Structure of ovaries and oogenesis in the snow scorpionfly Boreus hyemalis(Linne) (Mecoptera : Boreidae)

The ovaries of the snow scorpionfly, Boreus hyemalis (Mecoptera: Boreidae) are panoistic and comprise 7-8 ovarioles. Each ovariole consists of a termi nal filament, elongated vitellarium, and ovariole stalk (=pedicel) only; in adult specimens, functional germaria are absent. Five consecutive stages o f oogenesis i.e., early, mid- and late previtellogenesis, vitellogenesis, a nd choriogenesis have been distinguished in imagines. Oocyte nuclei (= germ inal vesicles) of previtellogenic oocytes contain numerous polymorphic mult iple nucleoli (or nucleolar masses), endobodies, and chromatin aggregations . Next to the nuclear envelope, large accumulations of nuage material are l ocalized, The ooplasm of late previtellogenic oocytes is differentiated int o transparent (perinuclear) and opaque (peripheral) regions. Ultrastructura l investigations have revealed that within the latter, abundant ribosomes a s well as mitochondria, elements of endoplasmic reticulum, Golgi complexes, annulate lamellae, symbiotic bacteroids, lipid droplets and distinctive ac cumulations of membrane-free clathrin-like cages are present. Early- and mi d previtellogenic oocytes are invested with flat somatic cells that gradual ly transform into a follicular epithelium. In the vicinity of 3-cell juncti ons, neighbouring follicular cells are joined by narrow intercellular bridg es. During late previtellogenesis, numerous microvilli develop on the oocyt e surface. They interdigitate with morphologically similar but less frequen t microvilli of the follicular cells. Concurrently, first endocytotic vesic les appear in the cortical ooplasm. In the context of presented results, th e phylogenetic relationships between mecopterans (boreids) and fleas are di scussed. (C) 1998 Elsevier Science Ltd. All rights reserved.