J. I. Ronny Larsson and Marianne Lebbad
The microsporidia constitute a phylum of small, spore-forming unicellular parasites. New species are being described in rapid succession, and it is obvious that about 1000 species known today are only a minor fraction. The real number probably exceeds one million. Their history started in 1857 with Nosema bombycis, the famous agent of the 'pébrine' disease of the silk moth. However, before long it was apparent that microsporidia were not restricted to insect hosts. At present microsporidia have been found in all major groups of animals. They are especially common and important parasites of insects, crustaceans, and fish. Approximately 10% of the species are parasites of vertebrates, with about 100 species from fish. Thus, far a little more than 10 species have been described from homoiothermous vertebrates (Canning and Lom 1986). Many of the species, at least of invertebrates, appear to be host specific.
Even if N. bombycis was originally described as a fungus, microsporidia have usually been considered to be protozoa. Cavalier-Smith (1983), apparently erroneously, included them among the primitive Archaezoa. Recent information suggests that the microsporidia have fungal affinities (discussed by Weiss and Vossbrinck 1999).
In the light microscopic era the minute size was a limitation. The first revolution came with the electron microscope, which revealed a strange and unique cytological world. The explosive appearance of new species in the last two decades is a product of electron microscopy, and ultrastructural documentation is nowadays necessary when describing new microsporidia. Recently, molecular biology has started a second revolution. So far this technique has not given us a huge number of new species, but it has facilitated identification and revealed relationships, and will undoubtedly contribute profoundly to our understanding of the evolution of microsporidia. Recently, classification and identification have been surveyed by Sprague (1977), Weiser (1977), Issi (1986), Larsson (1986, 1988, 1999), Canning (1989), and Sprague et al. (1992).
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