This article is about the parasitic fungi group. For the ascomycete fungi genus, seeMicrosporum. For the green algae genus, seeMicrospora. For the beetle family, seeMicrosporidae.
Microsporidia are a group ofspore-formingunicellularparasites. These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore.[7] They were once consideredprotozoans orprotists, but are now known to befungi,[8] or a sister group totrue fungi.[9] These fungal microbes are obligate eukaryotic parasites that use a unique mechanism to infect host cells.[7] They have recently been discovered in a 2017 Cornell study to infectColeoptera on a large scale. So far, about 1500 of the probably more than one million[10] species are named. Microsporidia are restricted to animalhosts, and all major groups of animals host microsporidia. Most infectinsects, but they are also responsible for common diseases ofcrustaceans andfish. 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 causemicrosporidiosis.
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 axenoma.[11]
Replication takes place within the host's cells, which are infected by means of unicellularspores. These vary from 1–40 μm, making them some of the smallesteukaryotes.[citation needed] Microsporidia that infectmammals are 1.0–4.0 μm.[12] They also have the smallest eukaryoticgenomes.
The terms "microsporidium" (pl. "microsporidia") and "microsporidian" are used as vernacular names for members of the group. The nameMicrosporidiumBalbiani, 1884[13] is also used as a catchall genus forincertae sedis members.[14]
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-denseexospore
a median, wide and seemingly structurelessendospore, containingchitin
a thin internalplasma membrane
In most cases there are two closely associatednuclei, forming adiplokaryon, but sometimes there is only one. The anterior half of the spore contains a harpoon-like apparatus with a long, thread-likepolar filament, which is coiled up in the posterior half of the spore. The anterior part of the polar filament is surrounded by apolaroplast, a lamella of membranes. Behind the polar filament, there is a posteriorvacuole.[11]
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, asporoplasm grows, dividing or forming amultinucleateplasmodium, before producing new spores. The life cycle varies considerably. Some have a simpleasexual life cycle,[16] while others have a complex life cycle involving multiple hosts and both asexual andsexual reproduction. Different types of spores may be produced at different stages, probably with different functions includingautoinfection (transmission within a single host).
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 astransovarial 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 mosquitoCulex salinarius Coquillett, andAmblyospora californica which infects the mosquitoCulex tarsalis Coquillett, provide typical examples of transovarial transmission of microsporidia.[17][18][19][20] Microsporidia, specifically the mosquito-infectingVavraia culicis, are being explored as a possible 'evolution-proof' malaria-control method.[21] Microsporidian infection ofAnopheles gambiae (the principal vector ofPlasmodium falciparum malaria) reduces malarial infection within the mosquito, and shortens the mosquito lifespan.[22] 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 thatMicrosporidia MB, a symbiont in the midgut and ovaries ofAn. arabiensis, significantly impaired transmission ofP. falciparum, had "no overt effect" on the fitness of host mosquitoes, and was transmitted vertically (through inheritance).[23]
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Microsporidian infections of humans sometimes cause a disease calledmicrosporidiosis. At least 14 microsporidian species, spread across eight genera, have been recognized as humanpathogens. These includeTrachipleistophora hominis.[24]
A hyperparasitic microsporidian,Nosema podocotyloidis, a parasite of adigenean which is itself a parasite of a fish.[25]
Microsporidia can infect a variety of hosts, including hosts which are themselves parasites. In that case, the microsporidian species is ahyperparasite, i.e. a parasite of a parasite. As an example, more than eighteen species are known which parasitizedigeneans (parasiticflatworms). These digeneans are themselves parasites in variousvertebrates andmolluscs. Eight of these species belong to the genusNosema.[25] Similarly, the microsporidian speciesToguebayea baccigeri is a parasite of a digenean, the faustulidBacciger israelensis, itself an intestinal parasite of a marine fish, the bogueBoops boops (Teleostei, Sparidae).[26]
Microsporidia have the smallest known (nuclear) eukaryoticgenomes. The parasitic lifestyle of microsporidia has led to a loss of manymitochondrial andGolgi genes, and even theirribosomal RNAs are reduced in size compared with those of mosteukaryotes. 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 manybacteria.[27]
TheRad9-Rad1-Hus1 protein complex (also known as the 9-1-1 complex) in eukaryotes is recruited to sites ofDNA damage where it is considered to help trigger the checkpoint-signaling cascade. Genes that code for heterotrimeric 9-1-1 are present in microsporidia.[28] In addition to the 9-1-1 complex, other components of theDNA repair machinery are also present indicting that repair of DNA damage likely occurs in microsporidia.[28]
The first described microsporidian genus,Nosema, was initially put byNägeli in the fungal groupSchizomycetes together with somebacteria andyeasts.[31][32] For some time microsporidia were considered as very primitive eukaryotes, placed in the protozoan groupCnidospora.[5] Later, especially because of the lack of mitochondria, they were placed along with the otherProtozoa such asdiplomonads,parabasalids andarchamoebae in theprotozoan-groupArchezoa.[33] 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.[34] 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.[35][36][37][38]
Work to identify clades is largely based on habitat and host. Three classes of Microsporidia are proposed by Vossbrinck and Debrunner-Vossbrinck, based on the habitat: Aquasporidia, Marinosporidia and Terresporidia.[39]
A second classification by Cavalier-Smith 1993:[40]
^Delphy, J. 1936. Sous-règne des Protozoaires. In: Perrier, R. (ed.).La Faune de la France en tableaux synoptiques illustrés, vol 1A. Delagrave: Paris.
^abCorliss JO, Levine ND (1963). "Establishment of the Microsporidea as a new class in the protozoan subphylum Cnidospora".The Journal of Protozoology.10 (Suppl):26–27.doi:10.1111/jeu.1963.10.issue-s3.
^Sprague, V. (1977). Classification and phylogeny of the Microsporidia. In:Comparative pathobiology. vol. 2, Systematics of the Microsporidia. Lee A. Bulla & Thomas C. Cheng (ed.). pp. 1–30. New York: Plenum Press,[1].
^abFranzen, C. (2005). How do Microsporidia invade cells?. Folia Parasitologica, 52(1–2), 36–40. doi.org/10.14411/fp.2005.005
^Hawksworth, David (2001). "The magnitude of fungal diversity: The 1.5 million spices estimate revisited".Mycological Research.105 (12): 1422.doi:10.1017/S0953756201004725.
^Andreadis TG, Hall DW (August 1979). "Development, ultrastructure, and mode of transmission of Amblyospora sp. (Microspora) in the mosquito".The Journal of Protozoology.26 (3):444–52.doi:10.1111/j.1550-7408.1979.tb04651.x.PMID536933.
^Andreadis TG, Hall DW (September 1979). "Significance of transovarial infections of Amblyospora sp. (Microspora:Thelohaniidae) in relation to parasite maintenance in the mosquito Culex salinarius".Journal of Invertebrate Pathology.34 (2):152–7.doi:10.1016/0022-2011(79)90095-8.PMID536610.
^Jahn GC, Hall DW, Zam SG (1986). "A comparison of the life cycles of two Amblyospora (Microspora: Amblyosporidae) in the mosquitoesCulex salinarius andCulex tarsalis Coquillett".Journal of the Florida Anti-Mosquito Association.57 (1):24–27.
^Fischer WM, Palmer JD (September 2005). "Evidence from small-subunit ribosomal RNA sequences for a fungal origin of Microsporidia".Molecular Phylogenetics and Evolution.36 (3):606–22.doi:10.1016/j.ympev.2005.03.031.PMID15923129.
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Data related toMicrosporidia at Wikispecies