The comparative approach on how and when vitellogenesis occurs in the diverse reproductive strategies displayed by aquatic and terrestrial lower vertebrates is presented in this chapter; moreover, attention has been paid to the multihormonal control of hepatic vitellogenin synthesis as it is related to seasonal changes and to vitellogenin use by growing oocytes. The hormonal mechanisms regulating vitellogenin synthesis are also considered, and the effects of environmental estrogens on the feminization process in wildlife and humans have been reported. It is then considered how fundamental nonmammalian models appear to be, for vitellogenesis research, addressed to clarifying the yolkless egg and the evolution of eutherian viviparity.

Multihormonal control of vitellogenesis in lower vertebrates.

In this study, the measurement both of peripheral gonadotropins (FSH and LH) and of sex steroids in male and female wild anuran, Rana esculenta, was performed during the annual reproductive cycle; moreover, the role of gonadotropins in the vitellogenic process and in ovarian steroidogenesis was investigated through in vitro experiments. LH plasma changes in males showed high values during autumn-winter months and during the mating period, when high androgen levels were found. Conversely, for the first time in male vertebrates, a clear correspondence between plasma FSH and estradiol-17β (E 2 ) was shown. In females, FSH peak values were found at the beginning of the mating period in parallel with those of plasma vitellogenin (VTG) and E 2 ; in contrast, high LH levels went together with ovarian weight (gonadosomatic index), which is considered a good marker for the plasma sequestration of VTG by growing oocytes. The in vivo results are corroborated by in vitro studies showing the direct effects of both FSH and LH in inducing hepatic VTG synthesis and release in the culture media. Lastly, although it is not yet known whether or not FSH and LH have separate functions in amphibians, it was clearly shown that they induce ovarian steroid production. These results are discussed in terms of the high seasonality previously demonstrated in this wild frog.

Gonadotropins and reproductive function in the anuran amphibian, Rana esculenta.

Effects of captivity stress on plasma steroid levels in the green frog, rana esculenta, during the annual reproductive cycle

Plasma vitellogenin and 17β-estradiol levels during the annual reproductive cycle of Podarcis s. sicula Raf

Sex steroid profile and plasma vitellogenin during the annual reproductive cycle of the crested newt (Triturus carnifex Laur.).

Analyse du developpement ovarien de R. esculenta en fonction de la vitellogenine hepatique induite par les œstradiols et de la capatation de la vitellogenine par les ovaires. Determination du role du processus d'hydratation dans la croissance ovarienne par analyses thermiques

Claudio Novara

Papers

Ovarian development and sex steroid-hormones during the reproductive-cycle of rana-esculenta complex