爬行动物的卵胎生进化：卵滞留的选择利益与生理制约

Viviparity is a reproductive mode evolving from oviparity through gradual increases in the length of intrauterine development until giving birth to young. Viviparous females are able to provide temperatures optimal for embryonic development via behavioral thermoregulation, thereby accelerating embryonic development and optimizing offspring phenotype. However, the selective advantages associated with viviparity can be offset by gestation costs. The evolutionary transition from oviparity to viviparity occurred in the past, presumably under similar environmental conditions experienced by extant oviparous forms. Thus, with suitable animals, it is possible to infer processes at work in evolutionary history from present-day phenomena. Squamate reptiles provide a very useful model system for studying the evolutionary transition from oviparity to viviparity because they exhibit a wide reproductive diversity well beyond a simple oviparity/viviparity dichotomy and because viviparity has evolved far more often in this group of animals than in all other non-mammalian vertebrates combined. It has been known in squamate reptiles that the evolution of viviparity is constrained by both ecological and physiological factors. However, while much has been known about ecological constraints on the evolution of viviparity, physiological constraints on this reproductive mode are poorly known. The selective advantages associated with an increased length of egg retention primarily result from the reduced exposure of embryos to adverse ecological factors. We therefore hypothesize that the length of egg retention per se is not a fitness-related trait. In this study, we will use Phrynocephalus przewalskii, an oviparous lizard, as the model system to test this hypothesis by manipulating (increasing or reducing) the length of egg retention. Were the hypothesis validate, we would be able to test the following three predictions that may have an implication for the way through which viviparity evolves from oviparity: Hatchlings from eggs with different embryonic stages at oviposition do not differ in phenotype and fitness if: (1) eggs are laid by females under the same thermal regimes and incubated in the same thermal environments; (2) if eggs are incubated under the same oxygen pressures; and (3) if eggs are under the same space constraints during incubation. This study aims to address questions of plasticity, physiological constraints and selective advantages of egg retention in squamate reptiles, and data generated from this will provide the clue for the gradual evolutionary transition from oviparity to viviparity in squamate reptiles.

繁殖模式由卵生向卵胎生转变是重要的进化事件，卵胎生进化过程中逐渐延长卵滞留的选择利益和面临的主要限制因素是学术界普遍关注且尚待解答的重要科学问题，但相关研究却十分匮乏，迫切需要选取合适的动物模型开展深入研究。本项目拟以荒漠沙蜥为材料，通过控制怀卵雌体的调温机会，测定其产卵前后的调温精度和体温调定点，检测雌体调温机会可得性对后代表型及适合度的影响；实验操纵缩短或延长卵滞留时间，检测各胚胎时期新生卵的孵化时间、孵化成功率和幼体表型及适合度差异；比较不同氧分压和空间制约水分吸收梯度下各胚胎时期新生卵孵化的胚胎发育速率、成功率和孵出幼体表型及适合度，检测氧气和水分可得性对卵滞留的制约作用。研究结果旨在探讨爬行动物卵滞留时间的表型可塑性、生理制约机制和进化选择利益，为阐述卵胎生的渐进式进化提供实验证据。

卵胎生进化及其适应意义一直是进化生物学领域倍受关注的核心研究内容之一。卵胎生是卵生物种通过逐渐延长受精卵体内滞留时间直至产出幼仔进化而来的繁殖模式，其进化过程中逐渐延长卵滞留的选择利益和面临的主要限制因素是尚待解答的重要科学问题，但相关研究却十分匮乏。动物繁殖模式从卵生到卵胎生的转变是重要的历史事件，迄今没有法相任何化石证据，故从现生物种中收集该进化事件发生过程的证据是重要的。有鳞类爬行动物中至少有100个独立进化支系，远比其它脊椎动物的总和更为丰富，因而是开展此研究的理想动物模型。本项目以荒漠沙蜥为材料，实验操纵缩短或延长卵滞留时间，研究卵滞留时间的表型可塑性及适合度关联性；在不同氧分压和水分梯度下孵化新生卵，检测氧气和水分可得性对卵滞留的制约作用。主要实验内容：在3个背景温度（20℃，24℃和28℃）下测定雌体怀卵前后的选择体温；实验操纵缩短或延长怀卵雌体卵滞留实验以获得胚胎时期不同的新生卵，检测各时期卵的孵化时间、孵化成功率、后代表型和适合度的差异；在不同氧分压和水分梯度下孵化新生卵，比较各处理组胚胎发育速率、成功率以及后代表型和适合度的差异。主要结果：1、背景温度显著影响荒漠沙蜥选择体温，28℃下蜥蜴选择体温最高、24℃次之，20℃下蜥蜴选择体温最低。孕期雌体选择体温显著低于非孕期雌体和雄体，提示雌体通过自主调温为能产出高适合度的后代而将体温维持在适宜温度；2、通过实验操纵获得胚胎时期在23-33期范围内的新生卵，受精卵滞留时长并非是一个直接与适合度相关的特征；3、水分可得性显著影响胚胎发育成功率、后代表型以及幼体早期适合度等，强烈受限组的上述特征或指标均差于不受限组和轻微受限组，表明通过空间制约胚胎吸水能力会对胚胎发育及生长产生不利影响；4、氧分压梯度能显著影响胚胎发育速率及成功率、后代表型以及幼体早期适合度等，低氧组的上述特征或指标均差于常氧组和高氧组，表明低氧环境会对胚胎发育及生长产生不利影响。本项目研究能通过人工实验操纵方法成功拓宽胚胎发育时期，即卵滞留时间具有可塑性，验证了卵滞留时间本身与后代和亲体适合度无关这一科学假设；并揭示了母体腹腔有限的空间和低氧环境是制约卵胎生进化的重要因素；亦为阐述爬行动物卵胎生的渐进式进化提供了实验证据。