Polypodium hydriforme is the only coelenterate adapted to intracellular parasitism in oocytes of acipenserid and polyodontid fishes. It occurs in both the Old and the New worlds, being parasitic in 12 species of Acipenseridae and in 1 species of Polyodontidae. Its earliest parasitic stages are binucleate cells that occur in previtellogenic oocytes. All embryonic and postembryonic development (which seems to be parthenogenetic) up to the budding stolon stage takes place inside fish oocytes and lasts several years. The planula and stolon have inverted germ layers. All parasitic stages are encircled with a highly polyploid unicellular trophamnion that is homologous to the second polar body. Before spawning, eversion of the stolon takes place inside the oocyte. At spawning, the everted stolons get into water and the free-living phase of the life cycle begins. The stolon fragments into individual specimens that can move and feed. They multiply by longitudinal fission (paratomy). In mid-summer they form 2 kinds of endodermal gonads. The so-called "female" gonads (2 ovaria, each with a gonoduct encircled with a common envelope) produce diploid cells that display no meiotic phenomena. The so-called "male" gonads have no gonoducts, but their sex cells undergo 2 meiotic divisions, giving rise to binucleate cells with unequal nuclei. The entire gonad becomes a gametophore with an ectodermal lid carrying nematocysts and containing many binucleate cells. Gametophores can be deposited onto the skin of prelarvae of fishes. How the parasite gets into young fish oocytes is not known.

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.

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In situ hybridization has been performed in sections through ovaries ofAcipenser ruthenus andAcipenser güldenstädti in order to detect the rDNA sequences. Hybridization resulted in specific labelling of the "caps" of extrachromosomal DNA present in pachytene oocyte nuclei and of the chromatin granules distributed beneath the nuclear envelope in early diplotene nuclei. In the same sections, the nuclei of all ovarian cells in both species (oogonia, leptotene, and zygotene stage oocytes, follicular cells, connective tissue cells) showed a very low, but similar labelling.Amplification of genes for rRNA thus occurs at the pachytene stage in early oogenesis ofAcipenseridae. No rDNA amplification could be detected in the previous stages.

Cytomorphological characters of Polypodium hydriforme-a unique cnidarian, adapted to intracellular parasitism inside oocytes of Acipenseriform fish-are reviewed and analysed. Polypodium's unique characters, those shared with the Myxozoa, and the characters shared with bilateral triploblastic animals are listed. The set of unique features has allowed us to allocate Polypodium to a separate cnidarian class Polypodiozoa Raikova, 1988. The combination of characters displayed by P. hydriforme suggests that this animal is not an aberrant cnidarian, as it was formerly believed, but a relic of a major cnidarian group (class Polypodiozoa) that has undergone reduction due to adaptation to parasitism.

No significant differences in macro- and micromorphology were found between the parasitic stolon and free-living polyps of Polypodium sp. obtained from infected eggs of the North American acipenseriform fish Polyodon spathula and corresponding developmental stages of Polypodium hydriforme Ussov, parasitic in the Volga sterlet (Acipenser ruthenus). Therefore, both the American and the European forms of Polypodium belong to the species P. hydriforme Ussov.

The fine structure of the stinging cells (nematocytes) and stinging capsules (nematocysts) is described for Polypodium hydriforme. a freshwater coclenterate with a prominent endoparasitie stage in its life cycle. All the nematocysts belong to the type of lesser glutinants (atrichous isorhiza) and fall into three size classes. The internal structure of the capsules is similar in the three classes. A novel type of organization of the cnidocil apparatus of the nematocysts is described. The cnidocil lacks a root fibre and its kinctosome sits directly on the operculum of the nematocyst, so that the entire cnidocil apparatus has a radial rather than bilateral symmetry. It is compared with that of other types of nematocytes and its similarity with the mechanoreccptors of the coelentcratcs is noted. The possible place of the Polypodium nematocytes in the evolution of the collar receptors of the Metazoa is discussed.

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