Effects of exogenous FSH on follicular recruitment in a viviparous lizard Niveoscincus metallicus (Scincidae)

In the viviparous skink Niveoscincus metallicus clutch size appears to be determined before vitellogenesis, and is not altered later by follicular atresia or embryonic loss. This suggests that the number of follicles recruited is determined by the endocrine environment early in the vitellogenic period. Through a series of experiments in which we manipulated gonadotropin concentrations by administering exogenous FSH, we aimed to investigate this hypothesis. Pre-vitellogenic females showed no response to exogenous ovine FSH. In early vitellogenic females, FSH induced follicular recruitment: follicles were enlarged and clutch size increased by recruitment of a second cohort of follicles; some females also ovulated. Females treated with FSH in mid-vitellogenesis had elevated mean plasma estradiol concentrations compared to controls; no follicular recruitment was observed, but most of these animals ovulated. Females treated with a range of doses of FSH in late vitellogenesis ovulated at least one month before natural ovulation, again without recruitment of extra follicles. It appears therefore that in Niveoscincus metallicus exogenous FSH can induce recruitment of additional follicles only if administered during early vitellogenesis. We conclude that in this species clutch size is determined by proximate environmental factors influencing gonadotropin levels early in follicular recruitment, and cannot be increased even if conditions become more favourable once vitellogenesis is established.