''Helicosporidium'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of colorless, pathogenic algae in the class

Trebouxiophyceae The Trebouxiophyceae, also known as trebouxiophytes, are a class of green algae, in the division Chlorophyta. Members of this class are single-celled, colonial, or multicellular and are found in freshwater, terrestrial or marine habitats worldwid ...

of the

green algae The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ...

. It is a

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

found in the gut of insects, and a close relative of ''

Prototheca ''Prototheca'' is a genus of algae in the family Chlorellaceae. While this genus is a member of the green algae, all ''Prototheca'' no longer have chloroplasts and therefore their photosynthetic ability. Some species can cause protothecosis in ...

''.

History of knowledge

The genus ''Helicosporidium'' was first described in England by David Keilin in 1921. He isolated the parasite from the ceratopogonid fly '' Dasyhelea obscura'', and named the species ''Helicosporidium 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 a

lepidopteran Lepidoptera ( ) or lepidopterans is an order of winged insects which includes butterflies and moths. About 180,000 species of the Lepidoptera have been described, representing 10% of the total described species of living organisms, making it ...

. ''Helicosporidium'' infections, although rare, have been discovered around the world and in a diverse range of host organisms. The unique morphology of ''Helicosporidium'' has made it easy to identify, but difficult to classify. ''Helicosporidium'' has at various points been considered to be a

protozoan Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically ...

or an

ascomycete Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The def ...

fungus A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

. It was not until

molecular phylogenetic Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

s demonstrated that ''Helicosporidium'' was a relative of the

that had lost its

plastid A plastid is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. Plastids are considered to be intracellular endosymbiotic cyanobacteria. Examples of plastids include chloroplasts ...

s and thus the ability to perform

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

. It is closely related and similar to ''

'', another non-photosynthetic genus that is parasitic.

Biology

The key morphological feature of ''Helicosporidium'' 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) stacked on each other, as well as a fourth cell which is elongated and filamentous; the fourth cell wraps around the other three.

Life cycle

Invertebrates become infected with ''Helicosporidium'' after ingestion, less commonly by wounds in their

cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...

. After entering the body, the cysts enter the gut lumen and undergo dehiscence, wherein the cysts split open and release the sporoplasms and filamentous cells. The sporoplasms develop into elongated cells, about 11.5 μm long, which divide to form four spherical vegetative cells. Vegetative cells of ''Helicosporidium'' are characterized by the production of two, four or eight daughter cells in an outer wall (also known as a pellicle), 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.

Hosts

''Helicosporidium'' is currently the only known genus of algae that infects

insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...

s (other algae such as ''

Coccomyxa ''Coccomyxa'' is a genus of green algae in the family Coccomyxaceae. This genus is defined by their small, elliptical to spherical shape, and the presence of a simple parietal chloroplast. These features, along with their occurrence in various l ...

'' infect invertebrates such as

mussel Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and Freshwater bivalve, freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other ...

s and

starfish Starfish or sea stars are Star polygon, star-shaped echinoderms belonging to the class (biology), class Asteroidea (). Common usage frequently finds these names being also applied to brittle star, ophiuroids, which are correctly referred to ...

). ''Helicosporidium'' is known to infect insects in three orders:

Diptera Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advance ...

Coleoptera Beetles are insects that form the Taxonomic rank, order Coleoptera (), in the superorder Holometabola. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 40 ...

, and

Lepidoptera Lepidoptera ( ) or lepidopterans is an order (biology), order of winged insects which includes butterflies and moths. About 180,000 species of the Lepidoptera have been described, representing 10% of the total described species of living organ ...

, but does not appear to be able to infect

orthoptera Orthoptera () is an order of insects that comprises the grasshoppers, locusts, and crickets, including closely related insects, such as the bush crickets or katydids and wētā. The order is subdivided into two suborders: Caelifera – gras ...

ns and

hymenoptera Hymenoptera is a large order of insects, comprising the sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are parasitic. Females typi ...

ns. Isolates of ''Helicosporidium'' are able to be horizontally transferred; for examples, isolates from dipterans can easily infect coleopterans and lepidopterans, and vice versa. It can also infect

mite Mites are small arachnids (eight-legged arthropods) of two large orders, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari. However, most recent genetic analyses do not recover the two as eac ...

s and collembolans, and has been detected in

trematode Trematoda is a Class (biology), class of flatworms known as trematodes, and commonly as flukes. They are obligate parasite, obligate Endoparasites, internal parasites with a complex biological life cycle, life cycle requiring at least two Host ( ...

s and cladocerans.

Genome

Plastids

Despite not performing photosynthesis, ''Helicosporidium'' retains

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

plastid-like organelles. The plastid has not been directly observed in ultra-thin sections, but its whole genome has been sequenced. The plastid genome is very small (about 37.5

kilo-base pair A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...

s long); it lacks all genes that code for proteins that function in photosynthesis, but also has very little

non-coding DNA Non-coding DNA (ncDNA) sequences are components of an organism's DNA that do not encode protein sequences. Some non-coding DNA is transcribed into functional non-coding RNA molecules (e.g. transfer RNA, microRNA, piRNA, ribosomal RNA, and reg ...

. Its

16S rRNA 16S ribosomal RNA (or 16Svedberg, S rRNA) is the RNA component of the 30S subunit of a prokaryotic ribosome (SSU rRNA). It binds to the Shine-Dalgarno sequence and provides most of the SSU structure. The genes coding for it are referred to as ...

sequences are similar to those of ''Prototheca'', and cluster with it in phylogenetic analyses. The overall structure of its genome is somewhat similar to those of

apicomplexa The Apicomplexa (also called Apicomplexia; single: apicomplexan) are organisms of a large phylum of mainly parasitic alveolates. Most possess a unique form of organelle structure that comprises a type of non-photosynthetic plastid called an ap ...

n parasites.

Mitochondria

The

mitochondrial A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

genome of ''Helicosporidium'' is highly similar to that of ''

''. However, its

intron An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e., a region inside a gene."The notion of the cistron .e., gen ...

s are very unusual in that their

group I intron A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...

s are ''trans''-spliced, and contain two

open reading frame In molecular biology, reading frames are defined as spans of DNA sequence between the start and stop codons. Usually, this is considered within a studied region of a prokaryotic DNA sequence, where only one of the six possible reading frames ...

s which may be degenerate maturase/

endonuclease In molecular biology, endonucleases are enzymes that cleave the phosphodiester bond within a polynucleotide chain (namely DNA or RNA). Some, such as deoxyribonuclease I, cut DNA relatively nonspecifically (with regard to sequence), while man ...

genes.

References

External links

AlgaeBase page for Helicosporidium
{{Taxonbar, from=Q5705244 Trebouxiophyceae genera Chlorellaceae Parasites of insects