Myxozoa

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Myxozoa
Triactinomyxon stage of Myxobolus cerebralis.
Triactinomyxon stage of Myxobolus cerebralis.
Scientific classification
Kingdom: Animalia
(unranked) Radiata
Phylum: Cnidaria
(unranked) Myxozoa
Grassé, 1970
Groups

Malacosporea
Myxosporea

The Myxozoa (etymology: Greek: myx- "slime" or "mucus" + zoa "animals") are a group of parasitic animals of aquatic environments. Over 1300 species have been described[1] and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. Infection occurs through valved spores. These contain one or two sporoblast cells and one or more polar capsules that contain filaments which anchor the spore to its host. The sporoblasts are then released as a motile form, called an amoebula, which penetrates the host tissues and develops into one or more multinucleate plasmodia. Certain nuclei later pair up, one engulfing another, to form new spores.

Phylogenetics

The Myxozoa were originally considered protozoan,[2] and were included among other non-motile forms in the group Sporozoa.[citation needed] As their distinct nature became clear through 18S ribosomal DNA (rDNA) sequencing, they were relocated in the metazoa. Further classification was hindered by conflicting evidence: although 18S rDNA suggested an affinity with Cnidaria,[3] other rDNA sampled,[4][5] and the HOX genes of two species,[6] were more similar to those of the Bilateria.

The discovery that Buddenbrockia plumatellae, a worm-like parasite up to 2 mm in length (relatively large), is a myxozoan[4] appeared to strengthen the case for a bilaterian origin, as the body plan is superficially similar. Nevertheless, closer examination reveals that Buddenbrockia is not longitudinally symmetrical by two ways, but four, casting doubt on this hypothesis.

Further testing has sourced the first three HOX genes found in previous research (Myx1-3) to the bryozoan Cristatella mucedo, and the fourth (Myx4) to Northern pike. This explained the confusion; original experiments had used contaminated tissue from host organisms, leading to false positives for a position among the Bilateria. More careful cloning of 50 coding genes from Buddenbrockia established the clade as severely modified members of the phylum Cnidaria, with medusozoans as their closest relatives. Similarities between myxozoan polar capsules and cnidarian nematocysts (stinging cells) had been drawn for a long time, but were generally assumed to be the result of convergent evolution.[7]

Taxonomists now recognize the outdated subgroup Actinosporea as a life-cycle phase of Myxosporea.[8]

Species

Some species of myxozoa include:

External links

References

  1. [1]
  2. Štolc, A. (1899). "Actinomyxidies, nouveau groupe de Mesozoaires parent des Myxosporidies". Bull. Int. l'Acad. Sci. Bohème. 12: 1–12.
  3. Smothers, J.F.; et al. (1994). "Molecular evidence that the myxozoan protists are metazoans". Science. 265 (5179): 1719–1721. doi:10.1126/science.8085160. Unknown parameter |month= ignored (help)
  4. 4.0 4.1 A.S. Monteiro; et al. (2002). "Orphan worm finds a home: Buddenbrockia is a Myxozoan". Mol. Biol. Evol. 19 (6): 968. Unknown parameter |month= ignored (help)
  5. J. Zrzavy & V. Hypsa (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. doi:10.1111/j.1096-0031.2003sampled .tb00305.x Check |doi= value (help). Unknown parameter |month= ignored (help)
  6. C. L. Anderson, E. U. Canning & B. Okamura (1999). "A triploblast origin for Myxozoa?". Nature. 392 (6674): 346–347. doi:10.1038/32801. Unknown parameter |month= ignored (help)
  7. E. Jímenez-Guri; et al. (2007). "Buddenbrockia is a cnidarian worm". Science. 317 (116): 116–118. doi:10.1126/science.1142024. Unknown parameter |month= ignored (help)
  8. Kent, M. L., Margolis, L. & Corliss, J.O. (1994). "The demise of a class of protists: taxonomic and nomenclatural revisions proposed for the protist phylum Myxozoa Grasse, 1970." Canadian Journal of Zoology 72(5):932-937.

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