In a 4-cell Caenorhabditis elegans embryo, two sister blastomeres called ABa and ABp are born with equivalent developmental potential, but eventually produce distinct patterns of cell fate. The different fates of ABa and ABp are specified at least in part by inductive interactions with neighboring blastomeres. Previous studies indicate that, at the 4-cell stage, a signal from the posterior-most blastomere, P-2, is required for ABp to produce at least one of its unique cell types. This P-2/ABp interaction depends on glp-1, a putative receptor for intercellular interactions. To investigate this early induction further, we isolated mutants in which ABp developed abnormally. We describe the effects of recessive mutations in apx-1, a maternal gene that appears to be required for P-2 to signal ABp. In embryos from mothers homozygous for mutations in apx-1 (apx-1 embryos), ABp fails to produce its characteristic cell types. Instead, ABp from apx-1 embryos develops more like its sister ABa: it produces ABa-like pharyngeal cells and it recapitulates ABa-like cell lineages. Because mutations in apx-1 affect the development of only the ABp blastomere, we suggest that the wild-type gene encodes a component of the P-2/ABp signalling pathway. To explain the observation that ABp in apx-1 embryos adopts an ABa-like fate, we propose a model in which the P-2 signal is required to break the initial equivalence of ABa and ABp. We performed two independent tests of this model. First, we examined ABp development in pie-1 mutant embryos, in which P-2 adopts the identity of another blastomere. We find that, in pie-1 embryos, ABp fails to produce its characteristic cell types and instead adopts a fate similar to that of ABa. We conclude that the changed identity of P-2 in pie-1 embryos prevents the P-2/ABp interaction. As a second test, we examined ABp development in wild-type embryos after physically removing P-2. These operated embryos produce extra pharyngeal cells, consistent with our proposal that a signal from P-2 breaks the initially equivalent developmental state of ABa and ABp. We discuss the possibility that apx-1 acts as a ligand in this glp-1-dependent signalling pathway.
