STEWART, J.R.*; THOMPSON, M.B.; East Tennessee State Univ., Johnson City; Univ. of Sydney, Australia: Similarity and Novelty in the Evolution of Reptilian Placentation

The placenta of squamate reptiles, which has evolved independently in more than 100 lineages, is a composite structure that arises as a functional interaction between the maternal uterus and extraembryonic membranes. The magnitude of parallel evolutionary transformations among squamates provides an unusual opportunity to study the evolution of independently derived similarity. We compared extraembryonic membrane and placental development of two squamate lineages, thamnophiine snakes and Pseudemoia lizards, in the context of recently published hypotheses of phylogenetic relationships. Although placentation evolved independently in these two lineages, each placental type, i.e., chorioallantoic or yolk sac, is probably functionally similar across lineages. Independently derived structural similarity is apparent in comparison of chorioallantoic placentae, but there are also novel structural differences between lineages in both chorioallantoic and yolk sac placentation. Smooth chorioallantoic placentae are homoplasies that evolved in parallel, likely to facilitate respiratory exchange. Histological and cytological evidence suggests that the yolk sac placenta of each lineage is specialized for histotrophic nutrient transfer, yet composition of these structures differs because of variation between ancestors. In addition, the omphalopleure that contributes to yolk sac placentation persists to later embryonic stages compared to oviparous outgroups, but the two lineages have evolved different structures that prevent replacement of the omphalopleure by the allantois. Each lineage has also evolved unique structural specializations of the chorioallantoic placenta.