Most microsporidia enter the new host through the gut, even if other routes of infection are known to occur. In the gut of the correct host, the conditions stimulate the spore to eject the polar filament (Figure 11.1D), which is everted like a finger of a glove. During this process the protein of the filament coils is rearranged to form an infection tube (Weidner 1976). In the laboratory some microsporidia discharge their filament easily when stimulated by hydrogen peroxide, certain salt solutions, or change of pH (Vavra and Maddox 1976). In the light microscopic era the artificial ejection of the polar filament was the proof of the microsporidian nature of the organism in study.

There are different theories for the ejection process (Dall 1983; Undeen 1990). According to one of them, which has so far not been proven, ionophore molecules in the plasma membrane participate in an exchange of cations or protons, causing an osmotic imbalance (Dall 1983). This leads to a rapid inflow of water into the spore. The posterior vacuole and the chambers of the polaroplast swell, and the increased pressure causes the explosive discharge of the polar filament. A second theory is based upon the observed decrease of the trehalose level during ejection. The disaccharid trehalose is degraded into smaller molecules causing an increased osmotic pressure (Undeen 1990).

The spore functions as a syringe. The tip of the newly formed tube penetrates a host cell and the parasite is injected safely into this (Figure 11.1D). The infectious stage (the sporoplasm) consists of the nucleus (or diplokaryon) of the spore and the cytoplasm. The plasma membrane of the sporoplasm is not the plasma membrane of the spore - this remains as the

Figure 11.1 Light and electron microscopic aspects of microsporidia. (A) Living, lightly pyriform spores of two size classes (Glugea anomala,* =

macrospore; phase contrast, bar = 10^m). (B) Living rod-shaped spores in eight-sporous sporophorous vesicles (Resiomeria odonatae; interference phase contrast, bar = 10^m). (C) Longitudinally sectioned mature spore exhibiting the characteristic organelles: layered spore wall, polaroplast divided into two regions, anisofilar polar filament (Trichoctosporea pygopellita; transmission electron microscopy, bar = 0.5^m). (D) Mature spores; one spore has extruded the polar filament and transformed it into an infection tube through which the infectious cell (the sporoplasm with nuclei coupled as diplokarya) has left the spore (Nosema tractabile; scanning electron microscopy, bar = 10^m). (E) Reproduction by multiple budding both in the vegetative phase (merogony) and the sporogony (Systenostrema corethrae; light microscopy, Giemsa stain, bar = 10^m). (F) Polysporous sporophorous vesicle (Vavraia holocentropi; scanning electron microscopy, bar = 5^m). Abbreviations: A = anchoring apparatus, D = diplokaryon, E = endospore, EX = exospore, F = polar filament, M = merogony, N = nucleus, PA = anterior part of polaroplast, PP = posterior part of polaroplast, S = sporogony, SP = sporoplasm, V = posterior vacuole.

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internal component of the spore wall. The new plasma membrane is formed from the unit membranes of the polaroplast during the ejection process.

Many microsporidia infecting insects combine horizontal transmission by spores, released into nature from the infected host, with vertical transmission from the female to the offspring (Becnel 1994; Becnel and Andreadis 1999). The oocytes are infected in the ovary and the larvae are already infected when they hatch.

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