| During the evolutionary transition from egg laying (oviparity) to live bearing (viviparity), several changes in maternal physiology must occur to support the developing fetus. The evolution of a placenta provides an interface between the mother and fetus, allowing exchange of gases, nutrients, and wastes. Because the diffusion-maximizing structure of the placenta also permits exchange of steroid hormones, species with relatively recently evolved placentation, such as many reptiles, may face the additional challenge of regulating maternal and fetal hormone levels. This dissertation investigated the endocrinology of the simple chorioallantoic placenta of the viviparous lizard, Sceloporus jarrovi, and examined its evolutionary significance.;In one experiment, maternal progesterone was elevated 100 fold using implants. The resulting increase in fetal and yolk progesterone was only about 2% of that in mothers, indicating that the placenta buffers transplacental progesterone diffusion. However, the small amount of maternal progesterone that did reach the fetal circulation had significant, sex-specific effects: progesterone increased more strongly in female than male fetuses, and fetal male testosterone was reduced to female-equivalent levels. These effects support the hypothesis that disruption of endocrine regulation is a significant problem during the evolution of viviparity.;A second experiment examined the relationship between maternal and fetal hormone levels in S. jarrovi versus its oviparous congener, S. graciosus. No clear relationship was identified for S. graciosus. However, consistent with findings from the first experiment, the data for S. jarrovi suggest that maternal hormones can inhibit fetal hormone secretion.;In vitro tissue incubation experiments showed that placental enzymes quickly metabolize progesterone and corticosterone, providing a possible mechanism for placental buffering of maternal-fetal hormone transfer. Placentae also rapidly synthesized progesterone, indicating that this tissue could be a source of circulating progesterone in vivo. Because progesterone and many of its metabolites influence uterine contractility, the balance between progesterone synthesis and conversion could also affect pregnancy maintenance.;The work presented here shows that, despite its rudimentary structure, the S. jarrovi placenta's endocrine role is well developed, potentially influencing sex-specific fetal development and uterine contractility. These findings suggest that reptilian placentae could have a more significant endocrine function than previously recognized. |
