Major Genes Influencing Prolificacy in Sheep: A Source of Basic Knowledge of Ovarian Function and a Tool for Genetic Improvement

For the meat sheep industry, the main selection criteria are based on growth rate, conformation traits, carcass weight and litter size. For this latter trait, numerous quantitative genetic studies have evidenced the segregation of mutations in major genes increasing prolificacy in various sheep populations worldwide. Indeed, in the last decades, identifying these mutations has become both a source of information for understanding the female reproductive function and a potential tool for genetic improvement. Up to now and by using molecular genetics, expressional and functional biology approaches, fourteen independent causal mutations were identified in only four genes named BMP15, GDF9, BMPR1B and B4GALNT2. These mutations are segregating in about twenty sheep populations originated from Oceania, Europe and Asia. Surprisingly, in most cases, heterozygous status of the mutation leads to an improved prolificacy, whereas, homozygous status of the mutation either leads to sterility or excessive prolificacy. Thus, a particular attention should be paid to avoid homozygous ewe generation in gene-assisted selection programs for prolificacy. Identification of oocyte-derived growth factors: BMP15, GDF9 and membrane receptor BMPR1B as major genes affecting female fertility in sheep has largely improved the knowledge on mammalian ovarian function. Indeed, the Bone Morphogenetic Protein (BMP) signaling system was evidenced as a major biological pathway controlling the ovary from the embryo to the adult and provides valuable candidates to study ovarian pathologies in human. Other prolificacy major genes in sheep are still to discover with novel basic knowledge perspectives and perhaps better relevance for genetic improvement.