Transcriptomic evidence that enigmatic parasites Polypodium hydriforme and Myxozoa are cnidarians

Abstract

Polypodium hydriforme and Myxozoa, represented in this study by Myxobolus cerebralis, are both enigmatic, intracellular parasites with very unusual life cycles and body plans, which has long made their phylogenetic placement unclear. It has been suggested that Polypodium hydriforme and Myxozoa have an affinity with cnidarians because of the presence of nematocyst-like structures in both organisms. Recently phylogenomic studies have lent support to the hypothesis that Myxozoa is cnidarian. However, the placement of Polypodium hydriforme and Myxozoa within cnidarian and in relation to each other remains unknown, and many questions about their evolution and transition to parasitism still remain. To address these questions, we have generated partial transcriptomes of M. cerebralis and Polypodium hydriforme, and searched within them for important families of developmental regulatory genes and nematocyst-specific genes. The Polypodium hydriforme transcriptome contained a much larger complement of both putative Hox/Parahox genes and Wnt-family genes, which may relate increased body plan complexity as compared with M. cerebralis. Both transcriptomes contained a number of minicollagen sequences, confirming their placement within Cnidaria.