Pituitary response to GnRH and ovariectomy in Booroola–Awassi and Awassi ewe lambs

Summary. Booroola–Awassi ewe lambs were heterozygous (F +) for a major gene F, influencing their ovulation rate, while Awassi lambs were non-carriers (++). Basal plasma FSH concentration (mean ± s.e.m.) in Booroola–Awassi ewe lambs at 4 weeks of age was significantly higher than in Awassi lambs of the same age (5·06 ± 0·60 and 2·04 ± 0·32 ng/ml respectively; P < 0·001). After GnRH administration, FSH increased from 3·89 ± 1·10 to 10·58 ± 1·30 ng/ml in Booroola–Awassi (N = 6) and from 1·87 ± 0·29 to 4·64 ± 0·33 ng/ml in Awassi (N = 6) ewe lambs (P < 0·05). Ovariectomy caused an increase in plasma FSH in Booroola–Awassi (N = 4) and Awassi (N = 4) ewe lambs. At 1 week after ovariectomy plasma FSH increased from 5·96 ± 1·02 to 7·06 ± 1·05 ng/ml in F+ and from 1·67 ± 1·06 to 5·21 ± 0·66 ng/ml in ++ ewe lambs, suggesting a stronger negative feed-back effect exerted by the ovaries of Awassi lambs. At 15 weeks after ovariectomy FSH values were similar in Booroola-Awassi (18·28 ± 1·96 ng/ml) and Awassi (16·07 ± 0·70 ng/ml) lambs. Although the overall pattern of pituitary response to ovariectomy was similar in the F + and ++ ewe lambs, Booroola–Awassi lambs had small ovaries (132·5 ± 24·9 mg) and follicular development did not proceed beyond the preantral stage in 3/4 animals, and Awassi lambs had large ovaries (600·0 ± 233·9 mg) (P < 0·05) with many preantral and antral follicles. The present results suggest that (1) the elevated plasma FSH concentration between 3 and 6 weeks of age in Booroola–Awassi compared to Awassi ewe lambs might be due to (a) greater responsiveness of the F + pituitary to GnRH and (b) reduced ovarian suppression of FSH release and (2) the ovaries of Booroola–Awassi lambs, although lacking antral follicles, secrete a factor that controls FSH values. We suggest that differences between the two genotypes can be attributed to the F gene or to other minor genes that came from the Booroola–Merino sires used to create the Booroola–Awassi progeny.