Regulation of vitellogenesis in reptiles

In non-mammalian vertebrates, embryonic nutrition is derived primarily from egg-yolk proteins (vitellogenins) synthesized in the liver and transported via the blood to the oocytes. We have characterized the vitellogenic cycle of two turtles (Chrysemys picta and Sternotherus odoratus). Plasma vitellogenin levels are correlated positively with ovarian growth and with levels of plasma estrogen' during the annual reproductive cycle. Vitellogenesis appears to be an estrogen-specific, dose-dependent response that can be inhibited by progesterone and testosterone. In addition, a pituitary component with properties similar to mammalian growth hormone appears to be an important synergist in the response to estrogen. Steroid-binding properties and intracellular distribution of an estrogen receptor in the turtle liver is reported, and the involvement of this receptor in vitellogenesis is discussed. Comparative studies using Anolis, Nerodia, and Chalcides indicate a similar estrogen-dependent vitellogenic response, but in the latter species the molecular weight of vitellogenin is much less than in other species studied. Molecular reduction and a functional role for other gonadal steroids may be implicated as important evolutionary steps in development of viviparous forms in which dependence upon yolk has been replaced by placentation.