The interrelationships of all major groups of Platyhelminthes: phylogenetic evidence from morphology and molecules

Abstract

We used a data matrix of 65 morphological characters from 25 ingroup and 6 outgroup taxa, and an alignment comprising complete 18S rDNA sequences from 82 species of parasitic and free-living Platyhelminthes and from 19 species of lower invertebrates to analyse phylogenetic relationships of various platyhelminth taxa. Of the 1358 unambiguously alignable molecular positions, 995 were variable and 757 were phylogenetically informative (parsimony criterion); complete 18S rDNA sequences ranged in length from 1755 to 2873 bp. Main conclusions are: Neodermata are monophyletic, and the Trematoda, Monogenea and Cestoda within them are monophyletic as well. The sister group of the Neodermata is all the other Neoophora; the Kalyptorhynchia, Typhloplanida, Dalyelliida and Temnocephalida form one clade, and the last three another. Monophyly of the Seriata is rejected, but Polycladida/Macrostomida/Haplopharyngida are monophyletic, as are the last two taxa. As a consequence, validity of the taxon Trepaxonemata is rejected. Further studies must show the correct position of the Acoela and Nemertodermatida. It is stressed that morphological and molecular data in some respects lead to contradictory results, for instance concerning the position of the Fecampiidae/Urastoma/Ichthyophagaand the relative position of the Lecithoepitheliata. Denser sampling of taxa for molecular data, complementary sequences from independent genes, and inclusion of additional morphological data are necessary to resolve these contradictions.