INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 1 - G.I. Shcherbak - 1995

SUBKINGDOM PROTOZOA, OR UNICELLULAR ANIMALS (PROTOZOA)

PHYLUM APICOMPLEXA (APICOMPLEXA)

CLASS SPOROZOEA (SPOROZOEA)

Subclass Gregarines (Gregarinia)

These are a distinct group of coelomic parasites of invertebrates. Most species inhabit the gut of Arthropods, predominantly insects. Some gregarines also parasitize other invertebrate groups, such as Annelids and Echinoderms. Certain species live in the body cavity, reproductive Organs, and other sites. There are about 500 known species of gregarines. Many species are characterized by an intracellular parasitic stage.

The size of gregarines ranges from 16 µm to 16 mm. The most complex structures are found in gregarines inhabiting the gut of arthropods; their body typically consists of three parts: an anterior epimerite, a middle protomerite, and a posterior deutomerite, separated by pellicular folds (Fig. 30). The single Nucleus is located in the deutomerite. The epimerite serves to attach the animal to the intestinal wall and often bears hooks or more complex appendages. A young individual initially parasitizes inside The Cell, feeding via a cytostome located on the epimerite. As it develops, the parasite emerges from the cell into the intestinal lumen while remaining anchored to cellular remnants via the epimerite. When sexual reproduction begins, the epimerite is shed.

This Structure is typical for a group of species united in the family Polycystidae (Corycella armata from the gut of whirligig beetle larvae, Clepsidrina blattarum from the cockroach gut, etc.). Representatives of another family, Monocystidae, which parasitize the body cavity, Gonads, and other organs, have a simpler structure; their body is not divided into an epimerite, protomerite, and deutomerite, and they often exhibit a worm-like shape.

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Gregarians exhibit a distinctive, smooth type of locomotion whose underlying mechanism remains poorly understood. It was once believed that gregarines secrete mucus that is expelled through numerous pores, flows backward, and solidifies to form a tiny stalk that propels the Organism forward. Later, researchers concluded that mucus is forcibly ejected from pores to generate jet propulsion. Electron Microscopy has revealed longitudinal folds and ridges On the surface of the gregarine pellicle, containing microfilaments within their walls and a layer of microtubules underneath. It was hypothesized that these ridges undergo undulatory movements, but studies of live animals demonstrated that these structures are actually stationary. Consequently, some scientists returned to the older hypothesis of mucus extrusion, suggesting that the secreted mucus is directed backward by the ridges.

Fig. 30. Gregarines — general appearance of Corycella armata from the intestine of a whirligig beetle (a), its epimerite embedded in an intestinal epithelial cell (b), and an individual that has shed its epimerite (c), Schneideria mucronata from dipteran larvae (d), Menospora polyacantha from dragonfly larvae (e), and a syzygy of Clepsidrina blattarum from a cockroach (f): 1 — epimerite; 2 — protomerite; 3 — deutomerite; 4 — nucleus

The life cycle of most gregarines lacks schizogony, alternating solely between sexual reproduction and spore formation (sporogony) (Fig. 31). The invasive stage is the oocyst containing sporozoites. In the host's intestine, the sporozoites shed their membranes and briefly parasitize epithelial Cells. Eventually, they enter the intestinal lumen or migrate into Body Cavities or organ spaces, where they reach sexual maturity (gamonts). The gamonts pair up without fusing (the syzygy stage) and become enclosed within a common membrane (the gamontocyst stage). Within each individual, successive mitotic divisions occur, producing numerous nuclei that position themselves near the gamont's surface. Cytoplasm localizes around these nuclei to form Gametes. Unused cytoplasm, along with some nuclei, forms a large residual body that subsequently degenerates.

Fig. 31. Life Cycle of the gregarine Stylocephalus longicollis:

1 — sporozoite; 2 — gamont; 3 — syzygy; 4 — gamontocyst; 5, 6 — gamete formation; 7 — formed gametes; 8 — gamete copulation; 9 — oocyst; 10–14 — development of sporozoites within the oocyst (sporogony); 15 — release of sporozoites from the oocyst

Within the gamontocyst, gametes from one gamont fuse in pairs with gametes from the other gamont, a process of isogamy or heterogamy. The resulting zygotes develop a thick, durable membrane, transforming into oocytes inside the gamontocyst. Within each oocyst, Meiosis occurs first, followed by mitosis, producing several sporozoites. This process is known as sporogony. Through the host's excreta or waste products, the gamontocyst is released into the external environment, where it is ingested by a new host, and the cycle repeats.

In some gregarines parasitizing marine polychaetes and insects, reproduction involves an alternation of sexual and asexual (schizogony) phases.

Recently, gregarines have been investigated for their potential use in the biological control of insect pests.