Understanding character polarity in Deep Metazoan phylogeny - Phylogenomics of Myzostomids

由于分子系统学的研究，最近出现了一种关于原口动物深部后生动物系统发育的新观点，将它们划分为蜕皮(蜕皮动物)和非蜕皮原口动物(通常被称为Lophotrochozoa)。肌口类包括棘皮动物上的一小群共生的原口动物，它们展示了具有环节动物和扁平动物典型特征的马赛克的身体平面。它们是薄壁生物，有附足、毛状体、环状突起和不完整的节段。因此，肌口类的分类地位自发现以来一直存在争议也就不足为奇了。最近基于几个基因的分子分析相当复杂，而不是解决了问题，这表明肌口类与扁形蠕虫有着密切的亲缘关系。在这里提出的项目中，我们的目标是通过系统基因组学的方法来阐明分生动物的分类地位：对于两个模式分生动物(环状肌口肌口和垫状肌口)，将建立表达序列标签(EST)的文库，并利用最先进的生物信息学工具与潜在的姊妹类进行比较。由于这两个焦点种起源于肌口纲内最远的两个亲缘关系最远的系统发育谱系，对它们的比较将使分类群内(肌口纲内)和分类群间(肌口纲与潜在姐妹类群之间)存在基因分歧。对于一系列重要基因，将对更大系列的肌口类物种进行深入分析。这些基因已经在SPP深部后生动物系统发育中的申请者中达成一致，因此将产生可供替代分析使用的可比基因组数据集。我们的系统基因组分析将与肌口体计划的全面超微结构分析相辅相成。虽然我们的近期目标是解决肌口类的分类位置，但我们最终的目标是为原生口体计划的一些基本特征的特征极性(多形性、无形性或汇聚性)提供证据，如副足、毛状体、圈和节段。

Mainly due to molecular phylogenetic studies, a new view on Deep Metazoan Phylogeny has recently emerged for the protostomia, dividing them into the molting (Ecdysozoa) and the nonmolting protostomes (most often called the Lophotrochozoa). The myzostomids comprise a small group of symbiotic protostomes on echinoderms, which exhibit a body plan with a mosaic of characters typical of annelides and platyzoans. They are parenchymous organisms with parapodia, chaetae, cirri, and an incomplete segmentation. It is therefore not surprising that the taxonomic position of myzostomids has been a matter of controversy since their discovery. Recent molecular analyses based on a few genes have rather complicated than resolved the picture, suggesting close affinities of myzostomids to platyhelminths. In the project proposed here, we aim at clarifying the taxonomic position of myzostomids by a phylogenomic approach: for two model myzostomids (Myzostoma cirriferum, Pulvinomyzostomum pulvinar) libraries of Expressed Sequence Tags (ESTs) will be generated and compared by state-of-the-art bioinformatic tools to potential sister taxa. As the two focus species derive from the two most distantly related phylogenetic lineages within myzostomids, comparison on them will allow relating intra-taxon (within myzostomids) to inter-taxon (among myzostomids and potential sister taxa) gene divergence. For a series of important genes, an in-depth analysis will be performed on a larger series of myzostomid species. These genes have been agreed on among applicants in the SPP Deep Metazoan Phylogeny , such that comparable genomic datasets will be generated for surrogate analysis. Our phylogenomic analysis will be complemented by a comprehensive ultrastructural analysis of myzostomid body plans. While our proximate goal is to resolve the taxonomic position of myzostomids, we ultimately aim at providing evidence for character polarity (plesiomorphic, apomorphic, or convergent) of some of the basic features of protostome body plans, such as parapodia, chaetae, cirri, and segmentation.