Molecular insights into deuterostome evolution from hemichordate developmental biology

Hemichordates, along with echinoderms and chordates, belong to the lineage of bilaterians called the deuterostomes. Their phylogenetic position as an outgroup to chordates provides an opportunity to investigate the evolutionary origins of the chordate body plan and reconstruct ancestral deuterostome characters. The body plans of the hemichordates and chordates are organizationally divergent making anatomical comparisons very challenging. The developmental underpinnings of animal body plans are often more conservative than the body plans they regulate, and offer a novel data set for making comparisons between morphologically divergent body architectures. Here I review the hemichordate developmental data generated over the past 20 years that further test hypotheses of proposed morphological affinities between the two taxa, but also compare the conserved anteroposterior, dorsoventral axial patterning programs and germ layer specification programs. These data provide an opportunity to determine which developmental programs are ancestral deuterostome or bilaterian innovations, and which ones occurred in stem chordates or vertebrates representing developmental novelties of the chordate body plan.

半索动物与棘皮动物和脊索动物一起，属于被称为后口动物的两侧对称动物谱系。它们作为脊索动物外类群的系统发育位置为研究脊索动物躯体模式的进化起源以及重建后口动物祖先的特征提供了机会。半索动物和脊索动物的躯体模式在组织结构上存在差异，这使得解剖学比较极具挑战性。动物躯体模式的发育基础往往比它们所调控的躯体模式更为保守，并为在形态上不同的躯体结构之间进行比较提供了新的数据集。在此，我回顾了过去20年中产生的半索动物发育数据，这些数据进一步检验了关于这两个类群之间所提出的形态亲缘关系的假说，同时也比较了保守的前后轴、背腹轴模式形成程序以及胚层特化程序。这些数据为确定哪些发育程序是后口动物或两侧对称动物祖先的创新，以及哪些发生在干群脊索动物或脊椎动物中并代表了脊索动物躯体模式的发育新特征提供了机会。