Effect of 17β-estradiol and progesterone on vitellogenesis in the spotted ray Torpedo marmorata Risso 1810 (Elasmobranchii: Torpediniformes): Studies on females and on estrogen-treated males

The influence of 17β-estradiol (E2) on vertebrate vitellogenesis is well ascertained. The aim of the present paper is to study the involvement of E2 and progesterone (P) in the induction and regulation of vitellogenesis in females and experimental E2-treated males of Torpedo marmorata. We analyzed females in various stages of the reproductive cycle and E2 experimentally treated males. The presence of vitellogenin was investigated in the plasma and in the liver by western blot and immunohistochemistry; its site of synthesis was investigated by in situ hybridization. The steroid levels in the plasma were measured by Enzyme Immunoassay. In treated males, E2 induces in the liver the synthesis of VTG which is then secreted into the bloodstream as a 205-kDa polypeptide, the same that is found in the plasma of non-pregnant vitellogenic females. In females, E2 is naturally present in the plasma and its level is correlated with VTG synthesis in the liver and with the female reproductive cycle. Indeed, large amounts of E2 are only found in mature vitellogenic females, whose liver is involved in VTG synthesis and secretion. By contrast, small amounts of E2 are evident in juveniles whose ovaries are lacking in vitellogenic follicles and in females preparing for ovulation. Low titers are also found in gravid females, whose liver is not engaged in VTG synthesis. We show that P, which is absent in untreated males and juvenile females, is evident in the blood serum of E2-treated males and sexually mature females. Interestingly, in treated males P appears in the plasma just 24 h after the first injection of E2 and its titer increases; a week after the last injections, the P level is similar to that recorded in non-gravid vitellogenic females. Finally, it is noteworthy that the highest titer of P was recorded in pregnant females. We demonstrate that in Torpedo vitellogenin synthesis, as in other vertebrates, is under the control of E2 but also that this synthesis is probably under the control of progesterone.