Fig. 3.

Differential requirement for N in the A compartment. Marked N^– clones in the dorsal cuticle (outlined with blue dashed lines). Clones arising in the posterior domain of A (a3-a6) do not make cuticle. We now correct our earlier report (Lawrence et al., 2002) when a6 was not included in that posterior zone: of 24 N^– clones found close to and on either side of the a6/P border in 11 abdomens only two were found in a6. These two exceptions were near the midline where N^– clones sometimes can be seen, even in a3 territory. Apart from these occasional exceptions, a3 epidermis is all unmarked (N⁺ cells) but has areas that lack bristles (indicated by arrows). These bald patches could be due to N^– clones that made neurons there and inhibited nearby bristles. We stained for neurons and found clusters of neurons of different sizes underlying the bald patches of cuticle. Of four bald patches, varying in size from one-third to most of a hemisegment, we saw underneath the cuticle five, eight, eight and ten clusters of neurons. No such clusters were seen in the normal territory flanking the bald patches. Perhaps, as in the notum of Drosophila, each N^– cell forms neurons that emit a strong Delta signal, inhibiting the formation of N⁺ bristles nearby (Heitzler and Simpson, 1991). However, clones form normally in the anterior domain of the A compartment and also in the P compartment and this image shows both.