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

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 alignablc molecular positions, 995 were variable and 757 were phylogenctically informative (parsimony criterion); complete 18S rDNA sequences ranged in length from 1755 to 2873 bp. Main conclusions are: Ncodermata are monophylctic, and the Trematoda, Monogenca and Cestoda within them are monophylctic as well. The sister group of the Ncodermata is all the other Ncoophora; the Kalyptorhynchia, Typhloplanida, Dalyelliida and Tcmnocephalida form one clade, and the last three another. Monophyly of the Seriata is rejected, but Polycladida/ Macrostomida/Haplopharyngida are monophylctic, as arc the last two taxa. As a consequence, validity of the taxon Trepaxonemata is rejected. Further studies must show the correct position of the Acocla and Nemertodermatida. It is stressed that morphological and molecular data in some respects lead to contradictory results, for instance concerning the position of the Fecampiidac/ Urastoma/Ichthyophaga and the relative position of the Lccithoepitheliata. Denser sampling of taxa for molecular data, complementary sequences from independent genes, and inclusion of additional morphological data are necessary to resolve these contradictions.