Microsporidia are a group of
spore-forming
unicellular
A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...
parasite
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
s. These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore. They were once considered
protozoans or
protist
A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
s, but are now known to be
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
, or a sister group to fungi.
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 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
insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three ...
s, but they are also responsible for common diseases of
crustaceans and
fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...
. The named species of microsporidia usually infect one host species or a group of closely related taxa. Approximately 10 percent of the species are parasites of vertebrates —several species, most of which are opportunistic, can infect humans, in whom they can cause
microsporidiosis
Microsporidiosis is an opportunistic intestinal infection that causes diarrhea and wasting in immunocompromised individuals ( HIV, for example). It results from different species of microsporidia, a group of microbial (unicellular) fungi.
In HI ...
.
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
Parasitic castration is the strategy, by a parasite, of blocking reproduction by its host, completely or in part, to its own benefit. This is one of six major strategies within parasitism.
Evolutionary strategy
The parasitic castration strateg ...
, 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
A xenoma (also known as a 'xenoparasitic complex') is a growth caused by various protists and fungi, most notably microsporidia. It can occur on numerous organisms; however is predominantly found on fish.
In most cases the host cell and nucl ...
.
[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
spores. These vary from 1–40 μm, making them some of the smallest
eukaryotes. Microsporidia that infect
mammals are 1.0–4.0 μm.
They also have the smallest eukaryotic
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...
s.
The terms "microsporidium" (pl. "microsporidia") and "microsporidian" are used as vernacular names for members of the group. The name ''Microsporidium''
Balbiani, 1884 is also used as a catchall genus for
incertae sedis
' () or ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertain ...
members.
Morphology
Microsporidia lack
mitochondria, instead possessing
mitosome
A mitosome is an organelle found in some unicellular eukaryotic organisms, like in members of the supergroup Excavata. The mitosome was found and named in 1999, and its function has not yet been well characterized. It was termed a ''crypton'' by ...
s. 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''.
Infection
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
Sporoplasm is an infectious material present in the cytoplasm of various fungi-like organisms, such as members of class ''Microsporidia''. Sporoplasm is defined as a mass of protoplasm that gives rise to or forms a spore. The protoplasmic body t ...
grows, dividing or forming a
multinucleate
Multinucleate cells (also known as multinucleated or polynuclear cells) are eukaryotic cells that have more than one nucleus per cell, i.e., multiple nuclei share one common cytoplasm. Mitosis in multinucleate cells can occur either in a coordina ...
plasmodium
''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a blood-feeding insect host which then injects parasites into a ver ...
, 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
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...
. Different types of spores may be produced at different stages, probably with different functions including
autoinfection (transmission within a single host).
Medical implications
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. Microsporidia, specifically the mosquito-infecting ''
Vavraia culicis'', are being explored as a possible 'evolution-proof' malaria-control method. 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).
Clinical
Microsporidian infections of humans sometimes cause a disease called
microsporidiosis
Microsporidiosis is an opportunistic intestinal infection that causes diarrhea and wasting in immunocompromised individuals ( HIV, for example). It results from different species of microsporidia, a group of microbial (unicellular) fungi.
In HI ...
. At least 14 microsporidian species, spread across eight genera, have been recognized as human
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
s. These include ''
Trachipleistophora hominis''.
As hyperparasites
Microsporidia can infect a variety of hosts, including hosts which are themselves parasites. In that case, the microsporidian species is a
hyperparasite
A hyperparasite, also known as a metaparasite, is a parasite whose host, often an insect, is also a parasite, often specifically a parasitoid. Hyperparasites are found mainly among the wasp-waisted Apocrita within the Hymenoptera, and in two oth ...
, 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
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
s and
molluscs. Eight of these species belong to the genus ''
Nosema''.
[ ] 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).
[ ]
Genomes
Microsporidia have the smallest known (nuclear) eukaryotic
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...
s. The parasitic lifestyle of microsporidia has led to a loss of many
mitochondrial and
Golgi genes, and even their
ribosomal RNA
Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosom ...
s 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
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
.
Horizontal gene transfer
Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). H ...
(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.
Classification
The first described microsporidian genus, ''
Nosema'', was initially put by
Nägeli in the fungal group
Schizomycetes
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
together with some
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
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
diplomonads,
parabasalids and
archamoebae in the
protozoan-group
Archezoa Archezoa was a kingdom proposed in the 20th century by Thomas Cavalier-Smith (1942–2021), and was believed to encompass eukaryotes which did not have mitochondria (and are therefore ''amitochondriate'') or peroxisomes (e.g. '' Giardia''). The c ...
. 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.
Since the mid-2000s microsporidia are placed within the Fungi or as a sister-group of the Fungi with a common ancestor.
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.
A second classification by Cavalier-Smith 1993:
* 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
See also
*
List of Microsporidian genera
* ''
Glugea'', a genus of microsporidia
* ''
Nosema apis'', a microsporidian parasite of bees
References
External links
BioHealthBase Bioinformatics Resource CenterDatabase of microspordia sequences and related information.
*
{{Taxonbar, from=Q132652
Fungus phyla
Fungi by classification