PLASMA-IMMUNOREACTIVE INHIBIN AND FSH IN PREPUBERTAL ASSAF AND BOOROOLA-ASSAF EWE LAMBS

Plasma immunoreactive inhibin (i-inhibin) was measured in Assaf and Bo oroola-Assaf (1/2 Booroola-1/2 Assaf) ewe lambs between the ages of 2 and 15 weeks and correlated with plasma FSH. 1-inhibin concentrations were similar in both genotypes and was maximal at 2 weeks of life and decreased thereafter. These levels (mean +/- s.e.) were 1.28 +/- 0.12 and 1.14 +/- 0.10 ng/ml at 2 weeks and 0.55 +/- 0.06 and 0.45 +/- 0.05 ng/ml at 15 weeks of age in Assaf (n=13) and Booroola-Assaf (n=13) re spectively. In Assaf ewes an inverse correlation between plasma inhibi n and FSH was observed at 4 and 15 weeks but not at 2 weeks of age. In Booroola-Assaf ewes, no significant correlation was found at any age studied. At 4 weeks of age, Assaf and Booroola-Assaf ewe lambs had ver y different ovarian morphology. Assaf lambs had large ovaries with a m ean diameter of 1.00 +/- 0.09 cm (mean +/- s.e) and many preantral and antral follicles present, while Booroola-Assaf had much smaller ovari es (0.66 +/- 0.03 cm, p < 0.05) with only a few antral follicles. It s eems likely that the Fec(B) allele from the Booroola Merino influenced the structure of the ovary although the contribution of other Boorool a genes cannot be ruled out. In summary, the plasma i-inhibin concentr ations decrease between 2 and 15 weeks of age irrespective of genotype . It is speculated that in prepubertal ewe lambs the ratio of bioactiv e to immunoreactive inhibin varies with age and between the genotypes with different ovarian development.