One of the major determinants for litter size in pigs is prenatal mortality. It occurs most frequently during the first few weeks of gestation and can be attributed to abnormalities in developmental processes during embryogenesis including trophoblastic elongation and blastocyst implantation. Improvement of litter size has been attempted by means of phenotypic selection. However, another promising approach in pursuit of this aim has been the use of genotypic information. Reproductive traits in general are well-suited for application of marker-assisted selection (MAS). The possibility of exerting selection criteria at the molecular level shortens the generation interval as the selection decision can take place early in the life of an animal. Moreover, in consideration of the sex-limited nature of reproductive traits, genotypic information allows for selection in the gender in which the trait cannot be directly observed. Accordingly, there has been considerable interest in mapping and identifying genes involved in the regulation of reproductive traits and in elucidating their expression patterns. This review offers a comprehensive, if not exhaustive, account of the efforts being made and the approaches currently used in this field. One approach has been to choose candidate genes a priori because of the physiological importance of the proteins they encode and their role in the reproduction of other mammals. The usefulness of candidate genes is then examined by association studies between genetic polymorphisms identified in the respective candidate genes and the phenotypic reproductive traits. The other approach discussed uses pre-existing or designed families for linkage analyses in order to map the location of quantitative trait loci (QTL) for the reproductive trait of interest. The results reported were not consistent among different studies but the QTL regions detected may be useful for identification of positional candidate genes in further molecular genetic studies. However, a better understanding of porcine reproduction requires that these functional genomic approaches are merged and integrated with detailed analyses of the proteome to establish linkages between predisposition and physiology.
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