Microsporidia

Overview
Microsporidia are parasites of animals, now considered to be extremely reduced fungi. Most infect insects, but they are also responsible for common diseases of crustaceans and fish, and have been found in most other animal groups, including humans and other mammals which can be parasitized by species of Encephalitozoon. 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. They also have the shortest eukaryotic genomes.

Microsporidia are unusual in lacking mitochondria and in having mitosomes. They also lack motile structures such as flagella. The spores are protected by a layered wall including proteins and chitin. Their interior is dominated by a unique coiled structure called a polar tube (not to be confused with the polar filaments of Myxozoa). In most cases there are two closely associated nuclei, forming a diplokaryon, but sometimes there is only one.

During infection, the polar tube penetrates the host cell (the process has been compared by Patrick J. Keeling to "turning a garden hose inside out"), and the contents of the spore are pumped through it. Keeling likens the system to a combination of "harpoon and hypodermic syringe", adding that it is "one of the most sophisticated infection mechanisms in biology".

Once inside the host cell, the 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). The 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 (Andreadis and Hall 1979a,b; Jahn et al. 1986; Becnel and Andreadis 1998).

Because they are unicellular, Microsporidia were traditionally treated as protozoa, and like other amitochondriate eukaryotes were considered to have diverged very early on. However, other genes place them alongside or within the Fungi, and this is supported by several chemical and morphological features. In particular they appear to be allied with the Zygomycota or Ascomycota.

Microsporidium was once the vernacular name for a member of the class Microsporea (Corliss and Levine 1963).

Classification

 * Subclass: Dihaplophasea
 * Order: Meiodihaplophasida
 * Superfamily Thelohanioidea
 * Family Thelohaniidae
 * Family Duboscqiidae
 * Family Janacekiidae
 * Family Pereziidae
 * Family Striatosporidae
 * Family Cylindrosporidae
 * Superfamily Burenelloidea
 * Family Burenellidae
 * Superfamily Amblyosporoidae
 * Family Amblyosporidae
 * Order Dissociodihaplophasida
 * Superfamily Nosematoidea
 * Family Nosematidae
 * Family Ichthyosporidiidae
 * Family Caudosporidae
 * Family Pseudopleistophoridae
 * Family Mrazekiidae
 * Superfamily Culicosporoidea
 * Family Culicosporidae
 * Family Culicosporellidae
 * Family Golbergiidae
 * Family Spragueidae
 * Superfamily Ovavesiculoidea
 * Family Ovavesiculidae
 * Family Tetramicridae
 * Subclass Haplophasea
 * Order Glugeida
 * Family Glugeidae
 * Family Pleistophoridae
 * Family Encephalitozoonidae
 * Family Abelsporidae
 * Family Tuzetiidae
 * Family Microfilidae
 * Family Unikaryonidae
 * Order Chyridiopsida
 * Family Chytridiopsida
 * Family Buxtehudiidae
 * Family Enterocytozoonidae
 * Family Burkeidae