COMBINATORIAL SPECIFICATION OF BLASTOMERE IDENTITY BY GLP-1-DEPENDENTCELLULAR INTERACTIONS IN THE NEMATODE CAENORHABDITIS-ELEGANS

Most somatic cells in the nematode Caenorhabditis elegans arise from A B, the anterior blastomere of the 2-cell embryo, While the daughters o f AB, ABa and ABp, are equivalent in potential at birth, they adopt di fferent fates as a result of their unique positions, One such differen ce is that the distribution of epidermal precursors arising from ABp i s reversed along the anterior-posterior axis relative to those arising from ABa, We have found that a strong mutation in the glp-1 gene elim inates this ABa/ABp difference, Furthermore, extensive cell lineage an alyses showed that ABp adopts an ABa-like fate in this mutant, This su ggests that glp-1 acts in a cellular interaction that makes ABp distin ct from ABa, One ABp-specific cell type was previously shown to be ind uced by an interaction with a neighboring cell, P-2. By removing P-2 f rom early embryos, we have found that the widespread differences betwe en ABa and ABp arise from induction of the entire ABp fate by P-2, Lin eage analyses of genetically and physically manipulated embryos furthe r suggest that the identities of the AB great-granddaughters (AB(8) ce lls) are controlled by three regulatory inputs that act in various com binations. These inputs are: (1) induction of the ABp-specific fate by P-2, (2) a previously described induction of particular AB(8) cells b y a cell called MS, and (3) a process that controls whether an AB(8) c ell is an epidermal precursor in the absence of either induction, When an AB(8) cell is caused to receive a new combination of these regulat ory inputs, its lineage pattern is transformed to resemble the lineage of the wild-type AB(8) cell normally receiving that combination of in puts, These lineage patterns are faithfully reproduced irrespective of position in the embryo, suggesting that each combination of regulator y inputs directs a unique lineage program that is intrinsic to each AB (8) cell.