MOLECULAR APPROACHES TO IMPROVED PIG FERTILITY

Geneticists have made limited progress in improving reproductive trait s in pigs through traditional selection and crossbreeding systems. Rec ent advances in molecular genetics and the human genome project have a llowed progress in gene identification and gene mapping in pigs. The p ig genetic linkage map now has over 1700 genetic markers, and about 20 0 of these are genes. Furthermore, comparative genomic maps are improv ing and are becoming useful tools. Molecular approaches such as candid ate gene identification and genomic scans permit new genes and chromos omal regions to be discovered which may influence reproduction. One si gnificant finding is that the oestrogen receptor locus (ESR) is associ ated with increased litter size. Differences between female Chinese pi g ESR BB and AA homozygotes have averaged 2.3 pigs born alive for firs t parity, and In commercial Large White crosses the differences are ab out 0.9 pigs per litter. Additional new genes have been discovered whi ch may significantly affect Litter size. Initial genome scans have rev ealed that there may be a gene or genes influencing ovulation rate and litter size on chromosome 8. Approaches such as positional comparativ e candidate gene analysis and eventually positional cloning will proba bly identify the genes controlling reproduction in pigs.