The border between the ultrabithorax and abdominal-A regulatory domains in the Drosophila bithorax complex

The bithorax complex in Drosophila melanogaster includes three homeobox-containing genes--Ultrabithorax (Ubx), abdominal--A (abd-A), and Abdominal-B (Abd-B)-which are required for the proper differentiation of the posterior 10 segments of the body. Each of these genes has multiple distinct regulatory regions; there is one for each segmental unit of the body plan where the genes are expressed. One additional protein- coding gene in the bithorax complex, Glut3, a sugar-transporter homolog, can be deleted without phenotype. We focus here on the upstream regulatory region for Ubx, the bithoraxoid (bxd) domain, and its border with the adjacent infraabdominal-2 (iab-2) domain, which controls abdA. These two domains can be defined by the phenotypes of rearrangement breakpoints, and by the expression patterns of enhancer traps. In D. virilis, the homeotic cluster is split between Ubx and abd-A, and so the border can also be located by a sequence comparison between species. When the border region is deleted in melanogaster, the flies show a dominant phenotype called Front-ultraabdominal (Fub); the first abdominal segment is transformed into a copy of the second abdominal segment. Thus, the border blocks the spread of activation from the bxd domain into the iab-2 domain.