Effects of the Booroola Fec gene on ovarian follicular populations in superovulated Romanov ewes pretreated with a GnRH antagonist

Endocrine control of follicular growth was studied in mature Romanov ewes carrying (RF+) or not carrying (R+2) the Booroola Fec gene during an oestrous cycle after gonadotrophin-dependent follicles were suppressed by treatment with an antagonist of GnRH (Antarelix, 0.5 mg per day) and superovulatory treatment was administered. The left ovary was removed after 10 days of treatment (saline or Antarelix) and the right ovary was removed at the end of the superovulatory treatment. Ewes of both genotypes treated with Antarelix had lower plasma LH concentrations than did controls from day 0 to day 10. The inhibitory effect of Antarelix on LH concentration increased with day of treatment. The variability in FSH concentrations during the initial 10 days was reduced by Antarelix treatment in both genotypes. Plasma FSH concentrations were higher in RF+ ewes than in R+2 ewes. In both genotypes, FSH concentrations varied significantly with day of treatment, with the lowest concentrations at day 8 and the highest concentrations at day 5. RF+ ewes had a greater total and atretic number of antral follicles 0.62-1.12, 1.12-2.00 and 2.00-3.00 mm in diameter (classes 2, 3 and 4) than did R+2 ewes before and after superovulatory treatment. After superovulatory treatment, the total number of atretic and non-atretic follicles > 3.00 mm in diameter (class 5) increased in both genotypes. Superovulatory treatment also increased the number of total and atretic class 4 follicles in RF+ only. Conversely, superovulatory treatment decreased the mean number of class 3 follicles in both genotypes, while the number of atretic follicles was decreased only in R+2 ewes. Antarelix treatment significantly reduced the percentage of follicles > 2.00 mm in diameter in RF+ but not in R+2 ewes. Antarelix treatment before superovulatory treatment increased the total number of class 4 follicles in both genotypes but the increase was more significant in RF+ than in R+2 ewes. These results indicate that Antarelix pretreatment favours a greater superovulatory response in Romanov ewes carrying the Fec gene because ovulatory follicles are recruited from a wider range of follicular size classes.