A role for the DP subunit of the E2F transcription factor in axis determination during Drosophila oogenesis

The E2F family of transcription factors contributes to cell cycle control b y regulating the transcription of DNA replication factors. Functional 'E2F' is a DNA-binding heterodimer composed of E2F and DP proteins. Drosophila c ontains two E2F genes (dE2F, dE2F2) and one DP gene (dDP). Mutation of eith er dE2F or dDP eliminates GI-S transcription of known replication factors d uring embryogenesis and compromises DNA replication. However, the analysis of these mutant phenotypes is complicated by the perdurance of maternally s upplied gene function. To address this and to further analyze the role of E 2F transcription factors in development we have phenotypically characterize d mitotic clones of dDP mutant cells in the female germline. Our analysis i ndicates that dDP is required for several essential processes during oogene sis, In a fraction of the mutant egg chambers the germ cells execute one ex tra round of mitosis, suggesting that in this tissue dDP is uniquely utiliz ed for cell cycle arrest rather than cell cycle progression. Mutation of dD P in the germline also prevents nurse cell cytoplasm transfer to the oocyte , resulting in a 'dumpless' phenotype that blocks oocyte development. This phenotype likely results from both disruption of the actin cytoskeleton and a failure of nurse cell apoptosis, each of which are required for normal c ytoplasmic transfer. Lastly, we found that dDP is required for the establis hment of the dorsal-ventral axis, as loss of dDP function prevents the loca lized expression of the EGFR ligand Gurken in the oocyte, which initiates d orsal-ventral polarity in the egg chamber. Thus we have uncovered new funct ions for E2F transcription factors during development, including an unexpec ted role in pattern formation.