CRAWFORD, Karen; St. Mary’s College of Maryland, St. Mary’s City, MD and Marine Biologcial Laboratory, Woods Hole, MA: MAPK in the D quadrant organizer of the squid embryo Loligo pealei

Classic ablation and isolation experiments performed on gastropod embryos demonstrate that the D macromere plays a critical role in axial patterning as the founder cell of the dorsal lineage. More recently, MAP kinase (MAPK) signaling cascades in the gastropod Ilyanassa obsolete have been linked to cell fate specification within the D macromere lineage and micromeres of the A, B, and C quadrants (Lambert and Nagy, 2001). To determine whether a similar signaling mechanism is present in squid, early cleavage and blastoderm stage embryos were studied by immunocytochemistry. First cleavage bisects the squid embryo along its mid-line, while second cleavage separates the anterior and posterior regions. One and two cell stage embryos did not exhibit MAPK expression. Following second cleavage, MAPK was observed in the two posterior blastomeres. With third cleavage, MAPK was found exclusively within the two posterior mid-line macromeres and persists in these cells and their descendents until just before 6th cleavage when MAPK expression is restricted to the 6 central micromeres destined to form the shell gland primordia at the dorsal-most tip of the embryo. Blocking early MAPK expression with the inhibitor U0126 results in embryos deficient in many of the tissues normally associated with the D lineage including: shell gland, heart, gills and mantle. These results provide the first molecular support for the classic notion that squid possess mirror image A, B, C, and D quadrants organized along their first cleavage furrow as described in the Cephalopoda by Adolf Naef (1928) and link the fundamental developmental mechanisms of the celphalopods to the molluscs. Supported by an ROA to KC from the NSF.