Fig. 8. Alternative deployment of the micromere-PMC GRN by NSM cells. (A) An expanded view of the PMC GRN [modified, with permission, from Oliveri et al. (Oliveri et al., 2008)]. Many regulatory components of the micromere-PMC GRN are normally deployed in NSM cells (black ovals). One key regulator not expressed by these cells is alx1 (dark blue box). alx1 controls a subcircuit that activates biomineralization and morphoregulatory genes via intermediaries such as dri, foxB (light blue boxes) and snail (not shown). (B-D) Ectopic deployment of alx1 during NSM transfating (L. variegatus). (B) An embryo at the mesenchyme blastula stage. alx1 mRNA expression (dark purple) is restricted to PMCs. (C) A mesenchyme blastula stage embryo immediately after the microsurgical removal of PMCs. (D) An embryo 6 hours after the microsurgical removal of PMCs. alx1 is expressed ectopically (dark purple) by NSM cells at the tip of the archenteron. Activation of alx1 in NSM cells occurs by a novel, pmar1-independent mechanism. (E) Expression of alx1 is sufficient to trigger NSM transfating. Co-injection of alx1 mRNA and a lineage tracer (green nuclear label) into one macromere at the 16-cell stage induces descendants of the labeled cell to adopt the PMC fate, as shown by immunostaining using a monoclonal antibody that recognizes MSP130 proteins, a family of PMC-specific cell surface proteins (red). White arrows mark transfated cells. Figure modified, with permission, from Ettensohn et al. (Ettensohn et al., 2007).