Genetic basis of prolificacy in Meishan pigs.

Research in France and in the UK confirms the prolificacy of the Chinese Meishan breed to be about three to four piglets greater than that of control Large White females. Crossbreeding studies clearly indicate that this breed difference is due to genes acting in the dam and not in the litter itself. There is high heterosis for litter size in F1 Meishan x Large White crossbred females, such that their litter size is similar to or greater than that of purebred Meishan females. There is some discrepancy between studies about whether the Meishan female has a higher ovulation rate than does the Large White breed and this can be attributed in large part to the different basis upon which breed comparisons have been made. Nevertheless, there may be real genetic differences between Meishan pigs exported to different countries. In young gilts at comparable numbers of oestrous cycles after puberty, the ovulation rate is similar in Meishan and Large White gilts, but in older gilts, and particularly in multiparous sows, Meishan pigs have a higher ovulation rate in British studies. Once comparisons of prenatal survival between breeds have been adjusted for any breed difference in ovulation rate, the main cause of prolificacy in Meishan pigs can be seen to be an enhanced level of prenatal survival. Crossbreeding studies show that this is controlled by the maternal genotype and not that of the embryos. The advantage in prenatal survival to the Meishan pig is clearly present in the post-attachment period (after day 20 of gestation), but may also be present earlier in gestation. Results from a study presented here suggest that Meishan sows have a higher uterine capacity than do Large White sows and this allows them to maintain their higher number of attached embryos through gestation. F1 Meishan x Large White crossbred females achieve their high litter size via a different route than do purebred Meishan females. These animals have a lower ovulation rate and fewer attached embryos than do purebred Meishan sows, but a very low level of fetal loss allows them to produce litters of similar size. The low level of fetal loss in F1 females appears to be due to the higher uterine capacity of F1 females compared with purebred Meishan sows.