Formation of the karyosome in developing oocytes of weevils (Coleoptera, Curculionidae)

A complex structure termed the karyosome forms in the nuclei of the developing oocytes of Anthonomus pomorum, Hylobius abietis and Phyllobius sp. (Coleoptera: Curculionidae). It is composed of highly condensed chromosomes, fused with an electron-dense granular material. There are two types of nuclear body associated with the karyosome. The smaller bodies are found in the immediate vicinity of the karyosome. The larger, and more electron-dense, bodies originate next to the condensing chromosomes. During vitellogenesis, the latter bodies disperse in the karyoplasm, and at least some of them locate in the characteristic irregular projections of the germinal vesicle. Morphologically, these projections resemble the accessory nuclei described in other insects. In the studied species, a proteinaceous sheath, the so-called karyosome capsule, surrounds the karyosome. The formation of the karyosome and its capsule occurs during previtellogenesis, so that these structures are fully formed at the onset of vitellogenesis. An extraction of the oocyte cytoplasm with Triton X-100 showed that the material constituting the karyosome capsule is filamentous. Staining with rhodamine-conjugated phalloidin reveals large amount of F-actin in the karyosome capsule.