Genes controlling ovulation rate in sheep.

Sheep provide a valuable model for studying the genetic control of ovulation rate. Recent progress includes the identification of mutations in BMP15 (bone morphogenetic protein 15) that increase ovulation rate in heterozygous carriers and block follicular development in homozygous carriers. The genes characterized to date appear to act principally within the ovary and result in earlier maturity of granulosa cells and reduced follicular size. There may also be other sites of action, and increased FSH concentrations appear to be important in the expression of the FecB phenotype. A new locus on the X chromosome in New Zealand Coopworth sheep increases ovulation rate by about 0.4 and is maternally imprinted. Results from studies in the Cambridge and Belclare breeds indicate that further genes remain to be characterized. Finding the first mutations leading directly to variation in ovulation rate is likely to speed up the identification and molecular analysis of these other genes. There is still much to learn about follicular development and the control of litter size from genetic models in sheep.