Most somatic cells in the nematode Caenorhabditis elegans arise from AB, the anterior blastomere of the 2-cell embryo, While the daughters of AB, ABa and ABp, are equivalent in potential at birth, they adopt different fates as a result of their unique positions, One such difference is that the distribution of epidermal precursors arising from ABp is reversed along the anterior-posterior axis relative to those arising from ABa, We have found that a strong mutation in the
glp-1 gene eliminates this ABa/ABp difference, Furthermore, extensive cell lineage analyses showed that ABp adopts an ABa-like fate in this mutant, This suggests that
glp-1 acts in a cellular interaction that makes ABp distinct from ABa, One ABp-specific cell type was previously shown to be induced by an interaction with a neighboring cell, P-2. By removing P-2 from early embryos, we have found that the widespread differences between ABa and ABp arise from induction of the entire ABp fate by P-2, Lineage analyses of genetically and physically manipulated embryos further suggest that the identities of the AB great-granddaughters (AB(8) cells) are controlled by three regulatory inputs that act in various combinations. These inputs are: (1) induction of the ABp-specific fate by P-2, (2) a previously described induction of particular AB(8) cells by a cell called MS, and (3) a process that controls whether an AB(8) cell is an epidermal precursor in the absence of either induction, When an AB(8) cell is caused to receive a new combination of these regulatory inputs, its lineage pattern is transformed to resemble the lineage of the wild-type AB(8) cell normally receiving that combination of inputs, These lineage patterns are faithfully reproduced irrespective of position in the embryo, suggesting that each combination of regulatory inputs directs a unique lineage program that is intrinsic