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Microsporidia are a group of spore-forming unicellular . These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore.Franzen, C. (2005). How do Microsporidia invade cells?. Folia Parasitologica, 52(1–2), 36–40. doi.org/10.14411/fp.2005.005 They were once considered or , but are now known to be fungi, or a sister group to fungus. These fungal microbes are obligate eukaryotic parasites that use a unique mechanism to infect host cells. They have recently been discovered in a 2017 Cornell study to infect Coleoptera (Beetle) on a large scale. So far, about 1500 of the probably more than one million species are named. Microsporidia are restricted to animal hosts, and all major groups of animals host microsporidia. Most infect , but they are also responsible for common diseases of 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.Ronny Larsson, Lund University (Department of Cell and Organism Biology) Cytology and taxonomy of the microsporidia 2004.
Replication takes place within the host's cells, which are infected by means of unicellular . These vary from 1–40 μm, making them some of the smallest . Microsporidia that infect are 1.0–4.0 μm. They also have the smallest eukaryotic .
The terms "microsporidium" (pl. "microsporidia") and "microsporidian" are used as vernacular names for members of the group. The name Microsporidium Balbiani, 1884Balbiani, G. 1884. Les Psorospermies des Articulés ou Microsporidies, pp. 150-168, 184. In: Leçons sur les sporozoaires. Paris: Doin, [2]. is also used as a catchall genus for incertae sedis members.Hoffman, G. (1999). Parasites of North American Freshwater Fishes, 2nd edn, University of California Press, Berkeley, California, USA, p. 89, [3].
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:
In most cases there are two closely associated cell nucleus, 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.
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, 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).
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 n. sp. which infects the mosquito Culex salinarius Coquillett, and Amblyospora which infects the mosquito Culex Coquillett, provide typical examples of transovarial transmission of microsporidia. Microsporidia, specifically the mosquito-infecting Vavraia culicis, are being explored as a possible 'evolution-proof' malaria-control method. (2025). 9780123747877 ISBN 9780123747877 Microsporidian infection of Anopheles gambiae (the principal vector of Plasmodium falciparum malaria) reduces malarial infection within the mosquito, and shortens the mosquito lifespan. 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).
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.
Node 1: The "Orphan lineage" includes Hamiltosporidium + Astathelohania. Neofabelliforma and Areospora are possible inclusions but support is weak.
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.
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.
South et al. (2024), quoting Corsano (2022), gives the following alternative topology for Microsporidia s.l.:
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.
Since the mid-2000s microsporidia are placed within the Fungi or as a sister-group of the Fungi with a common ancestor. 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.
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.
A second classification by Cavalier-Smith 1993:
Corsaro 2022 adds (not accepted by Index Fungorum or 2024 Outline):
In addition, there is the historical genus Microsporidium for holding species not otherwise classified.
Alternative topologies
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 with 68 single-copy genes. This branching order indicates:
Classification
The first described microsporidian genus, Nosema, was initially put by Nägeli in the fungal group Schizomycetes together with some bacteria and yeasts. For some time microsporidia were considered as very primitive eukaryotes, placed in the protozoan group Cnidospora. Later, especially because of the lack of mitochondria, they were placed along with the other Protozoa such as , and archamoebae in the -group Archezoa. 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. Furthermore, spore-forming organisms in general do have a complex system of reproduction, both sexual and asexual, which look far from primitive.
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