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MicrosporidiaMicrosporidia are important primary pathogens of insects. The smallest of eukaryotes, with unicellular spores and no mitochondria, they are obligate intracellular parasites. The taxonomic status of this group is problematic from the species level to the phylum level. Traditionally considered to be Protozoa, several current studies of various gene sequences closely align the microsporidia with primitive fungi (Keeling and Doolittle, 1996; Hirt et al., 1999). The most obvious morphological characteristic of microsporidia is the polar filament, a fine hollow tube that is coiled within the mature spore and which everts when the spore germinates in the midgut lumen of the host. This eversion is random in orientation but, if a midgut cell is punctured by a rapidly uncoiling filament, the spore contents are essentially injected into the cytoplasm of the cell and vegetative reproduction begins. Microsporidia can be either uninucleate or binucleate (diplokaryotic); some species have one or two or more nuclei in different life stages. Reproduction may be asexual, sexual, or both, depending on the species, and life cycles range from simple to very complex. Some species require intermediate hosts, and some may have multiple cycles of sporulation and multiple spore types (Johnson, et al. 1997), probably with different functions including autoinfection (cell-to-cell and tissue-to-tissue transmission within a host). After infection and reproduction in the midgut tissues, microsporidia may spread to other tissues. Tissue specificity is common in microsporidia, but there is great variability between species as to the level of specificity and which tissues are invaded. Some species invade nearly all tissues in a host. The fat body is the most commonly invaded tissue. Microsporidia cause disease in all vertebrate classes, almost all invertebrate classes, and ciliates, myxozoans and gregarines as well, but never plants. They are most common in arthropods, and then fish. Over half the described species have been isolated from Lepidoptera and Diptera, but this may be a function of which insect orders have received the most study. Many or most microsporidia are relatively host specific, usually infecting one or a few closely related hosts in the field, although their physiological (laboratory) host range is broader (Solter and Maddox, 1998). Transmission of microsporidia can be horizontal by oral ingestion, vertical, or both, or by mechanical inoculation by a parasitoid. Host survival depends on dosage, stage of host infected, virulence of the microsporidian species involved, and vitality of the host. Although some species may cause acute disease and death of the host, microsporidia are typically chronic pathogens. Effects may include reduced adult mating success, reduced longevity and fecundity, prolonged larval development, failure to successfully pupate, and deformities. The symptoms may or may not be obvious. The integument generally stays intact, although insects with transparent cuticles such as mosquitoes or those with heavily infected fat bodies swollen with spores may show obvious physical signs. Infective mature spores are produced in the thousands to over a billion per host, depending on the species involved and/or size of the host. Biological control: Although typically producing chronic effects, some species of microsporidia are very virulent. Some, like Nosema apis in honey bees and Nosema bombycis in silk worms, cause economically serious diseases in beneficial insects, however, microsporidia can also regulate insect populations and therefore offer promise as biological control agents. Their best use will probably be as augmentatively released or classical biological control agents, not as pesticides. The only microsporidian ever registered as a microbial pesticide is Nosema locustae, a pathogen of grasshoppers. It is used in rangeland areas for grasshopper control and is still produced in the U.S. Two other microsporidian species that are known to control populations of pest insects are Nosema fumiferanae, a naturally-occurring pathogen of the spruce budworm, and Nosema pyrausta , a naturally-occurring pathogen of the European corn borer, which was accidently introduced to the U.S. at the time of the host introduction or shortly thereafter. |
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