Localization of mRNAs to the oocyte is common in Drosophila ovaries.

Patterns of mRNA accumulation are sometimes dictated post-transcriptionally. Striking examples of mRNA localized to subcellular sites within cells have been described in animal oocytes, a variety of somatic cells in both animals and plants, and most recently in yeast (St Johnston, 1995; Bouget et al., 1996; Long et al., 1997; Takizawa et al., 1997). Some of the best characterized of the localized mRNAs come from the Drosophila ovary, where localization of three mRNAs--bicoid, oskar and gurken--within the oocyte defines the anteroposterior and dorsoventral body axes (reviewed in St Johnston, 1995). About 20 other mRNAs display a related pattern of localization, in which they become concentrated in the oocyte after transcription in the adjacent and interconnected nurse cells. Despite considerable work on the roles and mechanisms of mRNA localization, there have been no clear indications of the prevalence of this phenomenon, even in the much studied Drosophila system. Here we address this issue. We have examined the distributions of a random collection of individual Drosophila ovarian mRNAs, and find that a significant fraction, almost a tenth, are localized to the oocyte.