EVOLUTION OF AXIAL SKELETAL REGIONALISATION IN VERTEBRATES: INTEGRATING PHYLOGENY, ENVIRONMENT, AND DEVELOPMENTAL MECHANISMS

轴向骨骼（即椎骨和肋骨）的进化多样化是脊椎动物成功的关键组成部分。“鱼类”通常被认为具有两个区域的轴向骨骼，包括“前鳍”和“尾鳍”椎骨，而四足动物的脊柱则被划分为不同的颈椎、胸椎、骶骨和尾鳍区域。因此，传统的观点认为，从水到陆地的进化过渡与脊柱解剖特征的增加是一致的。然而，最近在一些活着的和化石鱼类群体中发现的惊人的轴向骨骼复杂性表明了另一种情况：脊椎动物中原始存在区域化的复杂轴向骨骼，随着对新环境和生态的适应，区域化的反复丧失或增强。这个项目将是第一个明确和定量地重建脊椎动物轴向骨骼复杂性的进化史，并测试这种复杂性的环境和发育相关性。为了了解脊柱的进化及其在新环境和生态多样化中的作用，我们将结合解剖学和调控遗传数据集，在现存和化石脊椎动物的广泛分类样本中进行轴向骨骼区域化的协调调查。1)我们将基于计算机断层扫描和直接标本测量得出的骨骼数据量化轴向骨骼区域化和复杂性。2)我们将使用比较系统发育方法重建主要脊椎动物分支的祖先状态，并测试轴向骨骼区划与环境和生态因素之间的相关性。3)我们将测试一种软骨鱼（鳐，Leucoraja erinacea）胚胎中定义羊膜-颈-胸和胸-腰过渡的基因的表达，我们将使用荧光细胞标记方法来测试这些基因是否描绘了该物种中以前未被识别的轴向骨骼区域。通过将我们的发现与其他谱系（如四足动物）的发现进行比较，我们将能够推断出脊椎动物进化史上最早节点之一的轴向骨骼发育机制。4)最后，我们将使用进化模型结合解剖学、进化和发育分析的结果来阐明在进化过渡到新环境中的椎体形态和发育机制之间的关系。围绕脊椎动物轴向骨骼进化的争论已经持续了几个世纪，目前的教条和“教科书情景”主要源于猜测，并受到分类单元采样不足的影响。我们的研究将在严格的系统发育框架内整合解剖学、发育和环境数据，以准确地重建脊椎动物轴向骨骼的进化和生态历史。

Evolutionary diversification of the axial skeleton (i.e. vertebrae and ribs) is a key component of the success of vertebrate animals. "Fishes" are generally thought to possess a two-region axial skeleton, with "precaudal" and "caudal" vertebrae, while tetrapods exhibit regionalisation of their vertebral column into distinct cervical, thoracic, sacral, and caudal regions. Conventional wisdom is, therefore, that the evolutionary transition from water to land coincided with increasing anatomical specialisations of the vertebral column. However, recent discoveries of striking axial skeletal complexity in some living and fossil fish groups suggest an alternative scenario: the primitive presence of a regionalised, complex axial skeleton in vertebrates, with repeated loss or augmentation of regionalisation as adaptation to new environments and ecologies. This project will be the first to explicitly and quantitatively reconstruct the evolutionary history of vertebrate axial skeletal complexity, and to test for environmental and developmental correlates of such complexity.In order to understand the evolution of the vertebral column and its role in diversifications into new environments and ecology, we will combine anatomical and regulatory genetic datasets to undertake a concerted investigation of axial skeletal regionalisation in a broad taxonomic sample of extant and fossil vertebrates. 1) We will quantify axial skeletal regionalization and complexity based on skeletal data derived from Computed Tomography and direct specimen measurement. 2) We will use comparative phylogenetic methods to reconstruct ancestral states for major vertebrate clades and to test for correlations between axial skeletal regionalisation and environmental and ecological factors. 3) We will test for expression of genes that define the amniote cervical-thoracic and thoracic-lumber transitions in embryos of a cartilaginous fish (the skate, Leucoraja erinacea), and we will use fluorescent cell labelling approaches to test whether these genes delineate previously unrecognised axial skeletal regions in this species. By comparing our findings with those from other lineages (e.g. tetrapods), we will be able to infer mechanisms of axial skeletal development at one of the earliest nodes in vertebrate evolutionary history. 4) Finally, we will use evolutionary modelling to combine results of anatomical, evolutionary, and developmental analyses to elucidate the relationships between vertebral morphology and developmental mechanisms in evolutionary transitions into new environments.Debate surrounding the evolution of the vertebrate axial skeleton has lingered for centuries, with current dogma and "textbook scenarios" stemming largely from speculation, and suffering from poor taxon sampling. Our study will integrate anatomical, developmental and environmental data, within a rigorous phylogenetic framework, in order to accurately reconstruct the evolutionary and ecological history of the vertebrate axial skeleton.