Understanding character polarity in Deep Metazoan phylogeny - Phylogenomics of Myzostomids

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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.