以蛇类为模型动物研究爬行类卵胎生选择劣势的补偿机制

Evolving from oviparity through gradual increases in the length of intrauterine development, viviparity offers pervasive benefits by lowering embryonic mortality, accelerating developmental rate, optimizing the phenotype of offspring, freeing females from having to find egg-laying sites, and providing the mother the chance to select the sex of her offspring in species with temperature-dependent sex determination, but it entails several costs of which the most important three are decreased fecundity or reproductive output, increased maternal mortality, and reduced genetic diversity among offspring as a consequence of reduced matings or clutches. This raises the question of whether viviparous females compensate for the costs of viviparity by increasing the amount of space available to hold eggs, by accelerating the rate of embryonic development to shorten gestation length, and/or by increasing the frequency of multiple paternity. To answer this question, one virtually needs to test three hypotheses. First, body plans should differ between oviparous and viviparous females, and in particular, selection should favor the evolution of a larger abdomen size (and thus, a larger body-volume) in viviparous females. Second, the rate of embryonic development should be higher in viviparous species, as the selective advantage accruing from reduced gestation length can be translated into enhanced maternal fitness in viviparous species because prolonged retention of eggs and impaired locomotor capacity during prenancy increase predation risks. Third, the frequency of multiple paternity should be higher in viviparous species, as the selective advantage accruing from increased multiple paternity can be translated into increased genetic diversity among offspring. Evidence from snakes has showed that viviparous females do compensate for reduced fecundity or reproductive output by producing more and larger offspring in single reproductive episodes than do oviparous females of the same body size, but they do so not by increasing the relative size of body-volume. Thus, for this group of animals the focus should be laid on the latter two hypotheses that have never been tested in any animal taxa. We will use three pairs of snakes of which each includes two coexisting species with different reproductive modes to test these two hypotheses by comparing heart and metabolic rates of embryos, offspring performance and multiple paternity between oviparous and viviparous species. We will also examine the frequency of multiple paternity in another 14 snakes using samples (hatchlings and their mothers) collected over the past decade to test whether the third hypothesis (the second hypothesis that will be tested in this study) mentioned above is generalisable to snakes. Our study is the first to address the question of whether reptiles can compensate for selective disadvantages of viviparity by modifying the rate of embryonic development and the frequency of multiple paternity.

卵胎生由卵生物种逐渐延长胚胎体内发育时间进化产生的繁殖模式，有母体死亡率高、生育力或繁殖输出低、后代遗传多样性低等选择劣势。卵胎生劣势补偿机制的关键科学问题是：卵胎生物种是否相对增大母体腹腔空间提高生育力或繁殖输出？是否相对提高胚胎代谢率缩短孕期降低母体捕食风险？是否增加同窝不同父权后代比例提高后代遗传多样性？已知卵胎生蛇比相同大小的卵生蛇产更多、更大的后代，但两者腹腔空间无异，故蛇类中须检验的假设是：1、卵胎生蛇应有较高的胚胎代谢率以缩短孕期降低母体捕食风险；2、卵胎生蛇应有较高的同窝不同父权后代比例以提高后代遗传多样性。申请人拟用3组、每组2种同属或同亚科繁殖模式不同但同域分布的蛇为材料，检验假设1和2。申请人将测定实验室现有的3科、14种其它蛇的后代父权，通过比较不同繁殖模式蛇的同窝不同父权比例检验假设2的普适性。从生理和后代遗传多样性角度探讨爬行动物卵胎生劣势补偿机制为原创性工作。

卵胎生的进化意义是进化生物学的中心议题之一。尽管具有降低胚胎死亡率、加速发育速率、优化后代表型、免除雌体寻觅产卵场的需要，以及在温度决定性别物种中提供母体选择后代性别的机会，卵胎生有提高母体死亡率、降低后代遗传多样性、生育力和繁殖输出等选择劣势。这提出一个尚待检验的假设：卵胎生物种是否通过（1）改变体型以增加容纳后代的腹腔空间、（2）加速胚胎发育速率以缩短孕期、（3）增加多父本发生频率以增加后代遗传多样性、（4）以上三种方式的某种组合以补偿卵胎生的选择劣势。本项目组用13种卵生蛇、9种卵胎生蛇开展实验，检验上述假设。从实验数据中，我们可以得出以下7个主要结论：第一、多次与雄性交配及相应的同窝多父本是蛇类中的普遍现象。22种蛇的总平均多父本比例是81.9%（眼镜蛇最低为40%，灰鼠蛇和渔异色蛇最高为100%）。卵生和卵胎生蛇多父本比例无差异。第二、生境利用和繁殖模式不同的蛇类维持代谢（能耗）不同。水栖和半水栖物种维持代谢高于陆栖物种，怀发育后期胚胎的雌体维持代谢大于非繁殖期和怀早期胚胎的雌体。第三、卵胎生蛇产更多更重的后代，借此补偿可能错失繁殖机会的损失。更具体而言，在相同的雌体大小下，卵胎生平均比卵生蛇多产2.8个后代，每窝平均比卵生蛇重约20克。第四、卵胎生蛇与卵生蛇体型无差异，证据源自体长相同的卵胎生和卵生蛇体长标准差无显著差异。第五、蛇类后代数量和大小权衡符合Noordwijk-De Jong经典预测，即繁殖资源分配与繁殖投入的方差比在后代数量和大小权衡更显著的种群中更大。第六、卵生到卵胎生的进化转变伴随维持代谢水平的下降。本研究是首次提供了繁殖模式的进化转变可改变有关物种维持代谢的证据。第七、胚胎内卵黄的分配与繁殖模式有关。具体而言，卵胎生物种产发育更完善的后代，但分配用于产后亲代呼吁的卵黄较少。概括而言，卵胎生蛇对其繁殖模式选择劣势的补偿性显著，但并非通过改变体型、胚胎发育速率、多父本发生频率等机制。