Baguñà Jaume, Martinez Pere, Paps Jordi, Riutort Marta
Departament de Genètica, Facultat de Biologia, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain.
Philos Trans R Soc Lond B Biol Sci. 2008 Apr 27;363(1496):1481-91. doi: 10.1098/rstb.2007.2238.
Conventional wisdom suggests that bilateral organisms arose from ancestors that were radially, rather than bilaterally, symmetrical and, therefore, had a single body axis and no mesoderm. The two main hypotheses on how this transformation took place consider either a simple organism akin to the planula larva of extant cnidarians or the acoel Platyhelminthes (planuloid-acoeloid theory), or a rather complex organism bearing several or most features of advanced coelomate bilaterians (archicoelomate theory). We report phylogenetic analyses of bilaterian metazoans using quantitative (ribosomal, nuclear and expressed sequence tag sequences) and qualitative (HOX cluster genes and microRNA sets) markers. The phylogenetic trees obtained corroborate the position of acoel and nemertodermatid flatworms as the earliest branching extant members of the Bilateria. Moreover, some acoelomate and pseudocoelomate clades appear as early branching lophotrochozoans and deuterostomes. These results strengthen the view that stem bilaterians were small, acoelomate/pseudocoelomate, benthic organisms derived from planuloid-like organisms. Because morphological and recent gene expression data suggest that cnidarians are actually bilateral, the origin of the last common bilaterian ancestor has to be put back in time earlier than the cnidarian-bilaterian split in the form of a planuloid animal. A new systematic scheme for the Bilateria that includes the Cnidaria is suggested and its main implications discussed.
传统观点认为，两侧对称生物起源于辐射对称而非两侧对称的祖先，因此只有单一的体轴且没有中胚层。关于这种转变如何发生的两种主要假说，一种认为是类似于现存刺胞动物浮浪幼虫或无肠扁形动物（浮浪幼虫-无肠幼虫理论）的简单生物，另一种认为是具有几种或大多数高等真体腔两侧对称生物特征的相当复杂的生物（原肠胚理论）。我们报告了使用定量（核糖体、核和表达序列标签序列）和定性（HOX 簇基因和 microRNA 集）标记对两侧对称后生动物进行的系统发育分析。得到的系统发育树证实了无肠扁形虫和纽形扁形虫作为两侧对称动物中最早分支的现存成员的位置。此外，一些无体腔和假体腔类群表现为早期分支的触手冠动物和后口动物。这些结果强化了这样一种观点，即两侧对称动物的祖先为小型、无体腔/假体腔、底栖生物，起源于类似浮浪幼虫的生物。由于形态学和最新的基因表达数据表明刺胞动物实际上是两侧对称的，因此最后一个共同两侧对称祖先的起源必须以浮浪幼虫状动物的形式追溯到比刺胞动物-两侧对称动物分化更早的时期。本文提出了一个包括刺胞动物的两侧对称动物新分类方案，并讨论了其主要意义。
