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Compared with amphibians, oviparous reptiles and birds lay large eggs that contain abundant yolk. Because the yolk is extracellular, it must be taken up by cells of the yolk sac and metabolized so that products of yolk digestion can be transported to the embryo to fuel development. In birds, the yolk is processed by cells that line the inside of the yolk sac. A very different developmental pattern recently has been demonstrated in lizards and snakes in which the yolk sac cavity is converted to a compact mass of blood vessels lined by endodermal cells. In this study, we used scanning electron microscopy to determine which of these developmental patterns (if either) occurs in a representative chelonian, the North American Snapping Turtle (Chelydra serpentina (Linnaeus, 1758)). Our observations reveal that yolk-filled endodermal cells progressively fill the yolk sac cavity. These cells become organized around anastomosing blood vessels, forming elongated strands that are morphologically well suited for yolk digestion and vascular transport of nutrients. This developmental pattern shares features with that of squamates, but it differs markedly from that of birds. These observations indicate that mechanisms of yolk processing in lizards and snakes have an evolutionary history that pre-dates the squamate clade. Key words: development, embryology, morphology, oviparity, turtles, Snapping Turtle, Chelydra serpentina. Comparativement aux amphibiens, les reptiles et oiseaux ovipares pondent des gros oeufs contenant un vitellus abondant. Parce que ce dernier est extracellulaire, il doit etre integre par les cellules du sac vitellin et metabolise pour que les produits de la digestion du vitellus puissent etre transportes a l'embryon pour lui fournir de l'energie pour son developpement. Chez les oiseaux, le vitellus est transforme par des cellules qui tapissent l'interieur du sac vitellin. La demonstration d'un motif de developpement tres different a recemment ete faite pour les lezards et serpents, chez lesquels la cavite du sac vitellin est convertie en une masse compacte de vaisseaux sanguins tapisses de cellules endodermiques. Nous utilisons la microscopie electronique a balayage pour determiner lequel de ces motifs de developpement (le cas echeant) se produit chez un chelonien representatif, la tortue serpentine (Chelydra serpentina (Linnaeus, 1758)). Nos observations revelent que des cellules endodermiques remplies de vitellus remplissent graduellement la cavite du sac vitellin. Ces cellules s'organisent autour de vaisseaux sanguins anastomoses, formant des brins allonges se pretant bien, sur le plan morphologique, a la digestion du vitellus et au transport vasculaire de nutriments. Ce motif de developpement partage des elements avec celui des squamates, mais est sensiblement different de celui des oiseaux. Ces observations indiquent que les mecanismes de transformation du vitellus chez les lezards et serpents ont evolue avant l'apparition du clade des squamates. [Traduit par la Redaction] Mots-cles : developpement, embryologie, morphologie, oviparite, tortues, tortue serpentine, Chelydra serpentina., Introduction Developmental and reproductive morphology of reptiles are subjects of active research by biologists seeking to answer fundamental questions about function and evolution. Most published work in this area has [...]

Historically, an understanding of viviparity and its evolution in Old World chameleons (Chamaeleonidae) has lagged behind that of other squamate families. Not only is reproductive information scarce or entirely absent for most chameleon species, but the literature reveals no consensus as to the frequency and ecological circumstances under which chameleon viviparity evolved. We integrated information on reproductive modes for nearly all chameleon species with recently published family‐scale phylogenetic and ecological analyses to clarify aspects of reproductive evolution in chameleons. Ancestral‐trait reconstructions, after accounting for phylogenetic uncertainty, indicated that viviparity has arisen a minimum of three times in Chamaeleonidae, with each origin of live birth in closed‐canopy forests. Our maximum‐likelihood optimization therefore did not support the previous hypotheses of one, two or four origins of viviparity in the family. Past claims that arboreality would not allow for evolution of viviparity were also not supported, nor was a recent suggestion that viviparity has reverted to oviparity. However, cold climates of high latitudes and elevations may have selected for viviparity in arboreal chameleons. While peritoneal pigmentation may facilitate viviparity, its role as an exaptation rather than an adaptation remains equivocal without data from a wider range of chameleon species. Based on a comprehensive review of reproductive modes throughout the family, our study has resolved the number of origins of viviparity in Chamaeleonidae and provided evidence that live birth evolved under arboreal conditions on three separate occasions in this enigmatic squamate group. This study also reveals the value of using phylogenetic analysis in a manner that is robust to uncertainty (rather than simple correlational approaches) when the goal is to reconstruct evolutionary sequences and selective pressures. [ABSTRACT FROM AUTHOR]