Practice question

LIN-39, C. elegans HOX5 ortholog, specifies vulval precursor cell competence and primary fate potential. It is expressed in central body region P3.p-P8.p during L1-L2, preventing fusion with hypodermal syncytium and promoting vulval competence by activating egl-17 and repressing fusion genes eff-1. Without LIN-39, all VPCs fuse prior to anchor cell signaling, yielding vulvaless phenotype even if LIN-3 present. LIN-39 also cooperates with Ras-MAPK to drive primary fate in P6.p by enhancing lin-3 responsiveness. LIN-12 Notch instead promotes secondary fate, and SKN-1 is embryonic factor, making LIN-39 central to VPC competence.