Meeting Abstract
37.3 Sunday, Jan. 5 08:30 Unequal second cleavage (of the CD blastomere) in the leech Helobdella austinensis ROGERS, D.V.; WEISBLAT, D.A.*; Univ. of California, Berkeley; Univ. of California, Berkeley weisblat@berkeley.edu
Among animals that develop via spiral cleavage (a diverse group that may be synonymous with the super-phylum Lophotrochozoa), D quadrant specification is the process by which the four-fold rotational symmetry of the early embryo is broken to establish the bilateral symmetry of the late embryo/adult. In equal cleavers (the presumed ancestral state), D quadrant specification involves stochastic interactions among cells of initially equipotent quadrants, and may occur as late as the 28-cell stage. In unequal cleavers, D quadrant specification entails the unequal segregation of D quadrant determinants beginning with the first cell division. Among clitellate annelids (oligochaetes and leeches), unequal cleavage entails formation of yolk-deficient, RNA-rich cytoplasm (teloplasm) at the animal and vegetal poles prior to first cleavage. An unequal first cell division segregates teloplasm to the CD blastomere; at second cleavage, the unequal division of cell CD segregates teloplasm to one blastomere at the 4-cell stage, defining it as the D quadrant macromere. The embryo is bilaterally symmetric along the AB-CD axis during the early 2-cell stage; symmetry is broken during mitosis by a rightward shift of the mitotic apparatus (MA). The MA itself is symmetric; its movement depends on non-muscle myosin activity–movement is blocked and the cleavage is equalized by treatment with blebbistatin (to inhibit non-muscle myosin) and ML-7 (to inhibit myosin activation). The rightward shift of the MA correlates with a change in the radius of curvature of the CD cell at the left vs. right side of the embryo. To determine the sub-cellular site of actomyosin activity, we are using time-lapse video analysis to ask if the radius of curvature in blebbistatin-treated embryos is more similar to the left side or the right side of unequally cleaving controls.
