Correction to Evans, N.M., Lindner, A., Raikova, E.V., Collins, A.G. and Cartwright, P. Phylogenetic placement of the enigmatic parasite, Polypodium hydriforme, within the phylum Cnidaria. BMC Evol Biol, 2008, 8:139.
Following the publication of this study  it was brought to our attention that the partial 28S rDNA sequence for Polypodium hydriforme used in some of our reported analyses was in fact a contaminant of an undetermined cnidarian species belonging to the genus Obelia. This contaminant was an isolated case involving a small batch of reagents and we are confident that no other sequences published in this work  are suspect. The 18S rDNA sequences were obtained in two laboratories using different reagents and different genomic DNA preparations derived from distinct tissue samples and are similar to a previously published 18s sequence from Polypodium.
The results of our 28S rDNA analyses (Additional file Two and Six, ) and our combined 18S and 28S rDNA analyses (Figure Two, Additional files One and Seven ) should be disregarded. We have reanalyzed the combined dataset including the Polypodium 18S and Obelia 28S as separate samples. The topologies with regards to the placement of Polypodium are consistent with those of our 18S rDNA only analyses (Figure ThreeB and Additional file ThreeB ).
Although the 18S rDNA analyses alone, excluding the long branch attractor Myxozoa, support our major conclusion that Polypodium hydriforme is a cnidarian (Figure ThreeB and Additional file ThreeB ) this placement should be viewed as tentative due to low bootstrap support (83% supporting a monophyletic Cnidaria that includes Polypodium) (Additional file ThreeB ). Further investigation of both the placement of Polypodium and the placement of Myxozoa within Metazoa are warranted.
We regret that this error was not discovered before publication and we apologize for any inconvenience this has caused. We gratefully acknowledge Maximiliano Maronna and Dr. Antonio Marques who, being the first to sequence 28S rDNA from Obelia, graciously brought this matter to our attention.