Subcellular localization of Bic-D :: GFP is linked to an asymmetric oocytenucleus

Bicaudal-D (Bic-D) is essential for the establishment of oocyte fate and su bsequently for polarity formation within the developing Drosophila oocyte. To find out where in the germ cells Bic-D performs its various functions we made transgenic flies expressing a chimeric Bic-D::GFP fusion protein. Onc e Bic-D::GFP preferentially accumulates in the oocyte, it shows an initial anterior localization in germarial region 2, In the subsequent egg chamber stages 1-6 BicD::GFP preferentially accumulates between the oocyte nucleus and the posterior cortex in a focus that is consistently aligned with a cra ter-like indentation in the oocyte nucleus. After stage 6 Bic-D::GFP fluore scent signal is predominantly found between the oocyte nucleus and the dors o-anterior cortex, During the different phases several genes have been foun d to be required for the establishment of the new Bic-D::GFP distribution p atterns. Dynein heavy chain (Dhc), spindle (spn) genes and maelstrom (mael) are required for the re-localization of the Bic-D::GFP focus from its ante rior to its posterior oocyte position. Genes predicted to encode proteins t hat interact with RNA (egalitarian and orb) are required for the normal sub cellular distribution of Bic-D::GFP in the germarium, and another potential RNA binding protein, spn-E, is required for proper transport of Bic-D::GFP from the nurse cells to the oocyte in later oogenesis stages. The results indicate that Bic-D requires the activity of mRNA binding proteins and a ne gative-end directed microtubule motor to localize to the appropriate cellul ar domains. Asymmetric subcellular accumulation of Bic-D and the polarizati on of the oocyte nucleus may reflect the function of this localization mach inery in vectorial mRNA localization and in tethering of the oocyte nucleus . The subcellular polarity defined by the Bic-D focus and the nuclear polar ity marks some of the first steps in antero-posterior and subsequently in d orsoventral polarity formation.