Dihaplophasea
Haplophasea 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. 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 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).

Because they are unicellular, Microsporidia were traditionally treated as protozoa. Like other amitochondriate groups they diverge early from other eukaryotes in rRNA trees, and so were considered some of the most primitive, separating before the appearance of mitochondria and flagella. However, this to be an artifact of rapid rRNA changes, and other genes place them alongside or within the Fungi. In paticular they appear to be allied with the Zygomycota or Ascomycota.

See also

• Nosema apis, a microsporidian parasite of bees

References

• Patrick J. Keeling et al. (2000). Evidence from Beta-Tubulin Phylogeny that Microsporidia Evolved From Within the Fungi. Molecular Biology and Evolution 17:23-31.
• Nature 414, 401 - 402 (22 November 2001); doi:10.1038/35106666