Ultracytology of a Tetrasporoblastic Microsporidium of the Caddis fly Holocentropus picicornis(Trichoptera, Polycentropodidae), with Description of Episeptuni inversum gen.et sp. nov. (Microspora, Gurleyidae)

Abstract

Summary The microsporidium Episeptum inversum gen. et sp. nov. (fam. Gurleyidae) is a parasite of the caddisfly Holocentropus picicornis (Polycentropodidae) in southern Sweden. Infection is restricted to the adipose tissue. Infected cells and their nuclei are hypertrophied, and in the final phase disintegrated. In the merogonial development multinucleate plasmodia with isolated nuclei give rise to numerous uninucleate merozoites, which mature to uninucleate sporonts. The tetranucleate sporogonial plasmodium is divided into four symmetrical lobes, and finally four rounded uninucleate sporoblasts are budded off. Sporogonial plasmodium and sporoblasts are enclosed in a sporophorous vesicle, produced by the sporont, with a thin electron-dense envelope and a fibrillar content. The vesicle is subpersistent and disappears at spore maturation. Mature spores are always free and ungrouped, even in situ in the host. Spores are pyriform, c. 2.1 X 2.5-3.2 Itm unfixed, 1.2-1.6 X 2.1-2.6,am fixed and stained. The membrane-lined posterior vacuole is visible also in unfixed spores, in an oblique position close to the posterior pole. The uninucleate spores have a polaroplast with two lamellar parts. The anterior lamellae are wide, the posterior ones narrow. The polar filament is anisofilar, with 3 wide (130-140 nm) anterior coils and 2 to 3 narrow (108-113 nm) posterior coils in a single layer close to the spore wall. The angle of tilt is acute, 30-35 degrees. The spore wall is 187-194 nm thick. The internal plasmalemma is c. 10 nm thick and the electron-translucent endospore - 100 nm. The plurilayered exospore has two c. 16 nm thick internal layers of somewhat different electron densities, and a 54 nm thick reticulate or chambered coat, with approximately 10 nm wide compartments and a 6 nm thick cover resembling a unit membrane. The ultracytology of the different developmental stages and the morphogenesis of the characteristic organelles are described. The new microsporidium is compared to the microsporidia of Trichoptera, to the three tetrasporoblastic microsporidian genera, and to the microsporidia with a specialized exospore.