Ryan Joseph F, Baxevanis Andreas D
Genome Technology Branch, National Human Genome Research Institute, National Institutes of Health, Bethesda, MD 20892, USA.
Biol Direct. 2007 Dec 13;2:37. doi: 10.1186/1745-6150-2-37.
The subkingdom Bilateria encompasses the overwhelming majority of animals, including all but four early-branching phyla: Porifera, Ctenophora, Placozoa, and Cnidaria. On average, these early-branching phyla have fewer cell types, tissues, and organs, and are considered to be significantly less specialized along their primary body axis. As such, they present an attractive outgroup from which to investigate how evolutionary changes in the genetic toolkit may have contributed to the emergence of the complex animal body plans of the Bilateria. This review offers an up-to-date glimpse of genome-scale comparisons between bilaterians and these early-diverging taxa. Specifically, we examine these data in the context of how they may explain the evolutionary development of primary body axes and axial symmetry across the Metazoa. Next, we re-evaluate the validity and evolutionary genomic relevance of the zootype hypothesis, which defines an animal by a specific spatial pattern of gene expression. Finally, we extend the hypothesis that Wnt genes may be the earliest primary body axis patterning mechanism by suggesting that Hox genes were co-opted into this patterning network prior to the last common ancestor of cnidarians and bilaterians.
两侧对称动物亚界包含了绝大多数动物，其中不包括四个早期分支的门：多孔动物门、栉水母动物门、扁盘动物门和刺胞动物门。平均而言，这些早期分支的门具有较少的细胞类型、组织和器官，并且被认为在其主要身体轴上的特化程度明显较低。因此，它们提供了一个有吸引力的外类群，可用于研究遗传工具包中的进化变化如何促成了两侧对称动物复杂身体结构的出现。本综述提供了两侧对称动物与这些早期分化类群之间基因组规模比较的最新情况。具体而言，我们在这些数据如何解释后生动物主要身体轴和轴向对称性的进化发展的背景下审视这些数据。接下来，我们重新评估动物型假说的有效性和进化基因组相关性，该假说通过特定的基因表达空间模式来定义动物。最后，我们扩展了Wnt基因可能是最早的主要身体轴模式形成机制这一假说，提出Hox基因在刺胞动物和两侧对称动物的最后共同祖先之前就被纳入了这个模式形成网络。
