Platyhelminth phylogenetics - a key to understanding parasitism?

The comparative method, the inference of biological processes from phylogen etic patterns, is founded on the reliability of the phylogenetic tree. In a ttempting to apply the comparative method to the understanding of the evolu tion of parasitism in the phylum Platyhelminthes, we have highlighted sever al points we consider to be of value along with many problems. We discuss f our of these topics. Firstly, we view the group at a phylum level, in parti cular discussing the importance of establishing the sister taxon to the obl igate parasite group, the Neodermata, for addressing such questions as the monophyly, parasitism or the endo or ectoparasitic nature of the early para sites. The variety of non-congruent phylogenetic trees presented so far, ut ilising either or both morphological and molecular data, gives rise to the suggestion that any evolutionary scenarios presented at this stage be treat ed as interesting hypotheses rather than well-supported theories. Our secon d point of discussion is the conflict between morphological and molecular e stimates of monogenean evolution. The Monogenea presents several well-estab lished morphological autapomorphies, such that morphology consistently esti mates the group as monophyletic, whereas molecular sequence analyses indica te paraphyly, with different genes giving different topologies. We discuss the problem of reconciling gene and species trees. Thirdly, we use recent p hylogenetic results on the tapeworms to interpret the evolution of strobila tion, proglottization, segmentation and scolex structure. In relation to th e latter, the results presented indicate that the higher cestodes are diphy letic, with one branch difossate and the other tetrafossate. Finally, we us e a SSU rDNA phylogenetic tree of the Trematoda as a basis for the discussi on of an aspect of the digenean life-cycle, namely the nature of the first intermediate host. Frequent episodes of host-switching, between gastropod a nd bivalve hosts or even into annelids, are indicated.