Bonnet Xavier, Naulleau Guy, Shine Richard
Am Nat. 2017 Mar;189(3):333-344. doi: 10.1086/690119. Epub 2017 Jan 24.
The parchment-shelled eggs of squamate reptiles take up substantial water from the nest environment, enabling the conversion of yolk into neonatal tissue and buffering the embryo against the possibility of subsequent dry weather. During development, increasing amounts of water are stored in the embryonic sacs (i.e., membranes around the embryo: amnion, allantois, and chorion). The evolution of viviparity (prolonged uterine retention of developing embryos) means that embryonic-sac fluid storage now imposes a cost (increased maternal burdening), confers less benefit (because the mother buffers fetal water balance), and introduces a potential conflict among uterine siblings (for access to finite water supplies). Our data on nine species of squamate reptiles and published information on three species show that the embryonic-sac fluids comprise around 33% of neonatal mass in viviparous species versus 94% in full-term eggs of oviparous squamates. Data on parturition in 149 vipers (Vipera aspis, a viviparous species) show that larger offspring store more fluids in their fetal sacs and that an increase in litter size is associated with a decrease in fluid-sac mass per offspring. Overall, the evolutionary transition from oviparity to viviparity may have substantially altered selective forces on offspring packaging and created competition among offspring for access to water reserves during embryonic development.
有鳞爬行动物的羊皮纸壳卵从巢穴环境中吸收大量水分，这使得卵黄能够转化为新生组织，并缓冲胚胎抵御后续干旱天气的可能性。在发育过程中，越来越多的水分被储存在胚胎囊中（即围绕胚胎的膜：羊膜、尿囊和绒毛膜）。胎生（发育中的胚胎在子宫内长时间保留）的进化意味着胚胎囊液储存现在带来了成本（增加母体负担），带来的益处减少（因为母亲缓冲胎儿的水平衡），并在子宫内的兄弟姐妹之间引入了潜在冲突（争夺有限的水源）。我们对九种有鳞爬行动物的数据以及已发表的关于三种物种的信息表明，胎生物种的胚胎囊液约占新生质量的33%，而卵生有鳞动物的足月卵中这一比例为94%。对149条蝰蛇（极北蝰，一种胎生物种）分娩的数据表明，较大的后代在其胎儿囊中储存更多的液体，并且窝卵数的增加与每个后代的囊液质量减少有关。总体而言，从卵生到胎生的进化转变可能极大地改变了对后代包装的选择压力，并在胚胎发育期间造成了后代之间争夺水源储备的竞争。
