Ovarian morphology and endocrine characteristics of female sheep fetuses that are heterozygous or homozygous for the inverdale prolificacy gene (fecX')

Abstract

The Inverdale gene (fecX'), located on the X chromosome, is a major gene affecting the ovulation rate of sheep. At each ovulation, ewes heterozygous (I+) for the fecX' gene ovulate, on average, one more egg than noncarriers (++), whereas ewes that are homozygous (II) for this gene are infertile and have 'streak' ovaries. Since formation of the ovary occurs in fetal life, it is possible that the fecX' gene influences ovarian development before birth. The aims of this study were to examine the effects of the fecX' gene on germ cell development, follicular formation and growth, and plasma gonadotropin concentrations at 5 different days of gestation (i.e., Days 40, 90, 105, 120, and 135) and also in adult life. The results suggest that one copy of the X- linked mutation in female fetuses leads to a retardation of germ cell development at Days 40 and 90 of gestation. However, from Day 105 of gestation, follicular formation and growth appear normal. By contrast, in females with two copies of the X-linked mutation, germ cell development and follicular formation appear normal, but thereafter follicular growth from the primary stage of development is impaired. During fetal life the plasma concentrations of FSH and LH, although not measurable at Day 40, were similar between all the genotypes at Day 105, 120, and 135 of gestation. The only exception was for LH at Day 90 in the I+ and II animals: in ewes with these genotypes the plasma concentrations of LH were similar but significantly lower (p < 0.01) than in the ++ genotype. In adult animals the plasma concentrations of FSH and LH were not different between the ++ and I+ genotypes, reflecting similar levels of ovarian follicular activity. However, in adult II animals, the plasma concentrations of FSH and LH were significantly higher (both p < 0.01) than in the ++ and I+ genotypes, reflecting the absence of normal secondary and antral follicles. In summary, these data show that the fecX' gene affects ovarian development before birth and that the nature of the effect is influenced by whether the female fetus is a homozygous or heterozygous carrier of the X-linked mutation.