Secretion of inhibin A and follicular dynamics throughout the estrous cycle in the sheep with and without the Booroola gene (Fec(B))

The identification of a single gene (Booroola Fec(B)) associated with a significant increase in the ovulation rate in sheep provides a powerful tool for the study of factors regulating the selection of preovulatory follicles. The ovarian secretion of dimeric inhibin A was investigated and related to the secretion of ovarian steroids, the concentration of gonadotropins, and the pattern of ovarian follicular development during the follicular and early luteal phases in ewes with an ovarian autotransplant with or without the Fec(B) gene. The secretion of inhibin A was related to the presence of large estrogenic follicles, being high during the follicular phase and falling after the LH surge (P < 0.05) before increasing during the early luteal phase (P < 0.05). There were no differences in the timing of the onset of the LH surge, the concentrations of FSH and progesterone, or the rates of inhibin A, estradiol, and androstenedione secretion during the follicular or luteal phase between ewes with and without the Booroola gene. Although the overall pattern of follicular development was similar between the genotypes, the ovulation rate was higher (P < 0.05) in the gene carrier ewes, and the ovulatory follicles and corpora lutea were significantly smaller (P < 0.05) than those in noncarrier animals. During the luteal phase, the diameter of the large follicles from the first wave was smaller (P < 0.05) in the gene carrier than in noncarrier ewes. Because there are no qualitative or quantitative differences in the pattern of secretion of pituitary gonadotropins or ovarian hormones between the two genotypes, we conclude that is likely that the Fec(B) gene exerts its action at the level of ovary.