F-actin is a component of the karyosome in neuropteran oocyte nuclei

The principles underlying the assembly of intranuclear compartments are only beginning to be understood. The karyosome is an organelle typical of oocyte nuclei. It represents the tightly packed oocyte chromosomes, arrested at the diplotene of meiotic prophase. It has been known from several insect orders that a prominent capsule of unknown materials is built around the karyosome in the course of previtellogenesis and vitellogenesis. Here we show that F-actin, detected by dye-coupled phalloidin, is a major molecular component of the karyosome capsule in Neuroptera. We investigated capsule formation in six species belonging to the family Chrysopidae. Though F-actin was present in the capsules of all six species there were striking interspecific differences in the morphological array of actin filaments and the developmental dynamics of actin deposition in the capsule. The potential biological significance of the karyosome capsule is discussed with respect to the presence of extrachromosomal rDNA in neuropteran oocytes and the molecular functions known from F-actin. Our results corroborate the still controversial hypothesis of a role for actin as a nuclear protein.