| 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 a genus of colorless, pathogenic algae in the class Trebouxiophyceae of the green algae. [2] [3] It is a parasite found in the gut of insects, and a close relative of Prototheca . [4]
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. Helicosporidium infections, although rare, have been discovered around the world and in a diverse range of host organisms. [5]
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 or an ascomycete fungus. It was not until molecular phylogenetics demonstrated that Helicosporidium was a relative of the green algae that had lost its plastids and thus the ability to perform photosynthesis. It is closely related and similar to Prototheca , another non-photosynthetic genus that is parasitic. [5]
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) [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 with Helicosporidium after ingestion, [5] less commonly by wounds in their cuticle. [7] 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. [5] [6]
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), [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 infects insects (other algae such as Coccomyxa infect invertebrates such as mussels and starfish). Helicosporidium is known to infect insects in three orders: Diptera, Coleoptera, and Lepidoptera, but does not appear to be able to infect orthopterans and hymenopterans. 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 mites and collembolans, and has been detected in trematodes and cladocerans. [5]
Despite not performing photosynthesis, Helicosporidium retains vestigial 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 37.5 kilo-base pairs long); it lacks all genes that code for proteins that function in photosynthesis, but also has very little non-coding DNA. [8] Its 16S rRNA sequences are similar to those of Prototheca, and cluster with it in phylogenetic analyses. [2] The overall structure of its genome is somewhat similar to those of apicomplexan parasites. [8]
The mitochondrial genome of Helicosporidium is highly similar to that of Prototheca . However, its introns are very unusual in that their group I introns are trans-spliced, and contain two open reading frames which may be degenerate maturase/endonuclease genes. [9]
The Apicomplexa 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 apicoplast—with an apical complex membrane. The organelle's apical shape is an adaptation that the apicomplexan applies in penetrating a host cell.
Chlorophyta is a division of green algae informally called chlorophytes.
The dinoflagellates are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also common in freshwater habitats. Their populations vary with sea surface temperature, salinity, and depth. Many dinoflagellates are photosynthetic, but a large fraction of these are in fact mixotrophic, combining photosynthesis with ingestion of prey.
A plastid is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms. Plastids are considered to be intracellular endosymbiotic cyanobacteria.
The green algae 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 (Embryophytes) have emerged deep within the charophytes as a sister of the Zygnematophyceae. Since the realization that the Embryophytes emerged within the green algae, some authors are starting to include them. The completed clade that includes both green algae and embryophytes is monophyletic and is referred to as the clade Viridiplantae and as the kingdom Plantae. The green algae include unicellular and colonial flagellates, most with two flagella per cell, as well as various colonial, coccoid (spherical), and filamentous forms, and macroscopic, multicellular seaweeds. There are about 22,000 species of green algae, many of which live most of their lives as single cells, while other species form coenobia (colonies), long filaments, or highly differentiated macroscopic seaweeds.
Zooxanthellae is a colloquial term for single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including demosponges, corals, jellyfish, and nudibranchs. Most known zooxanthellae are in the genus Symbiodinium, but some are known from the genus Amphidinium, and other taxa, as yet unidentified, may have similar endosymbiont affinities. "Zooxanthella" was originally a genus name given in 1881 by Karl Brandt to Zooxanthella nutricula which has been placed in the Peridiniales. Another group of unicellular eukaryotes that partake in similar endosymbiotic relationships in both marine and freshwater habitats are green algae zoochlorellae.
Charophyta is a group of freshwater green algae, called charophytes, sometimes treated as a division, yet also as a superdivision or an unranked clade. The terrestrial plants, the Embryophyta emerged deep within Charophyta, possibly from terrestrial unicellular charophytes, with the class Zygnematophyceae as a sister group.
Protothecosis, otherwise known as Algaemia, is a disease found in dogs, cats, cattle, and humans caused by a type of green alga known as Prototheca that lacks chlorophyll and enters the human or animal bloodstream. It and its close relative Helicosporidium are unusual in that they are actually green algae that have become parasites. The two most common species are Prototheca wickerhamii and Prototheca zopfii. Both are known to cause disease in dogs, while most human cases are caused by P. wickerhami. Prototheca is found worldwide in sewage and soil. Infection is rare despite high exposure, and can be related to a defective immune system. In dogs, females and Collies are most commonly affected.
The Archaeplastida are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group glaucophytes. It also includes the non-photosynthetic lineage Rhodelphidia, a predatorial (eukaryotrophic) flagellate that is sister to the Rhodophyta, and probably the microscopic picozoans. The Archaeplastida have chloroplasts that are surrounded by two membranes, suggesting that they were acquired directly through a single endosymbiosis event by phagocytosis of a cyanobacterium. All other groups which have chloroplasts, besides the amoeboid genus Paulinella, have chloroplasts surrounded by three or four membranes, suggesting they were acquired secondarily from red or green algae. Unlike red and green algae, glaucophytes have never been involved in secondary endosymbiosis events.
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 or terrestrial habitats worldwide. Many taxa in the Trebouxiophyceae form symbiotic relationships with other organisms; in particular, the majority of phycobionts within lichens are trebouxiophytes. A number of taxa have also lost the ability to photosynthesize, and have evolved to become parasitic; examples include Prototheca and Helicosporidium.
A microbial cyst is a resting or dormant stage of a microorganism, that can be thought of as a state of suspended animation in which the metabolic processes of the cell are slowed and the cell ceases all activities like feeding and locomotion. Many groups of single-celled, microscopic organisms, or microbes, possess the ability to enter this dormant state.
Protozoan infections are parasitic diseases caused by organisms formerly classified in the kingdom Protozoa. These organisms are now classified in the supergroups Excavata, Amoebozoa, Harosa, and Archaeplastida. They are usually contracted by either an insect vector or by contact with an infected substance or surface.
Ochrophytes, also known as heterokontophytes or stramenochromes, are a group of algae. They are the photosynthetic stramenopiles, a group of eukaryotes, organisms with a cell nucleus, characterized by the presence of two unequal flagella, one of which has tripartite hairs called mastigonemes. In particular, they are characterized by photosynthetic organelles or plastids enclosed by four membranes, with membrane-bound compartments called thylakoids organized in piles of three, chlorophyll a and c as their photosynthetic pigments, and additional pigments such as β-carotene and xanthophylls. Ochrophytes are one of the most diverse lineages of eukaryotes, containing ecologically important algae such as brown algae and diatoms. They are classified either as phylum Ochrophyta or Heterokontophyta, or as subphylum Ochrophytina within phylum Gyrista. Their plastids are of red algal origin.
Protozoa 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, protozoans were regarded as "one-celled animals".
Vaucheria litorea is a species of yellow-green algae (Xanthophyceae). It grows in a filamentous fashion. V. litorea is a common intertidal species of coastal brackish waters and salt marshes of the Northern Atlantic, along the coasts of Europe, North America and New Zealand. It is also found in the Eastern Pacific coasts of Washington state. It is found to be able to tolerate a large range of salinities, making it euryhaline.
Amoebidiidae is a family of single-celled eukaryotes, previously thought to be zygomycete fungi belonging to the class Trichomycetes, but molecular phylogenetic analyses place the family with the opisthokont group Mesomycetozoea. The family was originally called Amoebidiaceae, and considered the sole family of the fungal order Amoebidiales that included two genera: Amoebidium and Paramoebidium. However, Amoebidiidae is now monogeneric as it was recently emended to include only Amoebidium. Species of Amoebidium are considered obligate symbionts of freshwater-dwelling arthropod hosts such as midge larvae and water fleas (Daphnia). However, because Amoebidium species attach to the exoskeleton (exterior) of the host and grow in axenic culture, at least some species may be facultative symbionts.
Alexandrium is a genus of dinoflagellates. It contains some of the dinoflagellate species most harmful to humans, because it produces toxic harmful algal blooms (HAB) that cause paralytic shellfish poisoning (PSP) in humans. There are about 30 species of Alexandrium that form a clade, defined primarily on morphological characters in their thecal plates.
The Phragmoplastophyta are a proposed sister clade of the Klebsormidiaceae in the Streptophyte/Charophyte clade. The Phragmoplastophyta consist of the Charophyceae and another unnamed clade which contains the Coleochaetophyceae, Zygnematophyceae, Mesotaeniaceae, and Embryophytes. It is an important step in the emergence of land plants within the green algae. It is equivalent to the ZCC clade/grade, cladistically granting the Embryophyta.
Maullinia is a genus of intracellular, phytomyxid parasites found across the Southern Hemisphere though primarily in Chile, The Prince Edward Islands, South Africa, Australia, and New Zealand. These parasites infiltrate the cells of their brown algal hosts via cytoplasmic extensions called plasmodia that divide synchronously, becoming increasingly multi-nucleate and engulfing the host cell organelles as they grow. Eventually, as the plasmodia fill the entire cell volume, the host cells become hypertrophied and grow to 3- 4x their original size, showing up as swollen appendages or galls on the host tissue at a macroscopic level. These swollen regions will burst alongside the mature Maullinia plasmodia, releasing biflagellated zoospores to the inter- and extracellular space to disperse the infection further. Zoospores can come from sporangial plasmodia, as in M. ectocarpii, or from resting spores, as in M. braseltonii.
Chrompodellids are a clade of single-celled protists belonging to the Alveolata supergroup. It comprises two different polyphyletic groups of flagellates: the colpodellids, phagotrophic predators, and the chromerids, photosynthetic algae that live as symbionts of corals. These groups were independently discovered and described, but molecular phylogenetic analyses demonstrated that they are intermingled in a clade that is the closest relative to Apicomplexa, and they became collectively known as chrompodellids. Due to the history of their research, they are variously known in biological classification as Chromerida or Colpodellida (ICZN)/Colpodellales (ICN).