Context-dependent relationships between the BMPs gbb and dpp during development of the Drosophila wing imaginal disk

The Drosophila BMP5/6/7/8 homolog, glass bottom boat (gbb), has been shown to be involved in proliferation and vein patterning in the wing disk. To be tter understand the roles for gbb in wing development, as well as its relat ionship with the Drosophila BMP2/4 homolog decapentaplegic (dpp), we have u sed clonal. analysis to define the functional foci of gbb during wing devel opment. Our results show that gbb has both local and long-range functions i n the disk that coincide both spatially and functionally with the establish ed functions of dpp, suggesting that both BMPs contribute to the same proce sses during wing development. Indeed, comparison of the mutant phenotypes o f dpp and gbb hypomorphs and null clones shows that both BMPs act locally a long the longitudinal and cross veins to affect the process of vein promoti on during pupal development, and long-range from a single focus along the A /P compartment boundary to affect the processes of disk proliferation and v ein specification during larval development. Moreover, we show that duplica tions of dpp are able to rescue many of the phenotypes associated with gbb mutants and clones, indicating that the functions of gbb are at least parti ally redundant with those of dpp. While this relationship is similar to tha t described for dpp and the BMP screw (scw) in the embryo, we show that the mechanisms underlying both local and long-range functions of gbb and dpp i n the wing are different. For the local foci, gbb function is confined to t he regions of the veins that require the highest levels of dpp signaling, s uggesting that gbb acts to augment dpp signaling in the same way as scw is proposed to do in the embryo. However, unlike scw-dependent signals in the embryo, these gbb signals are not transduced by the Type I receptor saxopho ne (sax), thus, the cooperativity between gbb and dpp is not achieved by si gnaling through distinct receptor complexes. For the long-range focus along the A/P compartment boundary, gbb function does not appear to affect the h igh point of the dpp gradient, but, rather, appears to be required for low points, which is the reciprocal of the relationship between dpp and scw in the embryo. Moreover, these functions of gbb also do not require the Type I receptor sax. Given these results, we conclude that the relationships betw een gbb and dpp in the wing disk represent novel paradigms for how multiple BMP ligands signal during development, and that signaling by multiple BMPs involves a variety of different interligand relationships that depend on t he developmental context in which they act.