An Egalitarian-BicaudalD complex is essential for oocyte specification and axis determination in Drosophila.

Genetic experiments suggest that polarization of the oocyte is linked directly to the initial cell fate determination that singles out the oocyte from its 15 sister cells. Specification of oocyte cell fate as well as establishment and maintenance of a polarized microtubule network within the Drosophila oocyte require the activity of the egalitarian (egl) and BicaudalD (BicD) genes. We have isolated the egl gene and show that Egl protein colocalizes with BicD protein at all stages of oogenesis. Immunoprecipitation experiments show that both proteins are part of a protein complex. Egl and BicD proteins localize to the oocyte in three stages that correlate with the stepwise polarization of the oocyte. We propose that the Egl-BicD protein complex links microtubule polarity and RNA transport. During early oogenesis, the complex is required to transport factors promoting oocyte differentiation; during later stages of oogenesis the complex directs the sorting of RNA molecules required for anterior-posterior and dorsoventral patterning of the embryo.