Autocrine and paracrine Wingless signalling in the Drosophila midgut by both continuous gradient and asynchronous bursts of wingless expression

Wingless (Wg)/ Wnt signalling is a major regulator of homeostasis in both the mammalian and Drosophila intestine. In Drosophila the organisation and function of Wingless signalling in the adult intestine remain poorly understood. Here we characterise the pattern of expression of wg , the stabilisation of its effector Armadillo in the adult Drosophila midgut, and correlate them with the response of the cells to Wg signalling activation. We show that in normal homeostasis there is a gradient of Wingless signalling in the intestinal stem cell (ISC) and the undifferentiated progenitor cell (enteroblast, EB) populations along the posterior midgut, with a high point at the midgut-hindgut boundary (pylorus). This gradient results from a combination of two sources of Wingless: a distant source outside the epithelium (the pylorus) and a local one from the ISCs and EBs themselves. Altogether, our studies show that Wingless expression and signalling in the epithelium is not continuous, but operates through bursts that occur randomly in space and time.