Fig. 1. A summary of polarising gradients in the abdomen. On the left are the types of cuticle in the A (black, a1-a6) and in the P compartment (blue, p3-p1). The compartments are patterned by morphogen gradients; Hh in A and Wg in P (Struhl et al., 1997a; Lawrence et al., 2002), which set up the Ds gradients and also the activity gradients of Fj and Ft (Casal et al., 2002). Clones (ovals) that lack or overexpress a gene affect the polarity of the surrounding wild-type cells (arrows). ptc^- en^- clones (brown) constitutively activate the Hh transduction pathway and produce reversal of the wild-type cells behind the clones (but only near the middle of the A compartment, where they cause a large discrepancy in the Hh transduction pathway between the clone and the surround). Loss of ds reverses the polarity of cells anterior to those clones located at the back of the A compartment (where the level of Ds activity is high) but has no effect on clones located at the front (where Ds activity is low). Overexpression of Ds has the opposite effects: repolarising only behind clones located near the front of the A compartment. The effects of clones involving Fj and Ft are opposite in sign to those involving Ds. In contrast to the other genes, clones involving Fz have similar effects wherever they are situated. We conjecture there is an alteration in Fz activity that spreads out from the clones as the surrounding wild-type cells readjust their levels of Fz activity by an averaging process (haloes) (Lawrence et al., 2004). This difference of clonal behaviours points to a distinction between the Ds and Stan systems.