How does the F gene influence ovulation rates in booroola ewes? A 1990 perspective

K.P. McNatty, K.M. Henderson, J.S. Fleming, C.A. Price, I.J. Clarke

Wallaceville Animal Research Centre, MAF Technology, P.O. Box 40063, Upper Hutt, New Zealand

Proceedings of the New Zealand Society of Animal Production. 1990, 50: 135-140

The purpose of this paper is to summarise recent findings on the reproductive physiology of the Booroola ewe, with specific attention to factors influencing ovulation-rate (OR). The ovarian antral follicle populations, incidence of atresia, steroid secretion-rate and maximum OR potential in the different Booroola genotypes (FF, homozygotes; F+, heterozygotes; ++, non-carriers) are the same notwithstanding the fact that FF, F+ and ++ animals normal ovulate >=5, 3-4 and 1-2 follicles respectively during each oestrous cycle. Moreover, the Booroola F gene does not appear to modify the characteristics of the follicle- stimulating hormone (FSH) or luteinizing hormone (LH) receptors, or the gonadotrophin-sensitive components of the cyclic AMP generating system or the steroid or inhibin biosynthetic functions in follicular cells. It seems that the F gene influences follicular growth before antrum formation resulting in fewer granulosa cells at all phases of antral growth and smaller sized follicles at ovulation. In essence, the >=5, 3-4 or 1-2 presumptive preovulatory follicles in FF, F+ or ++ ewes are respectively 2.5-4.5 mm, 4-5mm and >5 mm in diameter just before ovulation but the total populations of oestrogen-secreting follicular or progesterone-secreting luteal cells in the different genotypes are the same. These genotypic differences in ovarian characteristics have not bee replicated by any of the known superovulation regimes.

At the level of the hypothalamus and pituitary no gene-associated differences have been noted in the concentrations of gonadotrophin- releasing hormone (GnRH) in the tissues or hypophyseal portal blood nor in the binding characteristics of GnRH to the pituitary gland. However F gene-specific differences have been noted in the plasma concentrations of FSH and LH in ovary-intact and ovariectomized (OVX) Booroola ewes (FF>++; P,0.05). The F gene-specific differences in FSH but not LH in OVX ewes can be replicated by exogenous GnRH treatment of OVX ewes deficient in endogenous GnRH following surgical disconnection of the hypothalamus from the pituitary gland but with the superior hypophyseal arteries being left intact. The greater frequency of high FSH values in ewes is also observed in both intact and castrate Booroola FF rams relative to ++ rams at P<=0.07. When considered altogether the evidence suggests that Booroola sheep with the F gene have higher mean FSH concentrations in plasma relative to ++ sheep. However the differences are small with considerable overlap between animals of the different genotypes. It therefore remains to be established whether modest increases in plasma FSH concentrations in ++ Booroola ewes are sufficient to modify follicular morphology and OR without altering steroid secretion.

Keywords: NZSAPAB; Ovarian follicles; pituitary, hypothalamus; granulosa cells; FSH; LH; sheep; Booroola gene

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