055. EVEN REPTILES DO IT, THE STRUCTURE AND FUNCTION OF PLACENTAE IN VIVIPAROUS LIZARDS

Abstract

Live birth (viviparity) has evolved independently more than 100 times in squamate reptiles (lizards and snakes). Most viviparous lineages are characterised by a simple placenta with squamous epithelia on both maternal and embryonic sides, and the remnants of an eggshell persist for some, if not all, of pregnancy. The embryos are predominantly lecithotrophic, with maternal-embryonic exchanges being limited mostly to inorganic ions, water and respiratory gases. Nevertheless, there is differentiation of a chorioallantoic placenta and a yolk sac (omphalo) placenta in all species. Complex placentae have evolved in the Squamata in only four or five lineages, all in the lizard family Scincidae (skinks). In species with complex placentation, the uterus differentiates to allow different functions in association with each embryonic membrane type. Species with complex placentae are characterised by a hypertrophy of maternal and embryonic cells, elaboration of the maternal surface, a reduction in yolk with a concomitant increase in placentotrophy and, in some, regional differentiation of the choriallantois into a placentome and a paraplacentome. Both the placentome and omphaloplacenta are organs of embryonic nutrition, but they transport nutrients by different mechanisms; the paraplacentome is a specialised gas exchange organ. The most placentotrophic species are in the South American genus Mabuya, where the females produce micro-lecithal eggs and the placenta is more complex than in any mammal, with four specialised structures for nutrient exchange and one for gas exchange. The number of independent origins of viviparity and the range of placental complexities exhibited by skinks enables us to infer the evolutionary trajectories that have resulted in microlecithal eggs and an almost complete reliance of placentotrophy from oviparous ancestors.