Blackburn Daniel G, Flemming Alexander F
Department of Biology, Trinity College, Hartford, Connecticut 06106, USA.
J Exp Zool B Mol Dev Evol. 2009 Sep 15;312(6):579-89. doi: 10.1002/jez.b.21234.
Current studies on fetal membranes of reptiles are providing insight into three major historical transformations: evolution of the amniote egg, evolution of viviparity, and evolution of placentotrophy. Squamates (lizards and snakes) are ideal for such studies because their fetal membranes sustain embryos in oviparous species and contribute to placentas in viviparous species. Ultrastructure of the fetal membranes in oviparous corn snakes (Pituophis guttatus) shows that the chorioallantois is specialized for gas exchange and the omphalopleure, for water absorption. Transmission and scanning electron microscopic studies of viviparous thamnophine snakes (Thamnophis, Storeria) have revealed morphological specializations for gas exchange and absorption in the intra-uterine environment that represent modifications of features found in oviparous species. Thus, fetal membranes in oviparous species show morphological differentiation for distinct functions that have been recruited and enhanced under viviparous conditions. The ultimate in specialization of fetal membranes is found in viviparous skinks of South America (Mabuya) and Africa (Trachylepis, Eumecia), in which placentotrophy accounts for nearly all of the nutrients for development. Ongoing research on these lizards has revealed morphological specializations of the chorioallantoic placenta through which nutrient transfer is accomplished. In addition, African Trachylepis show an invasive form of implantation, in which uterine epithelium is replaced by invading chorionic cells. Ongoing analysis of these lizards shows how integration of multiple lines of evidence can provide insight into the evolution of developmental and reproductive specializations once thought to be confined to eutherian mammals.
羊膜卵的进化、胎生的进化以及胎盘营养的进化。有鳞目动物（蜥蜴和蛇）是此类研究的理想对象，因为它们的胎膜在卵生物种中维持胚胎发育，在胎生物种中则参与胎盘形成。卵生玉米锦蛇（Pituophis guttatus）胎膜的超微结构显示，绒毛膜尿囊专门用于气体交换，而卵黄囊脏壁层则专门用于水分吸收。对胎生束带蛇属（Thamnophis，Storeria）蛇类的透射电子显微镜和扫描电子显微镜研究揭示了其在子宫内环境中进行气体交换和吸收的形态特化，这些特化是对卵生物种中发现的特征的修饰。因此，卵生物种的胎膜表现出形态分化以实现不同功能，这些功能在胎生条件下得到了招募和增强。胎膜特化的极致见于南美洲（美洲石龙子属）和非洲（喜山蜥属、真美蜥属）的胎生石龙子，在这些物种中，胎盘营养几乎提供了胚胎发育所需的全部营养。对这些蜥蜴正在进行的研究揭示了绒毛膜尿囊胎盘的形态特化，营养物质通过该胎盘进行传递。此外，非洲的喜山蜥表现出一种侵入性着床形式，即子宫上皮被侵入的绒毛膜细胞所取代。对这些蜥蜴的持续分析表明，整合多条证据线索如何能够为曾经被认为仅限于真兽类哺乳动物的发育和生殖特化的进化提供见解。
