These results reveal a diffusion-based, late-acting posterior localization mechanism for long-range transport of nanos mRNA. This mechanism differs from directed transport-based localization mechanisms in its reliance on bulk movement of RNA.

Localization of bicoid mRNA to the anterior of the Drosophila oocyte is essential to produce the Bicoid protein gradient that patterns the anterior-posterior axis of the embryo. Previous studies have characterized a microtubule-dependent pathway for bicoid mRNA localization during midoogenesis, when bicoid first accumulates at the anterior. We show that the majority of bicoid is actually localized later in oogenesis, when the only known mechanism for mRNA localization is based on passive trapping. Through live imaging of fluorescently tagged endogenous bicoid mRNA, we identify a temporally distinct pathway for bicoid localization in late oocytes that utilizes a specialized subpopulation of anterior microtubules and dynein. The directional movement of bicoid RNA particles within the oocyte observed here is consistent with dynein-mediated transport. Furthermore, our results indicate that association of bicoid with the anterior oocyte cortex is dynamic and support a model for maintenance of bicoid localization by continual active transport on microtubules.

Translational control of gene expression plays a fundamental role in the early development of many organisms. In Drosophila, selective translation of nanos mRNA localized to the germ plasm at the posterior of the embryo, together with translational repression of nanos in the bulk cytoplasm, is essential for development of the anteroposterior body pattern. We show that both components to spatial control of nanos translation initiate during oogenesis and that translational repression is initially independent of Smaug, an embryonic repressor of nanos. Repression during oogenesis and embryogenesis are mediated by distinct stem loops within the nanos 3′ untranslated region; the Smaug-binding stem-loop acts strictly in the embryo, whereas a second stem-loop functions in the oocyte. Thus, independent regulatory modules with temporally distinct activities contribute to spatial regulation of nanos translation. We propose that nanos evolved to exploit two different stage-specific translational regulatory mechanisms.