Ultrastructure of the Vitellogenic Egg Chambers of the Caddisfly Brachycentrus incanus (Insecta: Trichoptera)

Abstract

Ovarian structure and cellular composition of the oocyte and associated cells in Brachycentrus incanus (Insecta: Trichoptera) were examined by light and transmission electron microscopy. The ovary consisted of numerous ovarioles, each containing at least one maturing oocyte with its associated nurse cells and follicle cells. The ooplasm consisted primarily of electron-dense yolk bodies and lipid droplets. An oocyte nucleus was discernible only in preand early vitellogenic stages. Seven nurse cells, at the distal end of each oocyte, were connected through cytoplasmic bridges to the oocyte and to each other. Each large lobate nurse cell nucleus contained uniformly spaced clumps of heterochromatin and numerous nuclear pores; the cytoplasm contained mitochondria, a small amount of non-dilated granular endoplasmic reticulum (GER), agranular vesicles, and numerous free ribosomes. Follicle cells, which formed a layer around the oocyte, each contained a large oval nucleus with abundant heterochromatin and a moderate number of nuclear pores; the cytoplasm contained abundant dilated GER, mitochondria, Golgi bodies, and free ribosomes. Oocyte microvilli interdigitated with those of the follicle cells through a filamentous extracellular material of variable electron density. This is the first detailed description of egg chamber ultrastructure in this major insect order. The overall structure is very similar to that of other neopterous insect orders, including Lepidoptera, to which Trichoptera is generally considered to be closely related. Additional key words: follicle cells, nurse cells, oocytes, oogenesis, ovary Insects of the order Trichoptera, commonly known as caddisflies, have aquatic larval stages and winged terrestrial adults. Despite their ecological importance in freshwater habitats around the world (Lloyd 1921; Peckarsky et al. 1990), the development and reproduction of caddisflies have not been well described. Studies have been done on early embryogenesis (Kobayashi & Ando 1990), hydration properties and structure of the egg masses (Middleton 1977; Wood et al. 1982; Berte & Pritchard 1983), and female ovipositional behavior (Anderson 1973; Deutsch 1984; McGonigle & Howard 1987). The development (Dodson 1935) and morphology (Unzicker 1968; Nielsen 1980) of the female reproductive system and basic patterns of oogenesis (Gresson 1933) and ovariole structure (Le Lannic 1975; Denis & Le Lannic 1977) of various trichopteran species have been studied by light microscopy. However, caddisfly reproduction has not been studied using modern techniques or in light of recent advances in insect biology. Specifically, ultraa To whom correspondence should be addressed. structural studies on cellular aspects of mature trichopteran reproductive systems are lacking. The purpose of the present study was to examine the structure of the ovariole and the cellular composition of the egg chamber (the oocyte and its associated nurse and follicle cells) in Brachycentrus incanus HAGEN 1861 using light microscopy and transmission electron microscopy. The results may help to clarify phylogenetic relationships between the Trichoptera and other insect orders. This is part of a more comprehensive series of studies on the biology of this species, which is among the most ecologically important species in the St. Lawrence River ecosystem.