Role of parasites within the phylogeny of eukaryotes

Recent advances in molecular phylogenetics and morphological cladistics have considerably changed the picture we had on the tree of life (e.g. see Refs [1–16]). In particular, the alteration of the evolutionary relationships between eukaryotic organisms have split some traditional clades (e.g. Protozoa, Protophyta, Acoelomata, Aschelmintha) into highly divergent new branches, and have gathered formerly strongly distant organisms into new groups (e.g. Sporozoa with Dinoflagellata, Nematozoa with Arthropoda, Platyhelmintha with Mollusca and Annelida). Although the place that parasites occupy in this new phylogeny has already been discussed for Metazoa [14,15], a global view for eukaryotes, and the importance of parasitism itself, has not been analysed previously. The relationships of all eukaryotes and what proportion are parasitic are examined below; the discussion is based on two major references [11,13], which review the position and/or number of described species.