Introduction | 1 | 2 | 3

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In 1999, a Helicosporidium sp. was discovered in larvae of the black fly Simulium jonesi Stone & Snoddy (Simuliidae, Diptera). This isolate was demonstrated to grow under both in vitro and in vivo conditions (Boucias et al. 2001). In vivo, the ingested helicosporidial cysts dehisced in the midgut lumen and released the ovoid cells and the filamentous cells.The filamentous cells attached to the peritrophic matrix and penetrated the midgut epithelium initiating infection. The in vitro growth of Helicosporidium sp. was reminiscent of that reported for unicellular, achlorophytic algae belonging to the genus Prototheca. Both the genera Helicosporidium and Prototheca are characterized by a vegetative growth that consists of cell divisions inside a membrane. Four, eight or sixteen daughter cells are produced inside this pellicle and are eventually released. Such cell divisions result in the accumulation of both round daughter cells and empty pellicles. The suggestion that Helicosporidium was related to algae has been confirmed by molecular analysis (Tartar et al. 2002). The 18S, 26S, 5.8S regions of the Helicosporidium ribosomal DNA, as well as some partial sequences of the actin and tubulin genes, were sequenced. Comparative analyses of these nucleotide sequences were performed in order to evaluate the position of Helicosporidium sp. within the phylogeny of eukaryotes. All trees depicted Helicosporidium sp. as a green alga (Chlorophyta), and as sister taxon to the genus Prototheca (class Trebouxiophyceae). This association was always supported by significant bootstrap values. On the basis of this phylogenetic analysis, Helicosporidium sp. is clearly neither a protozoan nor a fungus, but represents the first described algal invertebrate pathogen.

Introduction | 1 | 2 | 3