Microsporidia
Intracellular parasitism results in extreme adaptations. ~ Swiss zoologist Karen Haag et al
Microsporidia are a phylum of unicellular, spore-forming, intracellular fungal parasites. There may be a million species. ~1,500 have been named.
Microsporidia are restricted to animal hosts. All animal phyla host microsporidia. Most infect insects, but they are also responsible for common diseases of crustaceans and fish. Only 10% are vertebrate parasites.
Known species of microsporidia are usually restricted to a specific host or closely related species. Some of the most opportunistic infect humans.
Microsporidia influence their hosts in various ways. All organs and tissues may be invaded, though generally by different microsporidia species.
Microsporidia use a specialized harpoon – polar tube – to insert themselves into the cells of their host.
Once infected, a microsporidian restructures the host cell, with the parasite seeking control of metabolism to survive and facilitate reproduction. The host cell provides protection from the host’s immune system. The cytoplasm is largely replaced with machinery that does the microsporidian’s bidding.
In extremity, a microsporidian rules its host cell completely, controlling metabolism and reproduction, forcing formation of an aberrant tissue growth termed a xenoma. Microsporidia are known to produce xenomas in earthworms, insects, crustaceans, and fish.
In dropping all but the absolute necessities, microsporidia are essentially eukaryotic viruses; an instance of convergent evolution in this regard. Microsporidia have the smallest known genome of all eukaryotes. They have no mitochondria. Their molecular evolution rate is viral in its rapidity. Like viruses, microsporidian reliance upon the molecular machinery of their hosts is as complete as can be.
