Genetic aspects of reproduction in sheep.

Maintenance of high levels of realized fertility (defined as the percentage of ewes that lamb) and appropriate levels of fecundity are critical for efficient sheep production. The optimal level of fecundity in most situations is well below the maximum attainable level and can be targeted by combining selection among and within breeds with use of an expanding array of single-gene mutations affecting ovulation rate and litter size. The heritability of litter size is approximately 0.10, allowing changes of up to 2%/year from simple mass selection. Mutations in several genes associated with the transforming growth factor beta superfamily (BMPRIB, GDF9 and sex-linked BMP15) can increase ovulation rates by 0.7-1.5 ova in heterozygous ewes. However, ewes that are homozygous for BMP15 or GDF9 mutations are sterile, so use of these mutations requires carefully structured breeding programmes. Improvements in fertility may be critical for autumn lambing or programmes that aspire to lamb throughout the year. Selection to improve fertility in spring matings has been successful; selected adult ewes have lambing rates of 80-85% in October and early November. The selected ewes have a dramatically reduced seasonal anestrus, and many ewes continue to cycle during spring and summer. Major genes affecting seasonal breeding have not been identified in sheep. Polymorphisms in the melatonin receptor 1a gene appear to be associated with seasonal breeding in some, but not all breeds. However, functional genomic studies of genes associated with circadian and circannual rhythms have potential to reveal additional candidate genes involved in seasonal breeding.