The phylogenetic placement of two enigmatic metazoan parasites: Polypodium hydriforme and Myxozoa

Abstract

The species Polypodium hydriforme and the phylum Myxozoa are both intracellular fish parasites. Both parasites possess putative nematocysts, the stinging structure characteristic of all cnidarians. This morphological evidence has historically supported, with some contention, a placement of these taxa within Cnidaria. Yet, current molecular data, in the form of 18S rDNA, has challenged this placement. Repeated phylogenetic analyses of 18S rDNA data alone have revealed myxozoans and Polypodium to be long-branched taxa, often falling sister to one another other, but consistently grouping, with albeit weak support, as sister to Bilateria. In the following work I reevaluated the most complete available molecular data, augmenting it where possible, in an effort to drawn stronger conclusions with regards to the phylogenetic placement of these two enigmatic, parasitic, putative cnidarian taxa. Results of a more comprehensive taxon sampling of 18S rDNA support Polypodium as a cnidarian and myxozoans as sister to Bilateria. This suggests that previous 18S rDNA analyses of Polypodium suffered from artifacts of long-branch attraction. Molecular sequence variation discovered within Polypodium is also reported. Finally, rDNA and phylogenomic data, which support two competing molecular phylogenetic hypotheses for Myxozoa's classification, are revisited here. New analyses reveal significant conflict within phylogenomic data purported to support a cnidarian placement. Combined analyses of the most comprehensive rDNA and phylogenomic data available, weakly support the controversial placement of Myxozoa as sister to Bilateria. While this placement remains tentative, results suggest that phylogenomic studies with limited taxonomic sampling should be interpreted cautiously. Comprehensive sampling remains one of the best ways to over come artifactual placements of long-branched clades.