Assembly and breakdown of Cajal bodies in accessory nuclei of Hymenoptera

In some species of insects, oocytes have vesicular organelles, termed accessory nuclei (ANs). The ANs form by budding off from the nuclear envelope of the oocyte and are filled with translucent matrix containing dense inclusions. One type of these inclusions contains coilin and small nuclear ribonucleoproteins (snRNPs) and is homologous to Cajal bodies. We describe the early events in the morphogenesis of Cajal bodies in the ANs (ANCBs) of the common wasp, Vespula germanica, and show that they contain survival of motor neurons (SMN) protein. We present evidence that in the wasp, ANCBs form by the gradual accumulation of aggregates composed of SMN and small nuclear RNAs. We also show that ANCBs break down and disperse within the ANs as the ANs, which initially surround the oocyte nucleus, localize to the oocyte cortex. The components of dispersed ANCBs are retained within ANs until the end of oogenesis, which suggests that their function may be required at the onset of embryonic development. Because the morphology and behavior of ANs and their Cajal body-like inclusions are conserved in two other hymenopteran species, these features might be characteristic of all hymenopterans.