DESCRIPTION OF THE PHYLUM MYXOZOA
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PHYLUM MYXOZOA LINKS
| Myxozoa (mix-o-ZO-a) is a combination of two Greek roots meaning slime animals [myxo (μύξα) slime; and zoa (ζώο) animal]. The reference is to the former association of this group with the slime molds. |
| INTRODUCTION TO THE MYXOZOA Members of the Myxozoa are intracellular parasites of vertebrates, mainly fishes but do occur in amphibians and certain reptiles. They are taken into a host animal by ingestion of the spore, from which amoeboid forms emerge and penetrate the gut. From there, the amoeba travels in the circulatory system until it arrives in the target organ. At that point, the amoeba penetrates a cell and grows as a plasmodium (text with tooltip) A plasmodium is a multinucleate cell formed by nuclear divisions without cytokinesis. . They produce distinctive spores that are multicellular and develop a nematocyst-like structure (see Figures 1-4). The identification of the Myxozoa as a group of animals is a spectacular example of the coalescence of evidence from many avenues: natural history, cell biology, development, ultrastructure, molecular biology, etc. Classical taxonomy had placed the Myxozoans in a group called the Sporozoa, a class within the Protozoa (e.g. Kudo 1966). Grell (1973) seemed at a loss to deal with the myxozoans (he called them Myxosporidia) and placed them together with the Microsporidia in an uncertain class of Protozoa called “Cnidosporidia”. With the advent of the 5-kingdom system, the classical sporozoan groups rose to phylum-level status within the new kingdom. In that system, Margulis and Schwartz (1988), Sleigh et al. (1984), and Weiser (1985a) raised the Cnidosporidia to phylum-level status. Later, evidence from cell biology, development, and ultrastructure showed that the cnidosporidian groups were very different and did not form a natural grouping. The Myxozoa were elevated to phylum-level status within the kingdom Protista (=Protochtista) [(Lom 1990; Weiser 1985a, and Margulis and Schwartz 1998)], and the Microsporidia were removed from them into a separate phylum (Weiser 1985b). Then, evidence began to mount that Microsporidia were not protists at all but belonged to a group of parasitic fungi (Fast and Keeling 2001; Keeling 2002; Sokolova et al. 2003; and Ragan et al. 2003). The association of Myxozoans with the animal kingdom was first proposed by Weill (1938, cited in Lom 1990) but began to enter the mainstream of systematic biology with Lom (1990) who supported the Weill hypothesis that the remarkable structural similarities between the polar capsules of the Myxozoa and the nematocysts of the cnidarians suggested that the Myxozoa evolved from that animal group. In particular, Weill (1938, cited in Lom 1990) proposed that the Myxozoa evolved from the Narcomedusae whose larvae resemble the Myxozoa in form and in lifestyle (that is, some of the Narcomedusae are also parasitic). Lom (1990) further claimed that the Myxozoa have the highest degree of cellular differentiation among any of the protist groups. This characteristic was also consistent with non-protist origins. Smothers et al. (1994) confirmed the structural evidence with molecular evidence that the Myxozoa are metazoans. Siddall et al. (1995) presented convincing evidence that the Myxozoa were coelenterate animals. Thus, we have placed them into the Animal Kingdom and allied them with the Cnidaria. |
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| FIGURE 1. Photomicrograph of Chloromyxum spores. Image from http://research.amnh.org/~siddall/myxozoa/myxo.html | FIGURE 2. Photomicrograph of a Henneguya spore. Image from http://research.amnh.org/~siddall/myxozoa/myxo.html | FIGURE 3. Photomicrograph of a Myxidium cell. Image from http://research.amnh.org/~siddall/myxozoa/myxo.html | FIGURE 4. Photomicrograph of a Myxobolus spores. Image from http://research.amnh.org/~siddall/myxozoa/myxo.html |
| LITERATURE CITED Fast, N. M. and P. J. Keeling. 2001. Alpha and beta subunits of pyruvate dehydrogenase E1 from the microsporidian Nosema locustae mitochondrion-derived carbon metabolism in microsporidia. Molecular and Biochemical Parasitology. 117(2):201-209. Grell, K. G. 1973. Protozoology. Springer-Verlag. New York. Kudo, R.R. 1966. Protozoology. 5th ed. Charles C. Thomas Publisher. Springfield. Keeling, P.J. 2002. Congruent evidence from α-tubulin and β-tubulin gene phylogenies for a zygomycete origin of microsporidia. Fungal Genetics and Biology. 38: 298-309. Lom, J. 1990. Myxozoa. In: Margulis, L., J.O. Corliss, M. Melkonian, and D.J. Chapman, eds. 1990. Handbook of the Protoctista; the structure, cultivation, habits and life histories of the eukaryotic microorganisms and their descendants exclusive of animals, plants and fungi. Jones and Bartlett Publishers. Boston. pp. 36-52. Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company. New York. Ragan, M.A., C.A. Murphy, and T. G. Rand. 2003. Are Ichthyosporea animals or fungi? Baysean phylogenetic analysis of elongation factor 1α of Ichthyophonus irregularis. Molecular Phylogenetics and Evolution. 29: 550-562. Siddall, M. E., D. S. Martin, D. Bridge, S. S. Desser, and D. K. Cone. 1995. The demise of a phylum of protists: phylogeny of Myxozoa and other parasitic Cnidaria. Journal of Parasitology 81:961-967. Sleigh, M.A., J.D. Dodge and D.J. Patterson. 1984. Kingdom Protista. In: Barnes, R.K.S., ed. A Synoptic Classification of Living Organisms. Sinauer Associates, Inc. Sunderland, Mass. Smothers, J. F., C. D. von Dohlen, L. H. Smith, Jr., and R. D. Spall. 1994. Molecular evidence that the myxozoan protists are metazoans. Science. 265:1719-1721. Sokolova, Y. Y., V. V. Dolgikh, E. V. Morzhina, E. S. Nassonova, I. V. Issi, R. S. Terry, J. E. Ironside, J. E. Smith, and C. R. Vossbrinck. 2003. Establishment of the new genus Paranosema based on the ultrastructure and molecular phylogeny of the type species Paranosema grylli Gen. Nov., Comb. Nov. (Sokolova, Selezniov, Dolgikh, Issi 1994), from the cricket Gryllus bimaculatus. Journal of Invertebrate Pathology. 84: 159-172. Weill, R. 1938. L’interpretation des Cnidosporides et la valeur taxinomique de leur cnidome. Leur cycle compare a la phase larvaire des Narcomeduses Cuninides. Travaux de la Station Zoologique de Wimereux. 13:727-744. Weiser, J. 1985a. Phylum Myxozoa. In: Lee, J.J., S.H. Hunter, and E.C. Bovee, eds. An Illustrated Guide to the Protozoa. Allen Press. Lawrence , Kansas . pp. 384-392. Weiser, J. 1985b. Phylum Microspora. In: Lee, J.J., S.H. Hunter, and E.C. Bovee, eds. An Illustrated Guide to the Protozoa. Allen Press. Lawrence , Kansas . pp. 375-383. |
| By Jack R. Holt and Carlos A. Iudica. Last revised: 02/02/2013 |
