Biol. Bull. 207: 166. (October 2004)
© 2004 Marine Biological Laboratory
MAP Kinase Expression Correlates With the Posterior Midline in Early Cleavage Stage Squid Embryos
Karen Crawford
Marine Biological Laboratory, Woods Hole, Massachusetts; and St. Mary’s College of Maryland, Maryland
Classical 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. Recently, MAP kinase (MAPK) signaling cascades in the gastropod Ilyanassa obsoleta have been linked to cell fate specification within the D macromere lineage and the micromeres of the A, B, and C quadrants (Lambert and Nagy, 2001). To determine whether a similar signaling mechanism is present in the long-finned squid, Loligo pealei, early cleavage stages and blastoderm stage embryos were studied by immunocytochemistry.
First cleavage bisects the squid embryo along its midline, 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. Expression was greatest along the first cleavage furrow or posterior midline, appearing more intensely in embryos fixed just prior to third cleavage. With third cleavage, MAPK was found exclusively within the two posterior midline macromeres. In living squid embryos, these cells undergo a delayed cleavage in comparison to the rest of the cells within the embryo. Departure from synchronous cleavage is characteristic of the D-lineage cells of gastropods. Just before sixth cleavage MAPK expression is restricted to the six central micromeres destined to form the shell gland primordia at the dorsal-most tip of the embryo. The D-lineage in gastropods also contributes early to the shell gland primordia. In later blastoderm stage embryos, MAPK expression is found in just two posterior cells; it fades while, with the onset of epiboly, an outer MAPK positive ring of cells begins to form. 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 Cephalopoda, by Adolf Naef in 1928.
Supported by a Faculty Development Grant and Aldom-Plansoen Distinguished Endowed Professorship in Contemporary Studies to K.C. from St. Mary’s College of Maryland, and by Robert Baker (MBL and NYU).
