Snakes exhibit a diverse array of body shapes despite their characteristically simplified morphology. The most extreme shape changes along the precloacal axis are seen in fully aquatic sea snakes (Hydrophiinae): “microcephalic” sea snakes have tiny heads and dramatically reduced forebody girths that can be less than a third of the hindbody girth. This morphology has evolved repeatedly in sea snakes that specialize in hunting eels in burrows, but its developmental basis has not previously been examined. Here, we infer the developmental mechanisms underlying body shape changes in sea snakes by examining evolutionary patterns of changes in vertebral number and postnatal ontogenetic growth. Our results show that microcephalic species develop their characteristic shape via changes in both the embryonic and postnatal stages. Ontogenetic changes cause the hindbodies of microcephalic species to reach greater sizes relative to their forebodies in adulthood, suggesting heterochronic shifts that may be linked to homeotic effects (axial regionalization). However, microcephalic species also have greater numbers of vertebrae, especially in their forebodies, indicating that somitogenetic effects also contribute to evolutionary changes in body shape. Our findings highlight sea snakes as an excellent system for studying the development of segment number and regional identity in the snake precloacal axial skeleton.

中文翻译：

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椎骨进化和本体遗传学：在微头海蛇中极端体形差异的发展基础。

尽管蛇具有特征性的简化形态，但它们仍显示出各种各样的体形。在完全水生的海蛇（Hydrophiinae）中，可以看到沿泄殖腔前轴的最极端的形状变化：“微头”海蛇的头很小，前体周长大大减少，不到后体周长的三分之一。这种形态在专门研究洞穴中鳗鱼的海蛇中已经反复演化，但是先前尚未对其发展基础进行过研究。在这里，我们通过检查椎骨数量变化和出生后个体发育增长的进化模式来推断海蛇体形变化的潜在发展机制。我们的结果表明，小头畸形物种通过胚胎和出生后阶段的变化而发展出其特征形状。个体发生的变化导致小头畸形物种的后体相对于成年前体达到更大的尺寸，这表明异时性转变可能与顺势作用有关（轴向区域化）。但是，小头足类物种的椎骨数量也更多，尤其是在它们的前体中，这表明体发生的作用也有助于身体形态的进化变化。我们的发现突出了海蛇是研究蛇泄殖腔前轴骨骼中节数和区域同一性发展的优秀系统。特别是在它们的前体中，这表明体发生的作用也有助于身体形态的进化变化。我们的发现突出了海蛇是研究蛇泄殖腔前轴骨骼中节数和区域同一性发展的优秀系统。特别是在它们的前体中，这表明体发生的作用也有助于身体形态的进化变化。我们的发现突出了海蛇是研究蛇泄殖腔前轴骨骼中节数和区域同一性发展的优秀系统。