a1 Roslin Institute (Edinburgh)†, Roslin, Midlothian EH25 9PS
A crossbreeding trial extending over three generations was used to investigate the genetic components contributing to the prolificacy of the Meishan breed in comparison with the Large White breed. Information on the number of teats and on body weight and litter size in the first two parities was recorded on purebred Meishan and Large White females and on reciprocal F1 and backcross females. Ovulation rate was also recorded for all litters, allowing the estimation of per litter prenatal survival. Crossbreeding parameters for direct, maternal and grandmaternal effects were estimated using restricted maximum likelihood analysis. There was a consistent advantage of three to four piglets born alive to the Meishan female compared with the Large White female. This was controlled by the maternal genotype, with no effect of the genotype of the litter itself. Both additive and heterosis effects were important, the contribution of additive maternal effects to the breed difference being similar across parities (4·0 (s.e. 1·1) and 4·2(s.e. 1·1), in the two parities respectively) and the maternal heterosis increasing slightly across parities (2·2 (s.e. 0·8) and 2·9 (s.e. 0·8), in the two parities respectively). The number born alive to F 1 females was similar to, or greater than, the number born alive to Meishan females. Ovulation rate was significantly higher in Meishan than in Large White females and this was controlled by additive gene effects which had a similar effect across parities, the weighted average of their contribution to the breed difference being 5·7 (s.e. 0·8) ova. Differences between the breeds in prenatal survival were small, although there was significant maternal heterosis, however, the maternal additive effect became significant after the inclusion of ovulation rate as a covariate. After adjustment for ovulation rate, the weighted average estimates across parity of the maternal additive contribution to the breed difference and the maternal heterosis for the proportional prenatal survival loere 0-14 (s.e. 0·05) and 0·13 (s.e. 0·03), respectively. This suggests that a combination of a high ovulation rate and especially a high level of prenatal survival for that ovulation rate led to the prolificacy observed in this sample of Meishan pigs. The inclusion of ovulation rate as a covariate in the analysis of number born alive confirms this view, as the maternal additive effect on litter size was only reduced by about one third and the heterosis effect was largely unchanged. Both numbers stillborn and mummified were increased in litters born to Meishan sows due to maternal additive effects, but the effects seemed largely a consequence of the increased ovulation rate as they became non-significant after its inclusion as a covariate in the model.
(Received February 15 1994)
(Accepted September 08 1994)
† An institute of the Biotechnology and Biological Sciences Research Council