INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOA

MICROSPORIDIANS

PHYLUM MICROSPORIDIA

CLASS MICROSPORIDIA (

The vegetative stages of microsporidians are uninucleate or binucleate cells lacking mitochondria, lysosomes, and reserve nutrients. The invasive stage is a unicellular spore, whose most characteristic structures are a tightly coiled polar tube capable of everting outward, and a polaroplast.

Structure and life cycle. The host is infected by a spore that typically enters the gut with food. The spore (Figs. 50, 51) is surrounded by a tough coat. Its anterior end features a polar disc connected to a spirally coiled polar tube. Inside the spore lies an amoeboid germ (sporoplasm) with one or two nuclei. A polaroplast is present in the anterior part of the spore, and a vacuole in the posterior. In the host's intestine, the polaroplast swells, sharply increasing the pressure inside the spore, which triggers the discharge of the polar tube. The amoeboid germ travels from the spore through the lumen of the polar tube into a cell, where it either parasitizes or is transported via the blood to other organs; within these cells, it develops into a trophozoite and subsequently a schizont. Vegetative stages reproduce by schizogony, or more rarely by binary fission, resulting in chains of cells. This is followed by sporogony, leading to the formation of unicellular spores. In some species, a sexual process in the form of autogamy (pairwise fusion of nuclei within cells) has been described during sporogony; in this case, the division of the resulting zygote is meiosis, and subsequent divisions are mitoses. Thus, the life cycle of microsporidians is accompanied by zygotic chromosomal reduction. The life cycles of microsporidians are not yet sufficiently studied. Some species may exhibit alternation of hosts (for example,

blood-sucking mosquitoes and cyclopoid copepods).

Fig. 50. Diagram of microsporidian spore ultrastructure (after Issi):

1 - polar disc; 2 - polaroplast; 3 - polar tube; 4 - nucleus;

5, 6, 9 - spore coats; 7 - cell membrane; 8 - posterior vacuole

Cells infected with microsporidians, along with their nuclei, increase in size. Microsporidians lack their own mitochondria; they utilize the ATP of the host cell, which is why they are referred to as "energy parasites." In arthropods, they disrupt normal metamorphosis by increasing the number of molts, thereby delaying the transition to the adult stage, and alter host behavior.

Certain species of microsporidians are harmful to sericulture (Nosema bombycis, the causative agent of pebrine in the silkworm) and apiculture (N. apis causes nosemosis in bees). Over 80 species parasitize freshwater and marine fish, including commercial species. They also cause mortality in commercial crustaceans (crayfish, crabs, shrimp, etc.) and mollusks (mussels). Five species of microsporidians that parasitize warm-blooded vertebrates are capable of infecting humans (most commonly Encephalitozoon cuniculi). It has been found that 30% of AIDS patients die specifically from microsporidiosis.

Fig. 51. Myxosporidian spores (after Dogiel and Polyansky, modified): a - structural diagram; b - spore of Ceratomyxa; c - Mitraspora; d - Davisia;

e - Leptotheca; f - Myxidium; g - Chloromyxum; h - Hexacapsula; i - Henneguya;

1 - polar capsule; 2 - polar filament; 3 - polar capsule nucleus; 4 - amoeboid germ; 5 - germ nuclei; 6 - valve nucleus

However, microsporidians can also be beneficial to humans. For instance, about 70% of the species of these protozoans parasitize arthropods, particularly blood-sucking insects, forest pests, and agricultural crop pests. In many countries around the world, microsporidians are being studied with the aim of using them in the biological control of harmful invertebrates through the development of biological agents based upon them.