Imaginal tissues of Drosophila melanogaster exhibit different modes of cell proliferation control

The highly conserved regulatory mechanisms that control progression of a cell through the cell cycle do not, alone, explain the programmed control of cell proliferation during animal development. Additional controls must coordinate the cell cycle regulators with developmental regulatory events. Here we report studies of cell cycle control in the imaginal tissues of Drosophila melanogaster, specifically in situations where cell cycle progression is regulated by varying the length of the G2 phase. We show that G2-phase arrest in late larval wing imaginal disks requires transcriptional control of stg, a mitotic inducer that encodes a D. melanogaster homologue of the Schizosaccharomyces pombe p80cdc25 phosphatase. In a second study, string transcriptional regulation was also shown to be important for G2-phase regulation in eye disk cells posterior to the morphogenetic furrow. Finally, unlike all other situations described to date, string transcriptional regulation was found not to be the cause of G2 arrest in abdominal histoblasts, these cells being refractory to ectopic expression of stg. This study further establishes string as an important regulator of G2 phase during D. melanogaster development, but also reveals that at least one additional mechanism is utilized to control G2-phase length and thus cell proliferation in different developmental contexts.