School of Biological Sciences, University of Sydney, A08 Heydon-Laurence Building, Sydney, New South Wales 2006, Australia.
Reproduction. 2013 Nov 20;147(1):R15-26. doi: 10.1530/REP-13-0309. Print 2014 Jan.
Squamate reptiles (lizards and snakes) are an ideal model system for testing hypotheses regarding the evolution of viviparity (live birth) in amniote vertebrates. Viviparity has evolved over 100 times in squamates, resulting in major changes in reproductive physiology. At a minimum, all viviparous squamates exhibit placentae formed by the appositions of maternal and embryonic tissues, which are homologous in origin with the tissues that form the placenta in therian mammals. These placentae facilitate adhesion of the conceptus to the uterus as well as exchange of oxygen, carbon dioxide, water, sodium, and calcium. However, most viviparous squamates continue to rely on yolk for nearly all of their organic nutrition. In contrast, some species, which rely on the placenta for at least a portion of organic nutrition, exhibit complex placental specializations associated with the transport of amino acids and fatty acids. Some viviparous squamates also exhibit reduced immunocompetence during pregnancy, which could be the result of immunosuppression to protect developing embryos. Recent molecular studies using both candidate-gene and next-generation sequencing approaches have suggested that at least some of the genes and gene families underlying these phenomena play similar roles in the uterus and placenta of viviparous mammals and squamates. Therefore, studies of the evolution of viviparity in squamates should inform hypotheses of the evolution of viviparity in all amniotes, including mammals.
蜥蜴和蛇类等有鳞目爬行动物是检验有关羊膜动物中胎生（活体生产）进化假说的理想模式生物。胎生在有鳞目动物中已进化了超过 100 次，导致生殖生理学发生重大变化。至少所有胎生的有鳞目动物都表现出由母体和胚胎组织贴合形成的胎盘，这些胎盘与形成真兽类哺乳动物胎盘的组织同源。这些胎盘有助于胚胎附着在子宫上，并交换氧气、二氧化碳、水、钠和钙。然而，大多数胎生的有鳞目动物仍然几乎完全依赖蛋黄获取有机营养。相比之下，一些物种至少部分依靠胎盘获取有机营养，表现出与氨基酸和脂肪酸运输相关的复杂胎盘特化。一些胎生的有鳞目动物在怀孕期间也表现出免疫能力降低，这可能是为了保护发育中的胚胎而进行免疫抑制的结果。最近使用候选基因和下一代测序方法的分子研究表明，至少有一些基因和基因家族在胎生哺乳动物和有鳞目动物的子宫和胎盘中有类似的作用。因此，对有鳞目动物胎生进化的研究应该为所有羊膜动物（包括哺乳动物）胎生进化的假说提供信息。
