Genes controlling ovulation rate in sheep

Sheep provide a valuable model for studying the genetic control of ovulatio n 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 gene s characterized to date appear to act principally within the ovary and resu lt in earlier maturity of granulosa cells and reduced follicular size. Ther e may also be other sites of action, and increased FSH concentrations appea r to be important in the expression of the FecB phenotype. A new locus on t he X chromosome in New Zealand Coopworth sheep increases ovulation rate by about 0.4 and is maternally imprinted. Results from studies in the Cambridg e and Belclare breeds indicate that further genes remain to be characterize d. Finding the first mutations leading directly to variation in ovulation r ate is likely to speed up the identification and molecular analysis of thes e other genes. There is still much to learn about follicular development an d the control of litter size from genetic models in sheep.