Vertebrate vitellogenesis: molecular model for multihormonal control of gene regulation

The stimulation of yolk protein synthesis by estrogen is a characteristic of female non-mammalian vertebrates; in mammals, however, vitellogenesis has been suppressed as a corollary of the evolution of viviparity. It is our hypothesis that progesterone has a dual role in this phylogenetic trend; a) to inhibit myometrial contraction and thus set the stage for internal development of embryos and associated placentation, b) to inhibit yolk protein synthesis as placentation became an efficient direct supply of nutrients to the fetus. We have presented evidence that in the reptiles, the central vertebrate group from which the ancestors of modern mammals evolved, the control of yolk protein synthesis is complex, involving both pituitary and ovarian steroids (estradiol, testosterone and progesterone). This system provides an excellent model for the multihormonal contents of gene regulation involving both + and - controls.