Submitted to: Society for Invertebrate Pathology Annual Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 8/18/2002
Publication Date: 8/18/2002
Citation: BECNEL, J.J. CHARACTERISTICS OF THE MICROSPORIDIA; REASONS TO PONDER THAT MICROSPORIDIA ARE HIGHLY EVOLVED FUNGI. PROCEEDINGS OF THE ANNUAL MTG. OF THE SOCIETY FOR INVERTEBRATE PATHOLOGY. 2002. p.248-250.
Technical Abstract: Nosema bombycis, the first described species of microsporidia and etiological agent of Pebrine disease in silkworms, was originally identified as yeast and placed in the Schizomycetes (Naegeli, 1857). Balbiani (1882), after a series of studies on this organism, decided that it lacked essential characters of the Schizomycetes but had affinities to the Sporozoa Leuckart, 1879. He established the taxon "Microsporidies" for N. bombycis and the few known but unnamed species of microsporidia and the group has not been considered related to the Fungi since that time. Recent molecular analyses, however, have determined that the group possesses several gene sequences that suggest this group is closely related to the Fungi (Keeling, 2001; Hirt et al., 1999). Since then, other biological features of the microsporidia and fungi have been analyzed to determine common traits or distinct differences that may provide additional insight on this relationship. However, these comparisons are confounded by widespread convergence and rapid divergence of these groups. Therefore, the following discussion is an attempt, in a very general way, to examine some of the similarities and differences between microsporidia and fungi. It is hoped this will stimulate investigations on both groups to help clarify the many points where information is unclear or lacking. The microsporidia are a large group of strictly obligate, intracellular parasites that infect most animal groups (from Protists to Man) but are not known to infect plants or fungi (Becnel and Andreadis, 1999; Vavra and Larsson, 1999). Only the spores of microsporidia are walled and spores examined thus far contain large amounts of trehalose. Vegetative growth is by non-motile amoeba-like stages (often multinucleate) with simple plasma membranes. Although variable is some respects, all microsporidian spores are definitively and uniquely characterized by containing a coiled polar filament. At germination, the polar filament is inverted to become a tube for transport of the sporoplasm into the host cell. The fungi constitute an extremely large and diverse group of heterotrophic organisms devoid of chlorophyll, have a cell wall, are non-motile (some species have motile reproductive cells) and reproduce by means of a tremendous variety of spore types (Alexopoulos et al., 1996). Fungi are usually filamentous and multicellular and glycogen is the primary carbohydrate storage product (trehalose in yeast and lichens). Obligate parasitic fungi infect plants, animals and in some cases even other fungi. Cytological Structures: Mitochondria, perixosomes, and lysosomes have not been identified in microsporidia but are known to occur in the fungi. Both groups contain paramural bodies. The Golgi apparatus of microsporidia are said to be of a special type mainly consisting of a vesicular meshwork without stacked cisternae. While some fungi possess typical stacked cisternae, most groups have Golgi which consists of a single cisternal element (Alexopoulos et al., 1996). Centrioles are absent in all microsporidia and most fungi. In microsporidia, spindle fiber attachment is to a spindle plaque located on the nuclear envelope with small associated polar bodies often connected to the spindle plaque by filaments. In the fungi, centrioles are present only in the Chytridomycota and are composed of nine triplets. In most true fungi, centrioles are replaced by spindle pole bodies (SPB) and multivesicular bodies. Ribosomes of the fungi are of the typical eukaryotic size of 80S while the ribosomes in the microsporidia are prokaryote-size (70S) consisting of a large (23S) and small (16S) subunits. They lack the 5.8S subunit but a homologuous region is found at beginning of the 23S subunit(Vossbrinck and Woese 1986). Invasion: Spores in both microsporidia and fungi initiate infection in a host. Some