Phylum MICROSPORIDIA Balbiani, 1882

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Introduction

Microsporidia are obligate intracellular parasites that lack mitochondria and form small unicellular spores. The spores have an inner chitinous wall and an outer proteinaceous wall.

Over 1,300 species have been described in invertebrates (especially insects) and lower (rarely higher) vertebrates. The parasites undergo cyclic merogony within host tissues followed by sporogony (often involving plasmotomy prior to sporoblastogenesis). Developmental stages may have single or paired nuclei (diplokaryotic) and they may be surrounded by a membranous sporophorous vesicle (pansporoblast) or lie free in the host cell cytoplasm. All spores contain a unique coiled polar tube which can be extruded to inject the infective sporoplasm into host cells. Infections may be disseminated throughout host tissues or they may cause focal lesions, inflammation and granulomas. Some species cause extensive hypertrophy of the host cell producing large xenomas.

Microsporidia were long considered to be a primitive basal group of protista due to their lack of organelles of energy transduction. Molecular phylogenetic studies, however, have revealed many similarities to bacteria (especially in ribosomes) and fungi (notably in biochemical pathways and structural components). They are now classified as a special fungal group within the supergroup Holomycota (Nucletmycea) in the Opisthokonta (cf. Adl et al. 2012).

Diagnosis

Microsporidia form unicellular spores with coiled polar tubes.

General References

Adl, S.M., Simpson, A.G.B., Lane, C.E., Lukes, J., Bass, D., Bowser, S.S., Brown, M.W., Burki, F., Dunthorn, M., Hampl, V., Heiss, A., Hoppenrath, M., Lara, E., Le Gall, L., Lynn, D.H., McManus, H., Mitchell, E.A.D., Mozley-Stanridge, S.E., Parfrey, L.W., Pawlowski, J., Rueckert, S., Shadwick, L., Schoch, C.L., Smirnov, A. & Spiegel, F.W. 2012. The revised classification of eukaryotes. Journal of Eukaryotic Microbiology 59: 429-493

Balbiani, G. 1882. Sur les microsporidies ou psorospermies des articules. Comptes Rendus de l'Academie des Sciences. Paris 95: 1168-1171

Canning, E.U. 1990. Phylum Microspora. Chapt. 5. pp. 53-72 in Margulis, L., Corliss, J.O., Melkonian, M. & Chapman, D.J. (eds). Handbook of Protoctista. Boston : Jones & Bartlett Publishers.

Sprague, V. 1977. Classification and phylogeny of the Microsporidia. In, Bulla L.A. & Cheng, T.C. Comparative pathobiology, Vol. 2: Systematics of the microsporidia. New York, NY : Plenum Press xi + 510 pp.

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