The early embryo expresses GLP-1, a receptor similar to Notch, in all the descendants of the AB blastomere. Exposure to ligand changes the fate of the AB descendants. At the 4-cell stage, P2 expresses the ligand APX-1 and contacts one AB descendant (ABp). At the 12-cell stage, MS (and E) provides a second signal and contacts a subset of ABa and ABp descendants. The ABa, but not ABp descendants respond to MS signaling, eventually turning on PHA-4 and forming the anterior pharynx. We would like to know at the molecular level how these two GLP-1-mediated interactions result in different outcomes. After numerous screens for embryonic lethal mutants lacking the anterior pharynx (the Aph phenotype), we found only two, very rare candidates that proved to be giant overlapping deficiencies extending over most of the right arm of III. Using these and other deficiencies we defined two separate intervals that contribute to the phenotype, and tested classes of genes that had family members in both intervals. Included in this list were four t-box transcription factors. Although RNAi of the candidate genes did not result in Aph phenotypes, we obtained partial rescue of the Aph phenotype with a transgene encoding one of the t-box transcription factors. We show that combined point mutations in this t-box (C24H11.3) and one of the three additional genes (Y47D3A.12) results in a penetrant Aph phenotype that closely resembles that of GLP-1 mutants defective in the 12-cell interaction. These t-box genes are expressed in all of the ABa descendants beginning at the 22-cell stage. This result indicates that their transcription is not regulated by the 12-cell, GLP-1-mediated interaction, and that these proteins may instead act in combination with a direct GLP-1 target. The fact that the genes are not expressed in ABp descendants led us to ask if they were repressed by the 4-cell, GLP-1-mediated interaction. Indeed, expression is activated in ABp descendants by a mutation in the
apx-1 gene or by RNAi depletion of
lag-1, the transcriptional effector of the GLP-1 pathway. These results raise the possibility that the ABp descendants fail to respond to the 12-cell, GLP-1 mediated interaction because the t-box genes are repressed by the 4-cell interaction. We are currently attempting to drive t-box expression in the ABp descendants to test this model.