Fusome asymmetry and oocyte determination in Drosophila.

Germline cysts containing 16 interconnected cells (cystocytes) are produced at an early stage of Drosophila oogenesis by progenitor cells known as cystoblasts that undergo four synchronous rounds of incomplete division. During cyst formation, a region of specialized, spectrin-rich cytoplasm called the fusome traverses the intercellular connections (ring canals), linking individual cystocytes. Subsequently, 15 cystocytes begin to transport specific RNAs and other components into the remaining cell, the future oocyte. We used fusome-specific antibodies to characterize the early stages of cyst formation. During the first cystoblast division, a spherical mass of fusome material (the "spectrosome") was associated with only one pole of the mitotic spindle, revealing that this division is asymmetric. During the subsequent three divisions, the growing fusome always associated with the pole of each mitotic spindle that remained in the mother cell, and only extended through the newly formed ring canals after each division was completed. These observations suggest that fusomes help establish a system of directional transport between cystocytes that underlies oocyte determination.