Phylogeny of Dalytyphloplanida (Platyhelminthes: Rhabdocoela): single escape from the marine environment?

Flatworms (Platyhelminthes) traditionally consist of the paraphyletic “turbellaria”\nand the parasitic Neodermata. The last two decades, molecular phylogenetic\nresearch has completely altered our views of flatworm systematics. However,\nphylogenetic relations within many turbellarian groups are far from understood.\nIn terms of species numbers, Rhabdocoela (± 1500 species) is one of the largest\nturbellarian taxa. Former studies on rhabdocoel relationships based on\nmorphological and molecular data have given new insights into the evolutionary\nhistory of this cosmopolitan and important meiofaunal taxon (Willems et al.\n2006). Rhabdocoels now consist of two monophyletic sistertaxa:\nKalyptorhynchia (± 500 species) and Dalytyphloplanida (± 1000 species), the\nlatter clade comprising all former “Typhloplanoida”, “Dalyellioida” and\nTemnocephalida interspersed throughout the phylogeny. Consequently, a\nnumber of new clades quite different from the traditional rhabdocoel taxa have\nbeen defined. For instance, the existence of a monophyletic freshwater clade,\ncomprising the bulk of the limnic dalytyphloplanids (Dalyelliidae, Typhloplanidae,\nTemnocephalida) was considered, suggesting a single invasion of the freshwater\nenvironment by a marine ancestor that was followed by a spectacular radiation.\nHowever, this could not be corroborated, because few taxa were used in these\nstudies and molecular analyses were based on 18S rDNA only. In addition, the\nphylogenetic position of some symbiotic Dalytyphloplanida also triggered new\nquestions on the origins of commensalism and eventually parasitism within freeliving flatworms.\nIn this study I try to address these questions by elaborating the\nphylogeny of Dalytyphloplanida including more taxa and using a widened\nmolecular approach. During the coarse of this work, dalytyphloplanids have been\nsampled in nearly all major zoogeographical regions (Palearctic, Nearctic,\nAfrotropics, Neotropics, Oriental, Australian, Pacific). Many of the collected taxa\nappeared to be new to science and a number of them have been formally\ndescribed in five !-taxonomical papers: (1) an overview of the marine\nrhabdocoels from Uruguay, with the description of two new genera and six new\nspecies, (2) the description and ecology of a new limnoterrestrial representative\nof Protoplanellinae from Alabama (USA), (3) an account of the Dalytyphloplanida\nfrom Andalusia (Spain), with the description of four new species, (4) the\ndescription of one new genus of Dalyelliidae and one new species of Gieysztoria\nfrom Alabama (USA), together with an overview of all other dalyelliid representatives collected in Ontario (Canada), and Michigan and Alabama (USA),\n(5) the erection of a new species group of Gieysztoria, the “Falcatae”, based on\na comparative morphological study and the description of five new species from\nIndia, South Africa and Australia. After DNA extraction, amplification and\nsequencing, ribosomal DNA datasets of 156 18S rDNA and 125 partial 28S rDNA\ndalytyphloplanid sequences were aligned and analysed as individual genes and\nas a concatenated dataset in a maximum likelihood and Bayesian framework.\nTwo kalyptorhynch rhabdocoels were used as outgroup.\nThe previously found new dalytyphloplanid clades, i.e. Neotyphloplanida,\nNeodalyellida and Thalassotyphloplanida, are confirmed in our topologies as\nmonophyletic assemblages. Many of the traditional non-kalyptorhynch\nrhabdocoel families are not monophyletic (Trigonostomidae, Promesostomidae,\nByrsophlebidae, Typhloplanidae, Dalyelliidae, Provorticidae, Graffillidae). In\naddition, alternative hypothesis testing constraining these families in the\ntopologies and using the approximately unbiased test, also statistically rejects\nthe monophyly of these families. The enigmatic Kytorhynchidae are most likely\nbasal thalassotyphloplanids. Our phylogenies indicate that dalytyphloplanids\nhave their origins in the marine environment, but were able to massively\ncolonise a wide range of limnic and limnoterrestrial habitats, when a\nneotyphloplanid ancestor escaped its marine environment. The 18S and\ncombined phylogenies clearly support the existence of a large, species-rich,\nmonophyletic freshwater clade, Limnotyphloplanida n.c., comprising Dalyelliidae,\nTemnocephalida and most Typhloplanidae. Moreover, Temnocephalida can be\nconsidered ectosymbiotic Dalyelliidae as they are embedded within this group.\nAdditionally, some thalassotyphloplanids and neodalyellids also invaded limnic\nenvironments, albeit very sporadically and not followed by spectacular\nspeciation events as in Limnotyphloplanida n.c. Secondary returns to brackish\nwater and marine environments occurred relatively frequently in several\ndalyeliid and typhloplanid taxa. The distinct phylogenetic positions of some\nsymbiotic taxa (Umagillidae, Pterastericolidae, Graffillidae, Temnocephalida) also\nconfirm multiple origins for commensal and parasitic life strategies within\nDalytyphloplanida.\nThe above-mentioned results are thoroughly discussed in an ecological\nand taxonomical context. Finally, these phylogenies undoubtedly evoke much\nmore questions regarding the evolutionary history of this group. Some future\nconsiderations and possible research topics are briefly mentioned in the end.

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