snRNPs are present in the karyosome capsule in the weevil germinal vesicle

Within the oocyte nucleus of the apple blossom weevil, Anthonomus pomorum (Insecta, Coleoptera) highly condensed and transcriptionaly inactive chromosomes form the karyosome. During its formation, within the nucleoplasm numerous, variably sized spherical inclusions termed nuclear bodies occur. As oogenesis progresses, the karyosome is gradually surrounded by a prominent sheath, the karyosome capsule. The function and molecular composition of both the nuclear bodies and the karyosome capsule are largely unknown. Using cytochemical methods we demonstrate that DNA is confined to the karyosome and there is no extrachromosomal DNA accumulations within the nucleoplasm. In addition, none of the oocyte nucleus subdomains contain argyrophilic proteins. Our immunoEM study revealed that in contrast to similar structures in germinal vesicles in other insect species, the nuclear bodies of A. pomorum do not cross-react with antibodies recognising small nuclear ribonucleoproteins, coilin or the splicing factor SC-35. Unexpectedly, we found that as the karyosome capsule develops, mature small nuclear RNAs and proteins containing the Sm epitope associate with the capsule material. We suggest that the karyosome capsule is a storage site for small nuclear ribonucleoprotein particles, which may be used during early embryonic development.