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Key words: Microsporidia, Marssoniella, Cyclops vicinus, phylogeny, ultrastructure, Trichoptera
Abstract. Marssoniella elegans Lemmermann, 1900, a parasite of ovarial tissues of the copepod Cyclops vicinus Uljanin, 1875, was studied as a representative of aquatic-clade microsporidia which form "heteroinfectious spores" (spores not infective to the original host as opposed to "homoinfectious spores" which are infective for the original host) and which thus should require an alternate host. Several structural characters of this microsporidian are similar to those of copepod morphs of microsporidia infecting mosquitoes. However, small subunit ribosomal DNA phylogeny indicates that caddis flies (Insecta, Trichoptera) might be the alternate hosts of Marssoniella. Ultrastructural data obtained are used to redefine the genus Marssoniella Lemmermann, 1900 and its type species Marssoniella elegans.
Aquatic microcrustacea (copepods and cladocera) are frequent hosts of microsporidia. About nine genera of microsporidia and about 50 species have been described from copepods (Bronnvall and Larsson 2001), and 9 genera with about 30 species have been described from cladocera (Larsson et al. 1996). However, the study of microsporidia goes beyond recording microsporidian diversity and establishing microsporidian classification. The challenge is to understand the biology of microcrustacean microsporidia, especially the poorly known host specificity, maintenance in habitats, and modes of transmission. Several past and recent attempts to infect aquatic microcrustacea with spores isolated from a host and fed to the same host species have failed (Vávra 1964, Green 1974, Refardt et al. 2002, Ebert unpublished). This has demonstrated that some aquatic microsporidia are different from their terrestrial counterparts, in which case perorally fed spores usually cause infection in the original host species. No explanation for the "non-infectivity" of spores in some microcrustacea was available until 1985 when it was shown that several copepod microsporidia (now known to represent the genera Amblyospora, Hyalinocysta and Parathelohania) are in fact mosquito parasites which are using copepods as intermediate hosts. The discovery that some microsporidia have a dixenous life cycle, that they form several kinds of spores specific for the respective hosts, was a major breakthrough in our understanding of microsporidian host specificity, life cycles and microsporidian disease epizootiology (Andreadis 1985, Sweeney et al. 1985).
Phylogeny based on the analyses of small subunit rRNA genes (SSU rRNA) has demonstrated that the mosquito microsporidia that use copepods as secondary...