A comprehensive molecular phylogeny of dalytyphloplanida (platyhelminthes: rhabdocoela) reveals multiple escapes from the marine environment and origins of symbiotic relationships

In this study we elaborate the phylogeny of Dalytyphloplanida based on complete 18S rDNA (156 sequences) and partial 28S rDNA (125 sequences), using a Maximum Likelihood and a Bayesian Inference approach, in order to investigate the origin of a limnic or limnoterrestrial and of a symbiotic lifestyle in this large group of rhabditophoran flatworms. The results of our phylogenetic analyses and ancestral state reconstructions indicate that dalytyphloplanids have their origin in the marine environment and that there was one highly successful invasion of the freshwater environment, leading to a large radiation of limnic and limnoterrestrial dalytyphloplanids. This monophyletic freshwater clade, Limnotyphloplanida, comprises the taxa Dalyelliidae, Temnocephalida, and most Typhloplanidae. Temnocephalida can be considered ectosymbiotic Dalyelliidae as they are embedded within this group. Secondary returns to brackish water and marine environments occurred relatively frequently in several dalyeliid and typhloplanid taxa. Our phylogenies also show that, apart from the Limnotyphloplanida, there have been only few independent invasions of the limnic environment, and apparently these were not followed by spectacular speciation events. The distinct phylogenetic positions of the symbiotic taxa also suggest multiple origins of commensal and parasitic life strategies within Dalytyphloplanida. The previously established higher-level dalytyphloplanid clades are confirmed in our topologies, but many of the traditional families are not monophyletic. Alternative hypothesis testing constraining the monophyly of these families in the topologies and using the approximately unbiased test, also statistically rejects their monophyly.

Evolutionary biology of parasitic platyhelminths: The role of molecular phylogenetics

As our appreciation of the diversity within the flatworms has grown, so too has our curiosity about the ways in which these varied creatures are related to one another. In particular, the parasitic groups (trematodes, cestodes and monogeneans have been the focus of enquiry. Until recently, morphology, anatomy and life histories have provided the raw data for building hypotheses on relationships. Now, ultrastructural evidence, and most recently, molecular data from nucleic acid sequences, have been brought to bear on the topic. Here, David Blair, Andrés Campos, Michael Cummings and Juan Pedro Laclette discuss the ways in which molecular data, in particular, are helping us recognize the various lineages of flatworms.