Meeting Abstract
Yolk sac placentation is a distinctive characteristic of the evolution of squamate viviparity yet knowledge of the patterns of variation is limited because placental development has been described in relatively few lineages. Gerrhonotine lizards (Family Anguidae) are of interest in exploring placental evolution because viviparity is the predominant reproductive mode within this lineage. We studied placental ontogeny using light microscopy for an embryological series of the Mexican gerrhonotine lizard, Mesaspis viridiflava. The placental interface of this species is dominated by yolk sac placentation. The unusual placental structure is a specialization for maternal-fetal exchange incorporating elements of the yolk cleft/isolated yolk mass complex, a shared, derived trait for squamate reptiles. Although this extraembryonic membrane system is a prominent feature of placental evolution among squamates, the unique attributes of placentation in M. viridiflava are supported by a novel pattern of yolk sac vascular development. A prominent feature of the placenta is a zone of uterine and embryonic epithelial cell hyperplasia located at the upper shoulder of the yolk mass, often extending above the yolk mass. Phylogenetic variation in the yolk sac placenta is likely to reflect variation expressed in oviparous antecedents as well as specializations for maternal – fetal exchange arising subsequent to the evolution of viviparity. The yolk sac placenta of M. viridiflava exemplifies both characteristics. Placental specializations of this species are consistent with a pattern of matrotrophic embryonic nutrition and have evolved in a unique lineage specific developmental pattern.
