Phylogeny of the Platyhelminthes and the evolution of parasitism

Robust phylogenies provide the basis for interpreting biological variation in the light of evolution. Homologous features provide phylogenetically informative characters whereas homoplasious characters provide phylogenetic noise. Both provide evolutionary signal. We have constructed molecular and morphologically based phylogenies of the phylum Platyhelminthes using a recently revised morphological character matrix and complete 18S and two partial 28S rRNA gene sequences in order to evaluate the emergence and subsequent divergence of parasitic forms. In total we examine 65 morphological characters, 97 18S rDNA, 41 D1 domain 28S rDNA, and 49 D3–D6 domain 28S rDNA sequences. For the molecular data there were 748, 132 and 249 phylogenetically informative sites for the 18S, D1 and D3–D6 28S rDNA data sets respectively. Morphological and molecular phylogenetic solutions are incongruent but not incompatible, and using the principles of conditional combination (18S rDNA + morphology passing Templeton's test) they demonstrate: a single and relatively early origin for the parasitic Neodermata (including the cestodes, trematodes and monogeneans); sister-group status between the cestodes and monogeneans, and between these taxa and the trematodes (digeneans and aspidogastreans). The sister-group to the Neodermata is likely to be a large clade of neoophoran turbellarians, based on combined evidence, or a clade consisting of the Fecampiid+Urastomid turbellarians, based on morphological evidence alone. The combined evidence solution for the phylogeny of flatworms based on 18S rDNA and morphology is used to interpret morphological and life-history data and to support a model for the evolution and radiation of neodermatan parasites in the group.