The roles of EGF and Wnt signaling during patterning of the C. elegans Bγ/δ Equivalence Group

BACKGROUND: During development, different signaling pathways interact to specify cell fate by regulating transcription factors necessary for fate specification and morphogenesis. In Caenorhabditis elegans, the EGF-Ras and Wnt signaling pathways have been shown to interact to specify cell fate in three equivalence groups: the vulval precursor cells (VPCs), the hook competence group (HCG) and P11/12. In the VPCs, HCG and P11/12 pair, EGF and Wnt signaling positively regulate different Hox genes, each of which also functions during fate specification. In the male, EGF-Ras signaling is required to specify the Bγ fate within the Bγ/δ equivalence pair, while Notch signaling is required for Bδ fate specification. In addition, TGF-β signaling by dbl-1/dpp controls ceh-13/labial/Hox1 expression in Bγ. RESULTS: We show that EGF-Ras signaling is required for Bγ expression of ceh-13/labial/Hox1. The transcription factors lin-1/ETS and lin-31/Forkhead, functioning downstream of the EGF pathway, as well as sur-2/MED23 (a component of the Mediator complex) also control ceh-13 expression in Bγ. In addition, our results indicate that lin-44/Wnt, mom-2/Wnt and lin-17/Fz are necessary to maintain the division of Bγ along a longitudinal axis. We also show that dbl-1/dpp acts either in parallel or downstream of EGF pathway to regulate ceh-13/Hox1 expression in Bγ. Lastly, we find that a dbl-1/dpp null mutation did not cause any vulval or P12 defects and did not enhance vulval and P12 defects of reduction-of-function mutations of components of the EGF pathway. CONCLUSIONS: ceh-13/labial/Hox1 expression in Bγ is regulated by the EGF pathway and downstream factors lin-1/ETS lin-31/Forkhead and sur-2/MED23. Wnt signaling is required for proper Bγ division, perhaps to orient the Bγ mitotic spindle. Our results suggest that dbl-1/dpp is not required for VPC and P12 specification, highlighting another difference among these EGF-dependent equivalence groups.