To date, there is only one named species of Helicosporidia; Helicosporidium parasiticum. It has been initially described and named by Keilin (1921), who detected this protist in larvae of Dasyhelea obscura Winnertz (Diptera: Ceratopogonidae) collected in England. He examined the new parasite thoroughly and attempted to infer its life history from his observations. He described a vegetative growth characterized by very active multiplications of helicosporidial cells inside the body cavity of the host, and noticed that these “schizogonic multiplications” were followed by the formation of what he called spores. Keilin noted that the spores (cysts) are very easily recognized: they consist of the assembly, inside an external membrane, of three ovoid cells (named by Keilin “sporozoites”) and one peripheral, spiral, filamentous cell. These features, especially the highly characteristic filamentous cell, have since remained the principal diagnostic for identification of a Helicosporidium sp. Keilin was able to describe and characterize structurally the new genus Helicosporidium, and the new species H. parasiticum. He was also able to present a hypothetical life cycle of this protist based on his microscopy observations. He suggested that the spores (or cysts) break open in the host hemocoel, releasing the filamentous cell and the three “sporozoites”, which he proposed are the infective forms of H. parasiticum. Keilin believed that H. parasiticum belonged to the Protozoa, and compared this isolate with members of various clades: Cnidiosporidia (which, at that time, included Microsporidia such as Nosema bombycis Naegeli), Haplosporidia, Serumsporidia and Mycetozoa. He concluded that the genus Helicosporidium differed markedly not only from all these groups, but also from all the protists known at that time. He finally proposed that Helicosporidium “forms a new group, which may be temporarily included in the group of the Sporozoa”.
Following the discovery of another isolate of Helicosporidium parasiticum in a larva of Hepialis pallens (Hepialidae, Lepidoptera), another taxonomic position was proposed for the group Helicosporidia. Based on observation of this new isolate as well as the original specimen described by Keilin, Weiser (1970) claimed that the Helicosporidia are best placed among the lower Fungi. He argued that the spore characteristics are too much different from what is found in Protozoa, but are similar in some aspects to primitive Fungi, such as insect pathogens of the genus Monosporella, classified as Nematosporoideae inside the Saccharomycetaceae (primitive Ascomycetes).
In the ealy 1970s Helicosporidium parasiticum was isolated from larvae and adults of the beetle Carpophilus mutilatus Erichson (Nitidulidae, Coleoptera). This isolate was infectious per os to 18 species of arthropods, including three orders of insects (Lepidoptera, Coleoptera, Diptera) and one family of mites (Kellen and Lindegren 1973). Contrarily, species of Orthoptera, Hymenoptera and Diptera were not susceptible to this isolates. The spores (cysts), present in the host artificial diet, were ingested and released the three round cells and the filamentous cells in the host midgut. After 24 h, helicosporidial cells appeared in the host hemolymph and grew vegetatively. The vegetative growth is characterized by cell division that occurs within a pellicle. After division, the pellicle ruptures and releases the daughter cells (4 or 8). Empty pellicles and daughter cells eventually fill the entire host hemocoel. Daughter cells then develop into spores, in which the filamentous cell differentiates and encircles the three round cells. Additional studies reported the presence of Helicosporidium sp. in new host species, such as crustaceans, mites and collembola, and trematodes.
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.