How do Dachsous and Fat polarise cells in the larval abdomen of Drosophila?

We investigate the mechanisms of planar cell polarity (PCP) in the Drosophila larva. The epidermis displays an intricate pattern of polarity and is excellent for the study of one system of PCP, the Dachsous/Fat system; partly because the Starry Night/Frizzled system plays no discernable role in the larva. Measurements of the amount of Dachsous reveal a peak near the rear of the anterior compartment. Localisation of Dachs and orientation of ectopic denticles reveal the polarity of every cell in the segment. We discuss how well these findings evidence our gradient model of Dachsous activity. Several groups have proposed that Dachsous and Fat fix the direction of PCP via oriented microtubules that transport PCP proteins to one side of the cell. We test this proposition in the larval cells and find that most microtubules grow perpendicularly to the axis of PCP. We find no meaningful bias in the polarity of those microtubules aligned close to that axis. We also reexamine published data from the pupal abdomen and fail to find evidence supporting the hypothesis that microtubular orientation draws the arrow of PCP.