Microsporidia
Spore-forming parasitic fungi with unique polar tube infection mechanism.
Microsporidia are single-celled, spore-forming fungi that live as parasites. They were originally mistaken for protozoans or protists, but genetic evidence now places them within the fungal kingdom—or as a close sister group to true fungi. These microbes are obligate parasites, meaning they cannot survive without a host, and they infect only animals. Every major animal group has at least some species that host microsporidia. Insects are the most common targets, but crustaceans and fish also frequently suffer from microsporidian diseases. Of the roughly 1,500 named species (out of an estimated million or more), about 10 percent infect vertebrates. Several of these, mostly opportunistic, can cause a disease called microsporidiosis in humans.
The infection process relies on a unique structure inside the spore: a coiled polar tube anchored at the spore’s tip. When the spore germinates—usually in a host’s gut—it builds up osmotic pressure until the spore wall bursts at its thinnest point. A posterior vacuole swells, forcing the polar filament to shoot out and pierce the gut epithelium. The filament then becomes a hollow tube, acting like a hypodermic needle to inject infectious material into the host cell. Once inside, the sporoplasm grows, either dividing directly or forming a multinucleate plasmodium, before producing new spores. Life cycles vary: some are simple and asexual, while others involve multiple hosts and both asexual and sexual stages. Different spore types may appear at different stages, sometimes allowing autoinfection within a single host.
Microsporidia lack mitochondria, instead carrying mitosomes, and they have no flagella or other motile structures. Their spores are highly resistant, surviving outside a host for years. Spore shape helps identify species—most are oval or pear-shaped, but rod-shaped and spherical forms occur, and a few genera have uniquely shaped spores. The spore wall has three layers: an outer electron-dense exospore, a thick middle endospore containing chitin, and a thin inner plasma membrane. Most spores contain two closely associated nuclei (a diplokaryon), though some have just one. The anterior half of the spore holds the harpoon-like apparatus: a polar filament surrounded by a membrane stack called the polaroplast, with a posterior vacuole behind it.
Infection affects hosts in various ways, and different microsporidian
- field
- Mycology, Parasitology
- known_for
- Obligate intracellular parasites with a unique polar tube infection mechanism; smallest eukaryotic genomes
- species_named
- About 1500
- estimated_species
- Probably more than one million
- spore_size_range
- 1–12 μm
Lore & Background
Microsporidia produce highly resistant spores capable of surviving outside their host for up to several years. The spore is protected by a three-layered wall: an outer electron-dense exospore, a median endospore containing chitin, and a thin internal plasma membrane. The anterior half of the spore contains a harpoon-like apparatus with a long, thread-like polar filament coiled in the posterior half. The spore germinates in the host's gut, building osmotic pressure until its wall ruptures at the apex, and the polar filament rapidly ejects infectious content into the host cell cytoplasm. Once inside, a sporoplasm grows, dividing or forming a multinucleate plasmodium before producing new spores. The life cycle varies considerably, with some having simple asexual cycles and others complex cycles involving multiple hosts and both asexual and sexual reproduction.
Reader's Guide
Microsporidia are significant as obligate eukaryotic parasites that infect a wide range of animal hosts, including insects, crustaceans, fish, and humans. They lack mitochondria, instead possessing mitosomes, and have the smallest known eukaryotic genomes, ranging from 2.5 to 11.6 Mb. Their unique infection mechanism—using a polar tube to inject sporoplasm into host cells—makes them a subject of study for biological control of insect pests and potential malaria control. For instance, microsporidian infection of Anopheles gambiae reduces malarial infection and shortens mosquito lifespan. In May 2020, researchers reported that Microsporidia MB, a symbiont in Anopheles arabiensis, significantly impaired transmission of Plasmodium falciparum without harming the mosquito. Microsporidia also act as hyperparasites, infecting other parasites such as digeneans. Their genomic and epigenetic architecture varies significantly across species, and horizontal gene transfer has occurred many times, with some genomes containing genes from animals, bacteria, and fungi.
Did You Know?
- Microsporidia lack mitochondria, instead possessing mitosomes.
- They have the smallest known eukaryotic genomes, ranging from 2.5 to 11.6 Mb.
- Microsporidia can survive outside their host for up to several years as highly resistant spores.
- Some microsporidian species are used in biological control of insect pests.
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