Microsporidia
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Microsporidia are a group of spore-forming unicellular parasites. These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore.<ref name="Franzen, C. 2005">Franzen, C. (2005). How do Microsporidia invade cells?. Folia Parasitologica, 52(1–2), 36–40. doi.org/10.14411/fp.2005.005</ref> They were once considered protozoans or protists, but are now known to be fungi,<ref>Template:Cite journal</ref> or a sister group to true fungi.<ref name=Silar2016>Template:Cite book</ref> These fungal microbes are obligate eukaryotic parasites that use a unique mechanism to infect host cells.<ref name="Franzen, C. 2005"/> They have recently been discovered in a 2017 Cornell study to infect Coleoptera (beetles) on a large scale. So far, about 1500 of the probably more than one million<ref name="Hawksworth, D. 2001">Template:Cite journal</ref> species are named. Microsporidia are restricted to animal hosts, and all major groups of animals host microsporidia. Most infect insects, but they are also responsible for common diseases of crustaceans and fish. The named species of microsporidia usually infect one host species or a group of closely related taxa. Approximately 10 percent of the known species are parasites of vertebrates—several species, most of which are opportunistic, can infect humans, in whom they can cause microsporidiosis.
After infection they influence their hosts in various ways and all organs and tissues are invaded, though generally by different species of specialised microsporidia. Some species are lethal, and a few are used in biological control of insect pests. Parasitic castration, gigantism, or change of host sex are all potential effects of microsporidian parasitism (in insects). In the most advanced cases of parasitism the microsporidium rules the host cell completely and controls its metabolism and reproduction, forming a xenoma.<ref name="lund">Ronny Larsson, Lund University (Department of Cell and Organism Biology) Cytology and taxonomy of the microsporidia Template:Webarchive 2004.</ref>
Replication takes place within the host's cells, which are infected by means of unicellular spores. These vary from 1–40 μm, making them some of the smallest eukaryotes.Template:Citation needed Microsporidia that infect mammals are 1.0–4.0 μm.<ref name=pmid15777637>Template:Cite journal</ref> They also have the smallest eukaryotic genomes.
The terms "microsporidium" (pl. "microsporidia") and "microsporidian" are used as vernacular names for members of the group. The name Microsporidium Balbiani, 1884<ref>Balbiani, G. 1884. Les Psorospermies des Articulés ou Microsporidies, pp. 150-168, 184. In: Leçons sur les sporozoaires. Paris: Doin, [1].</ref> is also used as a catchall genus for incertae sedis members.<ref>Hoffman, G. (1999). Parasites of North American Freshwater Fishes, 2nd edn, University of California Press, Berkeley, California, USA, p. 89, [2].</ref>
MorphologyEdit
Microsporidia lack mitochondria, instead possessing mitosomes. They also lack motile structures, such as flagella.
Microsporidia produce highly resistant spores, capable of surviving outside their host for up to several years. Spore morphology is useful in distinguishing between different species. Spores of most species are oval or pyriform, but rod-shaped or spherical spores are not unusual. A few genera produce spores of unique shape for the genus.
The spore is protected by a wall, consisting of three layers:
- an outer electron-dense exospore
- a median, wide and seemingly structureless endospore, containing chitin
- a thin internal plasma membrane
In most cases there are two closely associated nuclei, forming a diplokaryon, but sometimes there is only one.
The anterior half of the spore contains a harpoon-like apparatus with a long, thread-like polar filament, which is coiled up in the posterior half of the spore. The anterior part of the polar filament is surrounded by a polaroplast, a lamella of membranes. Behind the polar filament, there is a posterior vacuole.<ref name="lund"/>
InfectionEdit
In the gut of the host the spore germinates; it builds up osmotic pressure until its rigid wall ruptures at its thinnest point at the apex. The posterior vacuole swells, forcing the polar filament to rapidly eject the infectious content into the cytoplasm of the potential host. Simultaneously the material of the filament is rearranged to form a tube which functions as a hypodermic needle and penetrates the gut epithelium.
Once inside the host cell, a sporoplasm grows, dividing or forming a multinucleate plasmodium, before producing new spores. The life cycle varies considerably. Some have a simple asexual life cycle,<ref name="pmid17394631">Template:Cite journal</ref> while others have a complex life cycle involving multiple hosts and both asexual and sexual reproduction. Different types of spores may be produced at different stages, probably with different functions including autoinfection (transmission within a single host).
Medical implicationsEdit
In animals and humans, microsporidia often cause chronic, debilitating diseases rather than lethal infections. Effects on the host include reduced longevity, fertility, weight, and general vigor. Vertical transmission of microsporidia is frequently reported.
In the case of insect hosts, vertical transmission often occurs as transovarial transmission, where the microsporidian parasites pass from the ovaries of the female host into eggs and eventually multiply in the infected larvae. Amblyospora salinaria n. sp. which infects the mosquito Culex salinarius Coquillett, and Amblyospora californica which infects the mosquito Culex tarsalis Coquillett, provide typical examples of transovarial transmission of microsporidia.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Microsporidia, specifically the mosquito-infecting Vavraia culicis, are being explored as a possible 'evolution-proof' malaria-control method.<ref>Template:Cite book</ref> Microsporidian infection of Anopheles gambiae (the principal vector of Plasmodium falciparum malaria) reduces malarial infection within the mosquito, and shortens the mosquito lifespan.<ref>Template:Cite journal</ref> As the majority of malaria-infected mosquitoes naturally die before the malaria parasite is mature enough to transmit, any increase in mosquito mortality through microsporidian-infection may reduce malaria transmission to humans. In May 2020, researchers reported that Microsporidia MB, a symbiont in the midgut and ovaries of An. arabiensis, significantly impaired transmission of P. falciparum, had "no overt effect" on the fitness of host mosquitoes, and was transmitted vertically (through inheritance).<ref>Template:Cite journal</ref>
ClinicalEdit
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Template:Expand section Microsporidian infections of humans sometimes cause a disease called microsporidiosis. At least 14 microsporidian species, spread across eight genera, have been recognized as human pathogens. These include Trachipleistophora hominis.<ref>Template:Cite journal</ref>
As hyperparasitesEdit
Microsporidia can infect a variety of hosts, including hosts which are themselves parasites. In that case, the microsporidian species is a hyperparasite, i.e. a parasite of a parasite. As an example, more than eighteen species are known which parasitize digeneans (parasitic flatworms). These digeneans are themselves parasites in various vertebrates and molluscs. Eight of these species belong to the genus Nosema.<ref name=Toguebaye>Template:Cite journal Template:Open access</ref> Similarly, the microsporidian species Toguebayea baccigeri is a parasite of a digenean, the faustulid Bacciger israelensis, itself an intestinal parasite of a marine fish, the bogue Boops boops (Teleostei, Sparidae).<ref name="Miquel2022">Template:Cite journal Template:Open access</ref>
GenomesEdit
Microsporidia have the smallest known (nuclear) eukaryotic genomes. The parasitic lifestyle of microsporidia has led to a loss of many mitochondrial and Golgi genes, and even their ribosomal RNAs are reduced in size compared with those of most eukaryotes. As a consequence, the genomes of microsporidia are much smaller than those of other eukaryotes. Currently known microsporidial genomes are 2.5 to 11.6 Mb in size, encoding from 1,848 to 3,266 proteins which is in the same range as many bacteria.<ref name="Conradi2013">Template:Cite journal</ref>
Horizontal gene transfer (HGT) seems to have occurred many times in microsporidia. For instance, the genomes of Encephalitozoon romaleae and Trachipleistophora hominis contain genes that derive from animals and bacteria, and some even from fungi.<ref name="Conradi2013" />
DNA repairEdit
The Rad9-Rad1-Hus1 protein complex (also known as the 9-1-1 complex) in eukaryotes is recruited to sites of DNA damage where it is considered to help trigger the checkpoint-signaling cascade. Genes that code for heterotrimeric 9-1-1 are present in microsporidia.<ref name = Santos2022>Template:Cite journal</ref> In addition to the 9-1-1 complex, other components of the DNA repair machinery are also present indicting that repair of DNA damage likely occurs in microsporidia.<ref name = Santos2022/>
PhylogenyEdit
Phylogeny of Rozellomycota. Backbone from SSU rRNA in Wijayawardene et al. (2020);<ref name="Wijayawardene_2020">Template:Cite journal</ref> Mitosporidium, Morellospora, and Microsporidia s.l. from SSU rRNA in Corsaro et al. (2020);<ref name="Morellospora">Template:Cite journal</ref> internals of Microsporidia s.s. from SSU rRNA in Bojko et al. (2022).<ref name=Bojko>Template:Cite journal</ref>
Node 1: The "Orphan lineage" includes Hamiltosporidium + Astathelohania. Neofabelliforma and Areospora are possible inclusions but support is weak.<ref name=Bojko/>
Note 2: The presence of Enterocytozoonida (Mrazekiidae + Enterocytozoonidae) indicates a "splitter" view of Nosematida in Bojko et al. (2022). In the 2024 Outline, the two families are subsumed into Nosematida.
Note 3: Amblyosporida has also been split into Caudosporida (Caudosporidae) and a smaller Amblyosporida (Amblyosporidae + Gurleyidae). This is used with some doubt in Bojko et al. (2022). The Outline takes a lumper view.<ref name=Bojko/>
Note 4: Microsporida s.s. is the "classical" scope of the class used by the Outline and Bojko et al. (2022). Some authors such as Corsaro et al. (2020) defines a larger scope, noted as Microsporida s.l. here.
Alternative topologiesEdit
Bojko et al. (2022) also includes a different branching order recovered by both Cormier et al. (2021) and Wadi and Reinke (2020), using a phylogenomic (multilocus) approach<ref name=Bojko/> with 68 single-copy genes. This branching order indicates:<ref>Template:Cite journal</ref>
South et al. (2024), quoting Corsano (2022), gives the following alternative topology for Microsporidia s.l.:<ref name=South>Template:Cite journal</ref>
South et al. (2024) marks the above internal topology of Microsporidia s.l. as robust to newer genome-based (multiprotein) studies, namely de Albuquerque & Haag, 2023; Thomé et al., 2023; Williams et al., 2022.<ref name=South/>
ClassificationEdit
Template:Further The first described microsporidian genus, Nosema, was initially put by Nägeli in the fungal group Schizomycetes together with some bacteria and yeasts.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> For some time microsporidia were considered as very primitive eukaryotes, placed in the protozoan group Cnidospora.<ref name=corliss /> Later, especially because of the lack of mitochondria, they were placed along with the other Protozoa such as diplomonads, parabasalids and archamoebae in the protozoan-group Archezoa.<ref>Template:Cite journal</ref> More recent research has falsified this theory of early origin (for all of these). Instead, microsporidia are proposed to be highly developed and specialized organisms, which just dispensed functions that are needed no longer, because they are supplied by the host.<ref name="Keeling">Template:Cite journal</ref> Furthermore, spore-forming organisms in general do have a complex system of reproduction, both sexual and asexual, which look far from primitive.
Since the mid-2000s microsporidia are placed within the Fungi or as a sister-group of the Fungi with a common ancestor.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> To avoid destructive changes to lower classification, the International Code of Nomenclature for algae, fungi, and plants explicitly excludes Microsporidia since 2012, leaving it to the International Code of Zoological Nomenclature as is traditionally done.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Early work to identify clades is largely based on habitat and host. Three classes of Microsporidia are proposed by Vossbrinck and Debrunner-Vossbrinck in 2005, based on the habitat: Aquasporidia, Marinosporidia and Terresporidia.<ref>Template:Cite journal</ref>
A second classification by Cavalier-Smith 1993:<ref>Template:Cite journal</ref>
- Subphyla Rudimicrospora Cavalier-Smith 1993
- Class Minisporea Cavalier-Smith 1993
- Order Minisporida Sprague, 1972
- Class Metchnikovellea Weiser, 1977
- Order Metchnikovellida Vivier, 1975
- Class Minisporea Cavalier-Smith 1993
- Subphyla Polaroplasta Cavalier-Smith 1993
- Class Pleistophoridea Cavalier-Smith 1993
- Order Pleistophorida Stempell 1906
- Class Disporea Cavalier-Smith 1993
- Subclass Unikaryotia Cavalier-Smith 1993
- Subclass Diplokaryotia Cavalier-Smith 1993
- Class Pleistophoridea Cavalier-Smith 1993
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Corsaro 2022 adds (not accepted by Index Fungorum or 2024 Outline):Template:Cn
- Order Paramicrosporidiales Corsaro 2022
- Family Paramicrosporidiaceae Corsaro 2022
- Order Morellosporales Corsaro 2022
- Family Mitosporidiaceae Corsaro 2022
- Family Morellosporaceae Corsaro 2022
- Order Nucleophagales Corsaro 2022
- Family Nucleophagaceae Corsaro 2022
In addition, there is the historical genus Microsporidium for holding species not otherwise classified.
See alsoEdit
- List of Microsporidian genera
- Glugea, a genus of microsporidia
- Nosema apis, a microsporidian parasite of bees
ReferencesEdit
Template:Notelist Template:Reflist
External linksEdit
- Template:Wikispecies-inline
- BioHealthBase Bioinformatics Resource Center Database of microspordia sequences and related information.
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