Nuage morphogenesis becomes more complex: two translocation pathways and two forms of nuage coexist in Drosophila germline syncytia

We have developed a simple and reliable method of preserving antigen immunoreactivity with concomitant excellent retention of the cell ultrastructure. Using this method, we have been able to follow the origin and developmental stages of nuage accumulations within the nurse cell/oocyte syncytium in the ovary of the fruit fly, Drosophila melanogaster, at the ultrastructural level. We have found two morphologically and biochemically distinct forms of nuage material in the nurse cell cytoplasm: translocating accumulations of nuage containing the Vasa protein, termed sponge bodies and stationary polymorphic accumulations of nuage enriched in Argonaute and Survival of motor neuron proteins. Immunogold labeling combined with confocal fluorescent and ultrastructural analyses have revealed that the Vasa-containing nuage accumulations remain closely associated with the cisternae of the endoplasmic reticulum throughout their lifetimes. The migration mechanism of the Vasa-positive nuage appears distinct from the microtubule-dependent translocation of oskar ribonucleoprotein complexes. We postulate that these two distinct nuage translocation pathways converge in the formation of the polar granules within the polar/germ plasm of the oocyte posterior pole. We also provide morphological and immunocytochemical evidence that these polymorphic nuage accumulations correspond to the recently described cytoplasmic domains termed U body-P body complexes.