Cell cycle diversity involves differential regulation of Cyclin E activity in the Drosophila bristle cell lineage

In the Drosophila bristle lineage, five differentiated cells arise from a precursor cell after a rapid sequence of asymmetric cell divisions (one every 2 hours). We show that, in mitotic cells, this rapid cadence of cell divisions is associated with cell cycles essentially devoid of the G1-phase. This feature is due to the expression of Cyclin E that precedes each cell division, and the differential expression of the S-transition negative regulator, Dacapo. Thus, apart from endocycles (G/S), which occurred in two out of five terminal cells, two other cell cycles coexist in this lineage: (1) an atypical cell cycle (S/G2/M), in which the S-phase is initiated during the preceding telophase; and (2) a canonical cell cycle (G1/S/G2/M) with a brief G1 phase. These two types of cell cycle result from either the absence or very transient expression of Dap, respectively. Finally, we show that the fate determinant factor, Tramtrack, downregulates Cyclin E expression and is probably involved in the exit of the cells from the cell cycle.