Mapping quantitative trait loci for reproduction in pigs.

Abstract

Reproductive performance is a critical component of sustainable animal production systems. The low heritability of reproductive performance traits such as litter size, ovulation rate, and prenatal survival and their expression only in females limits improvement of these traits through traditional selective breeding programs. However, there is abundant evidence of genetic variation in these traits between pig breeds, which could be exploited to improve reproductive performance through selective breeding. The Chinese Meishan breed is one of the most prolific pig breeds known, displaying greater litter size than commercial Western breeds, such as Large White, through higher levels of prenatal survival for a given ovulation rate. But Meishan pigs have poor growth rates and high carcass fat content. However, increasing the number of viable and productive offspring per reproductive female reduces financial and environmental costs and improves the sustainability of the system. Thus, the superior Meishan alleles for reproduction traits are potentially commercially valuable. As only a fraction of the genes / loci that underpin the Meishan's superior reproductive performance have been identified to date, it is evident that the genetics of reproductive performance merits further investigation. In an earlier study we mapped a QTL (quantitative trait loci) with effects on embryo survival and litter size to the distal end of pig chromosome 8 (King et al. 2003). The objective of this study is to identify QTL affecting ovulation rate, teat number, litter size, number born alive and embryo survival, and characterize candidate gene(s) underlying such QTL. Our strategy to identify genetic markers for reproduction traits combines identifying QTL (regions of the genome linked to the phenotypes) through genome scans using interval mapping and testing genes recognized as candidates on both positional and physiological grounds. The QTL analyses involve testing for associations between variation in the trait(s) of interest and the inheritance of chromosomal segments from the parental animals. The inheritance of chromosomal segments through the QTL mapping population is tracked by genotyping the population for polymorphic genetic markers — microsatellites and single nucleotide polymorphisms (SNP5). The three-generation Roslin Institute Meishan x Large White F2 QTL mapping population was genotyped for ten additional markers across the QTL found previously on chromosome 8 and for 127 markers evenly spaced across the rest of the genome. The marker genotypes and trait data were lodged in the resSpecies database (www.resSpecies.org). Linkage maps were constructed using Multimap and Crimap (Green et al. 1990) and the resulting maps checked for anomalous double recombinants with the chrompic function. Anomalous genotypes were checked and corrected or omitted from the analysis. The marker orders in the linkage map exhibited good agreement with international reference linkage maps. QTL analyses were performed using the "fixed QTL allele" model on the GridQTL portal (www. gridqtl.org.uk/) (Seaton et al. 2002).