Schierwater Bernd, de Jong Danielle, Desalle Rob
Ecology and Evolution, Tierärztliche Hochschule Hannover, D-30559 Hannover, Germany.
Int J Biochem Cell Biol. 2009 Feb;41(2):370-9. doi: 10.1016/j.biocel.2008.09.023. Epub 2008 Oct 2.
The multicellular Metazoa evolved from single-celled organisms (Protozoa) and usually - but not necessarily - consist of more cells than Protozoa. In all cases, and thus by definition, Metazoa possess more than one somatic cell type, i.e. they show-in sharp contrast to protists-intrasomatic differentiation. Placozoa have the lowest degree of intrasomatic variation; the number of somatic cell types according to text books is four (but see also Jakob W, Sagasser S, Dellaporta S, Holland P, Kuhn K, and Schierwater B. The Trox-2 Hox/ParaHox gene of Trichoplax (Placozoa) marks an epithelial boundary. Dev Genes Evol 2004;214:170-5). For this and several other reasons Placozoa have been regarded by many as the most basal metazoan phylum. Thus, the morphologically most simply organized metazoan animal, the placozoan Trichoplax adhaerens, resembles a unique model system for cell differentiation studies and also an intriguing model for a prominent "urmetazoon" hypotheses-the placula hypothesis. A basal position of Placozoa would provide answers to several key issues of metazoan-specific inventions (including for example different lines of somatic cell differentiation leading to organ development and axis formation) and would determine a root for unraveling their evolution. However, the phylogenetic relationships at the base of Metazoa are controversial and a basal position of Placozoa is not generally accepted (e.g. Schierwater B, DeSalle R. Can we ever identify the Urmetazoan? Integr Comp Biol 2007;47:670-76; DeSalle R, Schierwater B. An even "newer" animal phylogeny. Bioessays 2008;30:1043-47). Here we review and discuss (i) long-standing morphological evidence for the simple placozoan bauplan resembling an ancestral metazoan stage, (ii) some rapidly changing alternative hypotheses derived from molecular analyses, (iii) the surprising idea that triploblasts (Bilateria) and diploblasts may be sister groups, and (iv) the presence of genes involved in cell differentiation and signaling pathways in the placozoan genome.
多细胞后生动物由单细胞生物（原生动物）进化而来，通常——但并非必然——比原生动物拥有更多的细胞。在所有情况下，根据定义，后生动物拥有不止一种体细胞类型，即与原生生物形成鲜明对比的是，它们表现出体细胞内分化。扁盘动物的体细胞内变异程度最低；根据教科书，其体细胞类型数量为四种（但也可参见雅各布·W、萨加塞尔·S、德拉波塔·S、霍兰德·P、库恩·K以及席尔沃特·B。扁盘动物（扁盘虫）的Trox - 2 Hox/ParaHox基因标记了上皮边界。《发育基因与进化》2004年；214卷：第170 - 175页）。基于这一原因以及其他几个原因，许多人认为扁盘动物门是最基础的后生动物门。因此，形态上组织最简单的后生动物，即扁盘虫Trichoplax adhaerens，类似于细胞分化研究的独特模型系统，也是一个关于著名的“原始后生动物”假说——扁盘假说——的有趣模型。扁盘动物的基础地位将为后生动物特有的一些关键问题（包括例如导致器官发育和轴形成的不同体细胞分化路线）提供答案，并将确定解开其进化之谜的根源。然而，后生动物基部的系统发育关系存在争议，扁盘动物的基础地位并未被普遍接受（例如席尔沃特·B、德萨莱·R。我们能确定原始后生动物吗？《综合与比较生物学》2007年；47卷：第670 - 676页；德萨莱·R、席尔沃特·B。一个甚至“更新的”动物系统发育。《生物论文》2008年；30卷：第1043 - 1047页）。在此，我们回顾并讨论：（i）长期以来关于扁盘动物简单身体结构类似于后生动物祖先阶段的形态学证据；（ii）一些源自分子分析的快速变化的替代假说；（iii）三胚层动物（两侧对称动物）和双胚层动物可能是姐妹群这一惊人观点；以及（iv）扁盘动物基因组中涉及细胞分化和信号通路的基因的存在。
