Department of Ecology and Evolutionary Biology, School of Biological Sciences, The University of Adelaide, Adelaide, South Australia, Australia.
Earth Sciences Section, South Australian Museum, Adelaide, South Australia, Australia.
Evol Dev. 2019 May;21(3):135-144. doi: 10.1111/ede.12284. Epub 2019 Feb 21.
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.
蛇类尽管具有典型的简化形态，但它们表现出了多样化的体型。在沿泄殖腔前轴的最极端形状变化中，可以看到完全水生的海蛇（Hydrophiinae）：“小头”海蛇的头部极小，前体周长显著减小，甚至可以小于后体周长的三分之一。这种形态已经在专门在洞穴中猎食鳗鱼的海蛇中多次进化，但它的发育基础以前尚未被研究过。在这里，我们通过检查脊椎数量的进化模式和产后个体发育生长，推断出海蛇体型变化的发育机制。我们的结果表明，小头物种通过胚胎和产后阶段的变化来发展其特征性形状。个体发育变化导致小头物种的后体在成年后相对于前体达到更大的尺寸，这表明可能与同源异形效应（轴向分区）有关的异时性转变。然而，小头物种的脊椎数量也更多，尤其是在前体，这表明体节发生效应也有助于身体形状的进化变化。我们的发现强调了海蛇作为研究蛇类泄殖腔前轴骨骼中体节数量和区域特征发育的优秀系统。
