Role of Bicaudal-D in patterning the Drosophila egg chamber in mid-oogenesis.

The Bicaudal-D (Bic-D) gene is required early in Drosophila oogenesis for the differentiation of an oocyte from one of a cluster of 16 interconnected germarial cells. To analyze the role of Bic-D later in oogenesis, we have constructed Drosophila lines in which Bic-D expression is under the control of the hsp70 promoter. In these flies, Bic-D activity can be induced early in oogenesis, allowing an oocyte to be made. Then, by shifting females to non-inducing conditions, Bic-D levels are depleted for the remainder of oogenesis. Using this system, we find that Bic-D is indeed required in the later stages of oogenesis. In ovaries from mutant females, oocyte growth is reduced, apparently due to defects in nurse-cell-to-oocyte transport. Smaller oocyte size results in the misalignment of follicle cells and the underlying germ line, leading to ventralization of dorsal follicle cells and to defects in centripetal cell migration. In addition, we show that Bic-D is required for the localization of specific mRNAs at both the anterior and posterior of the oocyte.