Movement of bristle precursors contributes to the spacing pattern in Drosophila

In Drosophila melanogaster, microchaetes (small bristles) are regularly spaced and form five straight rows in the acrostichal region of the adult notum. Microchaetes develop from sensory organ precursors that arise as single, evenly-spaced cells during pupal development. In this article we address the question of how the precursor cells remain aligned throughout pupal development, in spite of continued division of the intervening epidermal cells. Using in vivo imaging we show that bristle precursors move about continuously throughout development, covering distances of up to one or two cell diameters. During this process, they remain aligned in wild-type flies, suggesting that the movement may be regulated. Flies mutant for scabrous (sca) have a disorganised pattern of bristles with little or no alignment. In vivo observations of sca mutants indicated that the precursor cells move around more than in the wild type, but that, in spite of this the precursor cells and resulting bristles never become well aligned. They appear to follow a more complex path, suggesting that the movement is not co-ordinated. Moreover, analysis of the alignment of precursor cells in vivo in wild-type and sca mutant flies indicate that mutant animals are not able to maintain the pattern of precursor cells during development. Analysis of mosaic flies confirmed the time-lapse observations and showed furthermore that bristles preferentially move towards high levels of Scabrous. We suggest that, by altering the properties of epithelial cells in a graded fashion, Scabrous may provide cues that allow the precursors to remain evenly spaced after they have segregated.