A SPONGE-LIKE STRUCTURE INVOLVED IN THE ASSOCIATION AND TRANSPORT OF MATERNAL PRODUCTS DURING DROSOPHILA-OOGENESIS

Localization of maternally provided RNAs during oogenesis is required for formation of the antero-posterior axis of the Drosophila embryo. H ere we describe a subcellular structure in nurse cells and oocytes whi ch may function as an intracellular compartment for assembly and trans port of maternal products involved in RNA localization. This structure , which we have termed ''sponge body,'' consists of ER-like cisternae, embedded in an amorphous electron-dense mass. It lacks a surrounding membrane and is frequently associated with mitochondria. The sponge bo dies are not identical to the Golgi complexes. We suggest that the spo nge bodies are homologous to the mitochondrial cloud in Xenopus oocyte s, a granulo-fibrillar structure that contains RNAs involved in patter ning of the embryo. Exuperantia protein, the earliest factor known to be required for the localization of bicoid mRNA to the anterior pole o f the Drosophila oocyte, is highly enriched in the sponge bodies but n ot an essential structural component of these. RNA staining indicates that sponge bodies contain RNA. However, neither the intensity of this staining nor the accumulation of Exuperantia in the sponge bodies is dependent on the amount of bicoid mRNA present in the ovaries. Sponge bodies surround nuage, a possible polar granule precursor. Microtubule s and microfilaments are not present in sponge bodies, although transp ort of the sponge bodies through the cells is implied by their presenc e in cytoplasmic bridges. We propose that the sponge bodies are struct ures that, by assembly and transport of included molecules or associat ed structures, are involved in localization of mRNAs in Drosophila ooc ytes.