|Two specimens of free-living Polypodium|
Unusual characteristics have led to much controversy regarding the phylogenetic position of Polypodium within metazoans.
Polypodium has traditionally been considered a cnidarian because it possesses nematocysts, the stinging structures characteristic of this phylum. However, molecular phylogenetic studies using 18S rDNA sequence data challenged this interpretation, and indicated that Polypodium is a close relative to myxozoans and that 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, and myxozoans have in the mean time also been firmly classified within cnidarians.
Evans et al. (2008) 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. This support the placement of Polypodium within Cnidaria. This accords with the fact that Polypodium possesses nematocysts and a cnidarian-like body plan. Myxozoans are now recognized as cnidarians as well.
Description and life cycleEdit
Polypodium hydriforme is an endocellular parasite with unusual life cycle, peculiar morphology, and high rates of DNA evolution. Polypodium spends most of its life inside the oocytes of acipenseriform fishes (sturgeons and paddlefish). Its hosts include Acipenser ruthenus, Polyodon spathula and Scaphirhynchus platorynchus. During this time, Polypodium develops from a binucleate cell into an inside-out planuliform larva and then into an elongate inside-out stolon; the epidermal cell layer is located internal to the body and the gastrodermis is located externally. The embryo, larva and stolon are surrounded by a protective polyploid cell, which also functions in digestion. Just prior to host spawning, Polypodium everts to the normal position of cell layers, revealing tentacles scattered along the stolon. During eversion, the yolk of the host oocyte fills the gastral cavities of the parasite, supplying the future free-living stage with nutrients. Finally, upon emerging from the host egg in fresh water, the free-living stolon fragments into individual medusoid-like forms that go on to multiply by means of longitudinal fission, form sexual organs, and ultimately infect host fish with their gametophores.
Habitat of Polypodium hydriforme is freshwater.
Although the fresh water habitat of Polypodium is unusual for cnidarians, it is not unheard of, especially within hydrozoans. For instance, the model organism Hydra and the jellyfish Craspedacusta sowerbii are both exclusively fresh-water hydrozoans. Hydra and Craspedacusta sowerbii are distantly related and they are not closely related to Polypodium. Thus, it appears that in the evolution of cnidarians, invasion to fresh-water habitats has happened at least three separate times.
This article incorporates CC-BY-2.0 text from the reference.
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- Usov (1885). Trudy Obschtsch. est. Imp. Kasan. Univ. 14, Abt. 6: 1.
- Evans N. M.; Lindner A.; Raikova E. V.; Collins A. G.; Cartwright P. (2008). "Phylogenetic placement of the enigmatic parasite, Polypodium hydriforme, within the Phylum Cnidaria". BMC Evolutionary Biology. 8: 139. doi:10.1186/1471-2148-8-139. PMC 2396633. PMID 18471296.
- Zrzavý J. & Hypša V. (2003). "Myxozoa, Polypodium, and the origin of the Bilateria: The phylogenetic position of "Endocnidozoa" in light of the rediscovery of Buddenbrockia". Cladistics 19(2): 164–169. doi:10.1111/j.1096-0031.2003.tb00305.x
- E. Jímenez-Guri; et al. (July 2007). "Buddenbrockia is a cnidarian worm". Science. 317 (116): 116–118. doi:10.1126/science.1142024. PMID 17615357.