MORPHOGENESIS OF THE DROSOPHILA FUSOME AND ITS IMPLICATIONS FOR OOCYTE SPECIFICATION

The Drosophila oocyte develops within a cyst of 16 germline cells inte rconnected by ring canals. Polarized, microtubule-based transport of u nknown determinants is required for oocyte formation, but whether pola rity is established during or after cyst formation is not clear. We ha ve analyzed how polarity develops in stem cells and dividing cysts by following the growth of the fusome, a vesiculated cytoplasmic organell e, Our studies show that the fusome grows by a regular, polarized proc ess throughout the stem cell and cyst cell cycles. Each polarization c ycle begins in mitosis, when the fusome segregates to a single daughte r cell of each pair. Following mitosis, a 'plug' of fusomal material f orms in each nascent ring canal and gradually fuses with the pre-exist ing fusome. In stem cells, the ring canal is transient and closes down after the fusome is partitioned through it. In dividing cysts, as the fusome plugs move toward the pre-existing fusome, their associated ri ng canals also move, changing the geometry of the cyst. At the end of each cycle of cyst growth, the fusome remains asymmetrically distribut ed within the cyst; one of the two cells with four ring canals retains a bigger piece of fusome than any other cell, including the other cel l with four ring canals. Based on these observations, we argue that th e oocyte is specified at the first cyst division.