Limited oxygen availability in utero may constrain the evolution of live-birth in reptiles

Although viviparity (live birth) has evolved from oviparity (egg laying) at least 140 times in vertebrates, nearly 120 of these independent events occurred within a single reptile taxon. Surprisingly, only squamate reptiles (lizards and snakes) are capable of facilitating embryonic development to increasingly advanced stages inside the mother during extended periods of oviducal egg retention. Viviparity has never evolved in turtle lineages, presumably because embryos enter and remain in an arrested state until after eggs are laid, regardless of the duration of egg retention. Until now, the limiting factor that initiates and maintains developmental arrest has remained elusive. Here, we show that oviducal hypoxia arrests embryonic development. We demonstrate that hypoxia can maintain developmental arrest after oviposition and that subsequent exposure of arrested embryos to normoxia triggers resumption of their development. We discovered remarkably low oxygen partial pressure in the oviduc...

尽管胎生（viviparity）从卵生（oviparity）演化而来的事件在脊椎动物中至少发生了140次，但其中近120次独立演化事件都集中在单个爬行动物类群中。令人意外的是，仅有有鳞目爬行动物（squamate reptiles，蜥蜴与蛇类）能够在输卵管内卵滞留的延长周期中，支持胚胎在母体内发育至愈发成熟的阶段。龟类支系从未演化出胎生性状，推测其原因为无论卵滞留时长如何，胚胎都会进入并维持停滞状态，直至卵被产出。此前，启动并维持胚胎发育停滞的限制性因素始终未被探明。本研究证实，输卵管内低氧（hypoxia）可引发胚胎发育停滞。我们证明，低氧能够在卵产出后维持胚胎发育停滞状态，而停滞的胚胎后续暴露于常氧（normoxia）环境时，则会触发发育重启。我们在输卵管内检测到了极低的氧分压（oxygen partial pressure）……