Sensory mother cells are selected from among mitotically quiescent cluster of cells in the wing disc of Drosophila

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Abstract
The large sensory bristles on the head and thorax of Drosophila are arranged in a precise pattern. Each bristle is formed by the progeny of a sensory mother cell (SMC) which is generated in the imaginal wing disc. The proneural genes ac and sc play an important role in the correct positioning of these SMCs by conferring to reproducibly located clusters of cells the competence to become SMCs. Indirect evidence suggested that the mitotic state of a cell could also play an important role in SMC positioning. In order to analyze the relation between the formation of SMC and the mitotic activity in the developing wing disc, we used BrdU immunolabeling to monitor the mitotic activity in an enhancertrap transformant line, A101, where the SMCs can be visualized. Our results indicate that SMCs arise from clusters of mitotically quiescent cells (MQC), and that the cell that becomes a SMC was itself arrested at the G2 stage of the cell cycle. The emergence of MQCs follows a precise temporal and spatial pattern which is not affected by the absence of the ac and/or sc genes. We propose that the reproducible emergence of MQCs could be an important factor in the formation of SMCs.