The selector gene midline controls ventral leg pattern by both inhibiting Dpp signaling and specifying ventral fate
The patterning of Drosophila limbs is controlled in part by the regional expression of selector genes, which are transcription factors that mediate binary patterning choices. The T-box transcription factor gene midline (mid) (fly Tbx20) acts as a selector gene, causing cells to adopt a ventral pattern instead of dorsal. The ventral signal Wg (fly Wnt) specifies ventral fate by activating mid in all ventral cells. Dorsal fate is controlled by Dpp (fly BMP). Our work shows that mid specifies ventral fate via at least two pathways. In the first pathway, mid inhibits dorsalization by blocking Dpp signaling. Ectopic dorsal fate induced by tkvQD, an activated Dpp receptor, is inhibited by simultaneous expression of mid. As well, mid mutant cells have increased levels of phosphorylated Mothers-Against-Decapentaplegic (pMad), a readout of the level of Dpp signaling. Expression of mid reduces the levels of pMad accumulation. These results are consistent for the Dpp-target gene reporter dad-lacZ which displays increased expression in mid mutant cells and decreased expression when ectopic mid is present. Taken together, these results imply that the role of mid in dorsal inhibition is downstream of the Dpp receptor. We suspect that mid is interacting with genes involved in Mad phosphorylation, activation, or nuclear transportation and our research is currently investigating these possibilities. In the second pathway, mid directly promotes ventral fate. Genetic mosaics that lack mid and are blocked for Dpp signaling are not rescued to ventral fate in all but one ventral structure. Thus, mid also specifies ventral fate independent of Dpp signaling. Specification of ventral fate by mid requires a known repressing domain (eh1) and putative activating domains (TD1/2). Mid is a direct transcriptional repressor of several genes expressed in the ventral domain and a mid mutant in the eh1 domain is compromised in ventral fate specification in gain-of-function assays and rescue experiments. Although we have not identified genes activated by Mid, mutants in the mid TD1/2 domains are also compromised in ventral fate specification. We propose that mid specifies ventral fate though (1) inhibition of Dpp signaling and (2) coordinating the regulation of genes in the ventral leg.