Visualization of RNP dynamics and composition in Drosophila

Abstract

Localized transcripts direct asymmetric cellular processes in a wide range of biological contexts. A particularly well-studied model system has been the Drosophila oocyte, in which proper localization of mRNAs is required for successful embryo development. The germ plasm, a specialized cytoplasm enriched in ribonucleoprotein (RNP) particles, must accumulate at the posterior of the egg in order for primordial germ cells to properly segregate during embryo development.

An unanswered question has been how germ plasm RNPs maintain their localization in the oocyte: days or even weeks can pass between egg maturation and fertilization. While actin-based anchoring mechanisms have been previously implicated, we have discovered through high-resolution live imaging that, surprisingly, there is continued trafficking of germ plasm RNPs at the posterior of the oocyte. Further, this motility is required for long-term retention. Thus, we propose that anchoring may actually be a dynamic state in which motility provides robustness over long periods of developmental time (or in the face of environmental perturbations).

A second question has been how germ plasm RNPs are organized: are a number of RNA species organized into higher-order complexes or are distinct RNA species packaged into distinct granules? We have adapted a single molecule fluorescent in situ hybridization (smFISH) approach to address this question through the visualization of individual RNPs. We identified at least two "particle groups" containing distinct, non-overlapping sets of RNA species and proteins. We propose that these groupings reflect when the activity of contained transcripts is required (e.g. in the oocyte versus in the embryo).