SPECIFICATION OF ANTERIOR-POSTERIOR DIFFERENCES WITHIN THE AB LINEAGEIN THE C-ELEGANS EMBRYO - A POLARIZING INDUCTION

In a C. elegans embryo the third cleavages of descendants of the anter ior blastomere AB of the 2-cell stage create pairs of blastomeres that develop differently. By laser ablation experiments we show that the f ates of all the posterior daughters of this division depend on an indu ction occurring three cleavages before these blastomeres are born. The time of induction precludes a direct effect on cell fate. Alternative ly, we suggest that the induction creates a heritable cell polarity wh ich is propagated through several divisions. We suggest a model to dem onstrate how a signal could be propagated through several rounds of ce ll division. An important implication of our observations is that this early induction acts to specify blastomere identity, not tissue type. A detailed lineage analysis revealed that altering the inductive sign al alters complex lineage patterns as a whole. The induction described here, together with two inductions described previously can be used t o illustrate how the anterior portion of the C. elegans embryo can be successively subdivided into blastomeres with unique developmental pot ential.