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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.
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 reproduction|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).
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 reproduction|asexual]] life cycle,<ref name="pmid17394631">{{cite journal |author=Ironside JE |title=Multiple losses of sex within a single genus of Microsporidia |journal=BMC Evol. Biol. |volume=7 |issue= |pages=48 |year=2007 |pmid=17394631 |pmc=1853083 |doi=10.1186/1471-2148-7-48 |url=http://www.biomedcentral.com/1471-2148/7/48}}</ref> 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).


== Medical implications ==
== Medical implications ==

Revision as of 00:35, 25 February 2009

Microsporidia
Sporoblast of
Fibrillanosema crangonycis
Scientific classification
Kingdom:
incertae sedis
(unranked):
Microsporidia

Dihaplophasea

Haplophasea

The microsporidia constitute a phylum of spore-forming unicellular parasites. Loosely 1500 of the probably more than one million species are named now. Microsporidia are restricted to animal hosts, and all major groups of animals host microsporidia. Most infect insects, but they are also responsible for common diseases of crustaceans and fish. The distinguished species of microsporidia usually infect one specific host or a related group of hosts. Several species, most of which are opportunistic, also infect humans.

Approximately 10 per cent of the species are parasites of vertebrates, including in man.

After infection they influence their hosts in various ways and all organs and tissues are invaded. Some species are lethal, and a few are used in biological control of insect pests. Parasitic castration, gigantism, change of host sex are effects of microsporidian parasitism. In the most advanced cases of parasitism the microsporidium rules the host cell completely and controls its metabolism and reproduction, forming a xenoma. [1].

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.

Microsporidium was once the vernacular name for a member of the class Microsporea.[2]

Xenoma on flatfish caused by Glugea stephani

Anatomy

Microsporidia are lacking mitochondria and possess, instaed, mitosomes. They also lack motile structures such as flagella.
Microsporidia produce highly resistant spores to survive outside the host for up to several years. The spores are useful to distinct the 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. [1]

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 grows, dividing or forming a multinucleate plasmodium, before producing new spores. The life cycle varies considerably. Some have a simple asexual life cycle,[3] 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).

Medical implications

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.[4][5][6] [7]

Classification

For some time microsporidia were considered as very primitive eukaryotes, especially because of the lack of mitochondria, and placed along with the other protozoa diplomonads, parabasalia and archamoebae in the protist-group "Archezoa". More recent research has falsified this theory of early origin (for all of these). Yet 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 [8]. Furthermore, spore-forming organisms in general do have a complex system of reproduction, both sexual and asexual, which look far from primitive.

Nowadays microsporidia are placed within the Fungi or as a sister-group of the Fungi with a common ancestor [9] [10] [11] [12].

Forming of 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 [13].


One classification could be:

1. Subclass: Dihaplophasea

2. Subclass Haplophasea

See also

Weblinks

References

  1. ^ a b Ronny Larsson, Lund University (Department of Cell and Organism Biology) Cytology and taxonomy of the microsporidia 2004.
  2. ^ Corliss, J.O. and Levine, N.D. 1963. Establishment of the Microsporidia as a new class in the protozoan subphylum Cnidospora.. J. Protozool., 10 (Suppl.), 26-27.
  3. ^ Ironside JE (2007). "Multiple losses of sex within a single genus of Microsporidia". BMC Evol. Biol. 7: 48. doi:10.1186/1471-2148-7-48. PMC 1853083. PMID 17394631.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  4. ^ Andreadis, T. G., and Hall, D. W. 1979a. Development, ultrastructure, and mode of transmission of Amblyospora sp. (Microspora) in the mosquito. J. Protozool. 26, 444–452.
  5. ^ Andreadis, T. G., and Hall, D. W. 1979b. Significance of transovarial infections of Amblyospora sp. (Microspora: Thelohaniidae) in relation to parasite maintenance in the mosquito Culex salinarius. J. Invertebr. Pathol. 34, 152–157.
  6. ^ Jahn, G. C., Hall, D.W., and Zam, S. G. 1986. A comparison of the life cycles of two Amblyospora (Microspora: Amblyosporidae) in the mosquitoes Culex salinarius and Culex tarsalis Coquillett. J. Florida Anti-Mosquito Assoc. 57, 24–27.
  7. ^ Becnel, J. J. and Andreadis, T. G. 1998. Amblyospora salinaria n. sp. (Microsporidia: Amblyosporidae): parasite of Culex salinarius (Diptera: Culicidae), its life stages in an intermediate host and establishment as a new species. J. Invertebr. Pathol. 71:258-262.
  8. ^ Patrick J. Keeling and Claudio H. Slamovits Simplicity and Complexity of Microsporidian Genomes Eukaryot Cell. 2004 December; 3(6): 1363–1369.
  9. ^ Fischer WM, Palmer JD Evidence from small-subunit ribosomal RNA sequences for a fungal origin of Microsporidia. Mol Phylogenet Evol. 2005 Sep;36(3):606-22.
  10. ^ Liu YJ, Hodson MC, Hall BD. Loss of the flagellum happened only once in the fungal lineage: phylogenetic structure of kingdom Fungi inferred from RNA polymerase II subunit genes. BMC Evol Biol. 2006 Sep 29;6:74.
  11. ^ Gill EE, Fast NM. Assessing the microsporidia-fungi relationship: Combined phylogenetic analysis of eight genes. Gene. 2006 Jun 21;375:103-9. Epub 2006 Mar 20.
  12. ^ Soo Chan Lee, et al Microsporidia Evolved from Ancestral Sexual Fungi Current Biology, Volume 18, Issue 21, 1675-1679, 30 October 2008
  13. ^ Vossbrinck CR, Debrunner-Vossbrinck BA. Molecular phylogeny of the Microsporidia: ecological, ultrastructural and taxonomic considerations. Folia Parasitol (Praha). 2005 May;52(1-2):131-42
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