A review on candidate genes associated with sheep fertility traits: Implications for genetic improvement of indigenous sheep breeds in developing countries

Reproductive traits of sheep are under the control of both the environment and genetics. Moreover, most of these traits have low heritability, are sex-limited, and expressed later in the animal’s life, which makes it difficult to quantify in young animals and to apply direct selection to the traits. This calls for the need to identify and include the genetic bases of fertility traits in selection decisions to enhance the performance of fertility traits in sheep breeding programs. Therefore, several polymorphisms in many genes associated with fertility traits have been reported in several sheep breeds worldwide. The bone morphogenetic protein 15 (BMP15), bone morphogenetic protein receptor 1B (BMPR1B), and growth differentiation factor 9 (GDF9) genes are the major fecundity genes investigated in several sheep breeds studied. Polymorphisms on these genes have shown significant associations with fertility traits such as ovulation rate and litter size and have been used in selection programs. However, research findings focusing on candidate genes affecting fertility traits in indigenous sheep breeds from developing countries are lacking. This review has shown that the genes and the polymorphisms on the genes are variable between and within breeds of the sheep explored thus far, which calls for further efforts to examine sheep breeds in developing countries.