Morphological variation in the allantoplacenta within the genus Mabuya (squamata: Scincidae)

The type IV allantoplacenta has been described for the New World tropical scincids lizards of the genus Mabuya; it possesses the greatest morphological complexity known among viviparous squamates. Although a common morphological pattern has been observed in the few species of this lineage in which the allantoplacental morphology has been studied, some morphological variations may be present among species and populations. Here, we report morphological variation of the allantoplacenta of twelve populations of the genus Mabuya distributed in different geographical areas in northern South America using light microscopy. It is found that all the populations/species conserve a general arrangement of the placental structures. In the embryonic hemisphere there are a placentome, paraplacentome, and chorionic areolas; these structures are related to histotrophic nutrition. At the abembryonic hemisphere, there are absorptive plaques for histotrophic transfer and respiratory segments for gas exchange. However, in some populations some distinctive features in the placentome were found. The presence in the uterine syncytium of non syncytialized columnar cell groups, and invasive cells and apical projections of the chorionic cells directed toward the uterine syncytium, constitute a localized endotheliochorial placenta. Likewise, variations found in the abembryonic region include a greater morphological complexity, such as the folded and delimited absorptive plaques, and highly folded regions at the abembryonic pole (folded respiratory segments integrated with folded absorptive plaques). These specializations allow a larger surface for the passage of nutrients and respiratory exchange. Replication and the regionalized differentiation of the absorptive plaques were probably instrumental in the emergence of specialized structures for nutrient transport such as the placentome and the different types of absorptive plaques. These developmental processes appear to underlie the evolution of the placental complexity within thegenus Mabuya by the morphological variation of serial homologous structures.