Transport of Wingless in the Drosophila embryonic epidermis. S. Pfeiffer , C. Alexandre , J.P. Vincent. Div. Mammalian Development, N.I.M.R., London, U.K.

   Pattern formation in developing animals requires that cells exchange signals mediated by secreted proteins. How these signals spread is still unclear. It is generally assumed that they reach their target site either by diffusion or active transport. Here we demonstrate an alternative mode of transport for Wingless. In Drosophila embryos, wingless is transcribed in narrow stripes of cells abutting the source of Hedgehog. We show that these cells or their progeny often leave the domain of wingless expression, but only towards the anterior (never across the parasegment boundary, towards the posterior). As they do so, they no longer receive the Hedgehog signal and stop transcribing wingless. However, these cells can retain inherited protein product and carry it over a distance of up to 4 cells. In addition, Wingless can also spread independently of cell movement. The mechanism of such spread remains to be characterized. We have been able to uncouple the two mechanisms of Wingless movement and thus shown that either is sufficient to ensure a normal range of action. Interestingly, the parasegment boundary allows cells to carry Wingless only towards the anterior and this adds to other mechanisms ensuring an asymmetric range of Wingless in Drosophila embryos. Without this border, cells carrying Wingless would wander towards the posterior and disrupt segment polarity.