|dc.description.abstract||In contrast to advanced stages of oogenesis, the endocrine regulation of early stages of oocyte growth has received little attention. In oviparous vertebrates, particularly in teleost fish, the early stages of oogenesis are determinants of oocyte quality and fecundity. In advanced stages of the reproductive cycle, the existence of reciprocal relationships between the somatotropic axis and the reproductive axis has been well-documented in vertebrate models including several teleost species. In this context, insulin-like growth factor1, Igf1, has been identified as a mediating factor between growth and reproduction that promotes gonadal sex steroid synthesis. Stimulatory effects of Igf1 and an androgen, 11-ketotestosterone (11KT), have been recently reported on oocyte diameter from New Zealand shortfinned eels (Anguilla australis) during early stages of oogenesis, known as previtellogenesis, in vitro. However, no more information is available regarding the insulin-like growth factor system and its relationship with sex steroids in the eel during previtellogenesis. The present study, therefore, aimed to gain insight into the endocrine mechanisms associated with the role of Igf1 during the previtellogenic stage of oocyte growth in the New Zealand shortfinned eel.
I examined the molecular evolution and specific cellular expression of key members of the Igf system (Igf1, Igf2, Igf type1 receptor) in the shortfinned eel and showed that the primary structure of the Igf system members is highly conserved in the shortfineed eel, as in other teleosts (Chapter 2). For the first time, the existence of a new paralogous gene for igf1, i.e., insulin-like growth factor1 b, was reported in this study. Regarding the notion that eel is amongst the basal teleost fish, the observation of several duplicated genes, belonging to the Igf system in this study, provided further support in favour of the occurrence of genome duplication at the base of teleost fish radiation. Chapter 3 of this thesis examined the relationship between previtellogenic oocyte diameter and transcript abundances of intraovarian and extraovarian candidate genes belonging to the metabolic and reproductive axes. The results revealed a significant relationship between intraovarian components of the growth-metabolic axis (i.e., growth hormone receptor and Igf type1 receptor) and 11KT, with oocyte diameter. Increasing ovarian sensitivity to metabolic signals with advancing previtellogenic stage is thus likely. Furthermore, the level of 11KT, intraovarian growth differentiation factor9 (gdf9) and Igf type1 receptor were highlighted as predictors of oocyte diameter during previtellogenesis. To further evaluate the importance of Igf1 during previtellogenesis, I therefore assessed effects of Igf1 on ovarian steroidogenesis, in vitro and in vivo in Chapter 4. A stimulatory effect of Igf1 on mRNA abundance of ovarian steroidogenic proteins (P450 side chain cleavage and P450aromatase) was observed. Thereafter (Chapter 5), the effects of 11KT on regulation of the ovarian Igf system were examined. My observations did not detect a significant effect of 11KT on intraovarian igf1 or igf type1 receptor mRNA abundance, but the androgen significantly increased the abundance of mRNA for hepatic insulin-like growth factor mRNA binding protein 3.
In conclusion (Chapter 6), the results of this thesis suggest that Igf1, intraovarian growth factors (e.g., Gdf9) and 11KT are part of the regulatory mechanisms involved in controlling previtellogenic oocyte growth in teleost fish, or at least in the shortfinned eel.||