Induction of apoptosis in the germline and follicle layer of Drosophila egg chambers.

The reaper and head involution defective genes can induce apoptotic death in several Drosophila cell types, including portions of the embryo and eye. By a combination of FLP recombinase and the yeast Gal4/UAS transcription activation system, we expressed both cell death genes in discrete clones in the adult ovarian follicle cell layer. The expression of either reaper or head involution defective induced follicle cell apoptosis during all oogenic stages. Unexpectedly, the disruption of the follicle layer led to the induced degeneration of the nurse cells in an apoptotic manner, demonstrating a germline-somatic interaction required for germ cell viability. The germline apoptosis initiates at a specific time in oogenesis, coinciding with the beginning of vitellogenesis. This observation is intriguing given previous suggestions of a process to eliminate defective egg chambers at these same oogenic stages. The induce germline degeneration initiates with the transient formation of a network of filamentous actin around the nurse cell nucleus, in close association with a product of the adducin-related hu-li tai shao gene. This was immediately followed by nuclear condensation and DNA fragmentation, both characteristics diagnostic of apoptosis. Occurring concomitantly with the nuclear phenotypes were the disorganization of ring canals, and the degradation of Armadillo protein (a beta-catenin homolog) and filamentous actin. Germ cells degenerating as a normal consequence of oogenesis displayed a similar set of phenotypes, suggesting that a common apoptotic mechanism may underlie these different germline death phenomena.