks of Merino sheep, designated `C', `O', `T' and `B', which are, respectively, an unselected group and groups selected since 1954, for low, medium and high litter size. The genetic history of these flocks has been described by Turner (1969). In May 1973, eighty-nine ewes (aged 4 to 7 years) from these flocks, were kept with vasectomized rams and observed daily for evidence of oestrus. After experiencing one oestrous cycle of normal length, ewes in sub-groups from each flock were injected intramuscularly with either 100 μg synthetic Gn-RH (Hoechst) in 2-0 ml saline or 2-0 ml saline only. The injections were given at 09.00 hours on the morning of the day of oestrus. In a proportion of the ewes from each group, jugular plasma samples were collected immediately before, and 60 min after, injection. These were assayed for LH by the method of Goding & co-authors (1969). Laparotomy was performed under thiopentone sodium anaesthesia 96 to 120 hr after the day of oestrus, and the corpora lutea were counted to assess the number of ovulations. After a delay of approximately two oestrous cycles, the ewes from each flock

The growth of infant mice at two temperatures.

Summary. Mice, Mus musculus, were bred in permanently mated pairs, in two environments, at 21° C and −3° C, respectively. Observations on strain A2G/Tb are described in detail; others studied were strain C57BL/Tb, outbred laboratory mice, and wild mice bred in the laboratory. Second to fourth litters were observed at birth, 10 days and 21 days. Most deaths were due to losses of whole litters. Mortality was higher at −3° C than at 21° C, but not among A2G/Tb mice of a stock bred for many generations in the cold. Litter weights at birth, for a given litter size, were unaffected by cold, but there were fewer large litters at −3° C than at 21° C; hence mean litter weight at birth was lower in the cold. At 10 and 21 days, litter weights, except those of strain C57BL/Tb, were lower in the cold at most litter sizes. Individual weight at 3 weeks was unaffected by number of litter mates at litter sizes around the mode, and declined only at high litter sizes; hence the sizes of most litters were within the range which allowed most growth. The findings illustrate the importance of the uterine environment in determining litter size and survival in the nest.

Improvement of litter size in a strain of mice at a selection limit.

SUMMARYA strain of mice that had ceased to respond to selection for high litter size was inbred with continued selection. Depression of the mean proved the existence of residual genetic variance. Four lines survived the inbreeding, and one reached 20 generations with a mean equal to the original strain, thus disproving overdominance as a major cause of the residual variation. The four selected inbred lines were crossed and a new strain derived from the cross was maintained in parallel with the original strain. The new strain showed an improvement of 1·5 mice per litter over the original strain. Thus selection with inbreeding was able to achieve an advance beyond the limit attained by the original selection.The hypothesis that the residual variation was due to genes with simple dominance was tested by seeing if it could account for the observations with reasonable values of the relevant parameters. The improvement made by the inbreeding and crossing required the elimination of about 30 recessive genes with effects (homozygote difference) of 0·5 phenotypic standard deviations and gene frequencies of 0·2. Consideration of the mean levels of the selected inbred lines in conjunction with the rate of depression found on inbreeding without selection showed that the selection with inbreeding had eliminated about 75% of the segregating reces-sives. The number of genes contributing to the residual variance was therefore about 40. The additive variance generated by these genes was just consistent with the estimate of zero from the realized heritability. Consideration of the original selection showed that about half the genes could have been still segregating when the response ceased. The hypothesis therefore requires the number of genes in the base population to have been about 80. The number of genes required, though large, does not seem impossible, and the hypothesis of genes with simple dominance can account for all the observations.