Data from: 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 oviducts of gravid turtles and found that secretions produced by the oviduct retard oxygen diffusion. Our results suggest that an extremely hypoxic environment in the oviduct arrests embryonic development and may constrain the evolution of viviparity in turtles, with the reduced diffusive capacity of oviducal secretions possibly creating or contributing to this hypoxia. We anticipate that these findings will allow us to better understand the mechanisms underlying the evolutionary transition between reproductive modes.

尽管胎生（viviparity）从卵生（oviparity）的演化转变在脊椎动物中至少已发生140次，但其中近120次独立演化事件均集中于单一爬行类类群内。令人意外的是，仅有有鳞目爬行动物（squamate reptiles，即蜥蜴与蛇）能够在输卵管内长时间滞留卵的过程中，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，启动并维持这一发育停滞的限制因素始终未被探明。本研究表明，输卵管内的低氧（hypoxia）环境会阻滞胚胎发育。我们证实，低氧可在卵产出后维持胚胎的发育停滞状态，而当停滞的胚胎后续暴露于常氧（normoxia）环境时，会重新启动发育。我们在怀卵龟的输卵管中检测到了极低的氧分压，同时发现输卵管分泌物会延缓氧气扩散。本研究结果提示，输卵管内极端低氧的环境会阻滞胚胎发育，这可能限制了龟类胎生性状的演化；而输卵管分泌物降低的扩散能力，可能是造成或加剧这一低氧环境的原因。我们预期这些研究发现将帮助我们更好地理解生殖模式演化转变背后的机制。