Endocrine basis of prolificacy in the Booroola Merino

Recent research confirms that the ovulation rate of the sheep can be increased by short term elevation of plasma follicle stimulating hormone (FSH) concentrations during the early follicular phase. Studies of the Booroola Merino suggest that the F gene probably acts through this pathway. That is, there are gene specific differences in plasma FSH early in the follicular phase. There is no evidence that the ovary of the Booroola is especially sensitive to exogenous FSH. In contrast, increased ovulation rate in response to selection within the Finn breed (in Ireland) cannot be attributed to any changes in plasma FSH concentrations. The elevated plasma FSH in the Booroola female may result from a deficiency of ovarian inhibin, the major feedback regulator of FSH. This has been demonstrated by both bioassay of inhibin in ovarian cytosol and radioimmunoassay of inhibin in peripheral plasma. It is not yet known if this low inhibin is attributable to a derangement of the inhibin genes in the Booroola or to the fact that their follicles are smaller and produce less follicular fluid than those of other sheep. The increased ovulation rate of Merinos following immunization against inhibin confirms the importance of this hormone as a key regulator of ovarian function. At present there are no known effects of the Booroola gene on the gonad of the male or on the hormones that regulate testis function. (author). 14 refs, 4 figs, 6 tabs