| Helicosporidium | |
|---|---|
 | |
| Helicosporidium under a light microscope | |
Scientific classification | |
| Clade: | Viridiplantae |
| Division: | Chlorophyta |
| Class: | Trebouxiophyceae |
| Order: | Chlorellales |
| Family: | Chlorellaceae |
| Genus: | Helicosporidium Keilin, 1921[1] |
| Species | |
Helicosporidium is agenus of colorless, pathogenic algae in the classTrebouxiophyceae of thegreen algae.[2][3] It is aparasite found in the gut of insects, and a close relative ofPrototheca.[4]
The genusHelicosporidium was first described in England byDavid Keilin in 1921. He isolated the parasite from theceratopogonid flyDasyhelea obscura, and named the speciesHelicosporidium parasiticum. In 1931, the genus and species names were validated, it was placed in its own order Helicosporidia. In 1970,Helicosporidium was discovered from Argentina infecting alepidopteran.Helicosporidium infections, although rare, have been discovered around the world and in a diverse range of host organisms.[5]
The unique morphology ofHelicosporidium has made it easy to identify, but difficult to classify.Helicosporidium has at various points been considered to be aprotozoan or anascomycetefungus. It was not untilmolecular phylogenetics demonstrated thatHelicosporidium was a relative of thegreen algae that had lost itsplastids and thus the ability to performphotosynthesis. It is closely related and similar toPrototheca, another non-photosynthetic genus that is parasitic.[5]
The key morphological feature ofHelicosporidium is the presence of four-celled structures, termed cysts (also known as spores in the older literature). The cyst is barrel-shaped and contains three ovoid cells (called sporoplasms)[6] stacked on each other, as well as a fourth cell which is elongated and filamentous; the fourth cell wraps around the other three.[5]
Invertebrates become infected withHelicosporidium after ingestion,[5] less commonly by wounds in theircuticle.[7] After entering the body, the cysts enter the gutlumen and undergo dehiscence, wherein the cysts split open and release the sporoplasms and filamentous cells.[5][6]
The sporoplasms develop into elongated cells, about 11.5 μm long, which divide to form four spherical vegetative cells. Vegetative cells ofHelicosporidium are characterized by the production of two, four or eight daughter cells in an outer wall (also known as a pellicle),[5] and may undergo this cycle (termed autosporulation) a number of times. After about 3 to 6 days, the vegetative cells develop into cysts, secrete an outer spore wall (or pellicle), and differentiate into the three sporoplasms and filamentous cell.[6]
Helicosporidium is currently the only known genus of algae that infectsinsects (other algae such asCoccomyxa infect invertebrates such asmussels andstarfish).Helicosporidium is known to infect insects in three orders:Diptera,Coleoptera, andLepidoptera, but does not appear to be able to infectorthopterans andhymenopterans. Isolates ofHelicosporidium are able to be horizontally transferred; for examples, isolates from dipterans can easily infect coleopterans and lepidopterans, and vice versa. It can also infectmites andcollembolans, and has been detected intrematodes andcladocerans.[5]
Despite not performing photosynthesis,Helicosporidium retainsvestigial plastid-like organelles. The plastid has not been directly observed in ultra-thin sections,[5] but its whole genome has been sequenced. The plastid genome is very small (about 37.5kilo-base pairs long); it lacks all genes that code for proteins that function in photosynthesis, but also has very littlenon-coding DNA.[8] Its16S rRNA sequences are similar to those ofPrototheca, and cluster with it in phylogenetic analyses.[2] The overall structure of its genome is somewhat similar to those ofapicomplexan parasites.[8]
Themitochondrial genome ofHelicosporidium is highly similar to that ofPrototheca. However, itsintrons are very unusual in that theirgroup I introns aretrans-spliced, and contain twoopen reading frames which may be degeneratematurase/endonuclease genes.[9]