Microsporidia

Dictyocoela diporeiae.[13] A, meront and spore; B, spore wall; C, polar filament

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.[9]

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,[14] 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 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.[15][16][17][18]

Microsporidia, specifically the mosquito-infecting Vavraia culicis, are being explored as a possible 'evolution-proof' malaria-control method.[19] Microsporidian infection of Anopheles gambiae (the principal vector of Plasmodium falciparum malaria) reduces malarial infection within the mosquito, and shortens the mosquito lifespan.[20] 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 transmisison of P. falciparum, had "no overt effect" on the fitness of host mosquitoes, and was transmitted vertically (through inheritance).[21]

A hyperparasitic microsporidian, Nosema podocotyloidis, a parasite of a digenean which is itself a parasite of a fish[23]

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.[23]

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.[24]

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.[24]

The first described microsporidian genus, Nosema, was initially put by Nägeli in the fungal group Schizomycetes together with some bacteria and yeasts.[25][26] For some time microsporidia were considered as very primitive eukaryotes, placed in the protozoan group Cnidospora.[4] 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.[27] 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.[28] 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.[29][30][31][32]

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.[33]

Another proposed classification is:[34]

• Class Metchnikovellea Weiser 1977 emend. Cavalier-Smith 1993 [Manubrispora Cavalier-Smith 1998]
  • Order Metchnikovellida Vivier 1977
    • Family Amphiacanthidae
    • Family Metchnikovellidae Caullery & Mesnil 1897
• Class Microsporea Delphy 1936 ex Levine & Corliss 1963
  • Family Cougourdellidae Poisson 1953
  • Family Facilisporidae
  • Family Heterovesiculidae Lange et al. 1995
  • Family Myosporidae
  • Family Nadelsporidae
  • Family Neonosemoidiidae
  • Family Ordosporidae
  • Family Pseudonosematidae
  • Family Telomyxidae Leger & Hesse 1910
  • Family Toxoglugeidae
  • Family Tubulinosematidae
  • Subclass Haplophasea Sprague, Becnel & Hazard 1992
    • Order Chytridiopsida Weiser 1974
      • Family Chytridiopsidae Sprague, Ormieres & Manier 1972
      • Family Buxtehudiidae Larsson 1980
      • Family Enterocytozoonidae
      • Family Burkeidae Sprague 1977
      • Family Hesseidae Ormierees & Sprague 1973
    • Order Glugeida Issi 1986
      • Family Glugeidae Thelohan 1892 [Pleistophoridae Stempell 1909]
      • Family Gurleyidae Sprague 1977
      • Family Encephalitozoonidae
      • Family Abelsporidae
      • Family Tuzetiidae Sprague, Tuzet & Maurand 1977
      • Family Microfilidae
      • Family Unikaryonidae Sprague 1977
  • Subclass Dihaplophasea Sprague, Becnel & Hazard 1992
    • Order Meiodihaplophasida Sprague, Becnel & Hazard 1992
      • Superfamily Thelohanioidea Hazard & Oldacre 1975
        • Family Thelohaniidae Hazard & Oldacre 1975 [Coccosporidae]
        • Family Duboscqiidae Sprague 1977
        • Family Janacekiidae
        • Family Pereziidae Loubes et al. 1977
        • Family Striatosporidae
        • Family Cylindrosporidae
      • Superfamily Burenelloidea Jouvenaz & Hazard 1978
        • Family Burenellidae Jouvenaz & Hazard 1978
      • Superfamily Amblyosporoidea Weiser 1977
        • Family Amblyosporidae Weiser 1977
    • Order Dissociodihaplophasida Sprague, Becnel & Hazard 1992
      • Superfamily Nosematoidea Labbe
        • Family Nosematidae Labbe 1899
        • Family Ichthyosporidiidae
        • Family Caudosporidae Weiser 1958
        • Family Pseudopleistophoridae Sprague 1977
        • Family Mrazekiidae Leger & Hesse 1922
      • Superfamily Culicosporoidea Weiser 1977
        • Family Culicosporidae Weisser 1977
        • Family Culicosporellidae
        • Family Golbergiidae
        • Family Spragueidae Weissenberg 1976
      • Superfamily Ovavesiculoidea Sprague, Becnel & Hazard 1992
        • Family Ovavesiculidae
        • Family Tetramicridae

A third classification by Cavalier-Smith 1993:[35]

• Subphyla Rudimicrospora Cavalier-Smith 1993
  • Class Minisporea Cavalier-Smith 1993
    • Order Minisporida Sprague, 1972
  • Class Metchnikovellea Weiser, 1977
    • Order Metchnikovellida Vivier, 1975
• 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

• List of Microsporidian genera
• Glugea, a genus of microsporidia
• Nosema apis, a microsporidian parasite of bees

Data related to Microsporidia at Wikispecies

• BioHealthBase Bioinformatics Resource Center Database of microspordia sequences and related information.
• Microsporidia at the US National Library of Medicine Medical Subject Headings (MeSH)

Wikimedia Commons has media related to Microsporidia.