Phylogenetic placement of the enigmatic parasite, Polypodium hydriforme, within the Phylum Cnidaria.

Nathaniel M Evans,Ekaterina V Raikova,Allen G Collins,Alberto Lindner,Paulyn Cartwright

doi:10.1186/1471-2148-8-139

Abstract

BackgroundPolypodium hydriforme is a parasite with an unusual life cycle and peculiar morphology, both of which have made its systematic position uncertain. Polypodium has traditionally been considered a cnidarian because it possesses nematocysts, the stinging structures characteristic of this phylum. However, recent molecular phylogenetic studies using 18S rDNA sequence data have challenged this interpretation, and have shown that Polypodium is a close relative to myxozoans and together they share a closer affinity to bilaterians than cnidarians. Due to the variable rates of 18S rDNA sequences, these results have been suggested to be an artifact of long-branch attraction (LBA). A recent study, using multiple protein coding markers, shows that the myxozoan Buddenbrockia, is nested within cnidarians. Polypodium was not included in this study. To further investigate the phylogenetic placement of Polypodium, we have performed phylogenetic analyses of metazoans with 18S and partial 28S rDNA sequences in a large dataset that includes Polypodium and a comprehensive sampling of cnidarian taxa.ResultsAnalyses of a combined dataset of 18S and partial 28S sequences, and partial 28S alone, support the placement of Polypodium within Cnidaria. Removal of the long-branched myxozoans from the 18S dataset also results in Polypodium being nested within Cnidaria. These results suggest that previous reports showing that Polypodium and Myxozoa form a sister group to Bilateria were an artifact of long-branch attraction.ConclusionBy including 28S rDNA sequences and a comprehensive sampling of cnidarian taxa, we demonstrate that previously conflicting hypotheses concerning the phylogenetic placement of Polypodium can be reconciled. Specifically, the data presented provide evidence that Polypodium is indeed a cnidarian and is either the sister taxon to Hydrozoa, or part of the hydrozoan clade, Leptothecata. The former hypothesis is consistent with the traditional view that Polypodium should be placed in its own cnidarian class, Polypodiozoa.

Highlights

Polypodium hydriforme is a parasite with an unusual life cycle and peculiar morphology, both of which have made its systematic position uncertain
Polypodium hydriforme is an endocellular parasite whose unusual life cycle, peculiar morphology, and high rates of DNA evolution, have led to much controversy regarding its phylogenetic position within metazoans [1,2,3,4,5]
Sampled taxa All taxa used in this study are arranged taxonomically in Table 1. 155 sequences were obtained from GenBank. 45 new cnidarian sequences for 18S and 59 for 28S were generated for this study and deposited in GenBank

Summary

Introduction

Polypodium hydriforme is a parasite with an unusual life cycle and peculiar morphology, both of which have made its systematic position uncertain. Polypodium hydriforme is an endocellular parasite whose unusual life cycle, peculiar morphology, and high rates of DNA evolution, have led to much controversy regarding its phylogenetic position within metazoans [1,2,3,4,5]. Polypodium spends most of its life inside the oocytes of acipenseriform fishes (sturgeons and paddlefish) During this time, Polypodium develops from a binucleate cell into an inside-out planuliform larva and into an elongate inside-out stolon; the epidermal cell layer is located internal to the body and the gastrodermis is located externally [6,7,8]. Upon emerging from the host egg in fresh water, the freeliving stolon (Figure 1A) fragments into individual medusoid-like forms (Figure 1B) that go on to multiply by means of longitudinal fission, form sexual organs, and infect host fish with their gametophores [6,7,8,9]

Methods

Results

Discussion

Conclusion