AN INDUCTIVE INTERACTION IN 4-CELL STAGE C-ELEGANS EMBRYOS INVOLVES APX-1 EXPRESSION IN THE SIGNALING CELL

During the 4-cell stage of C. elegans embryogenesis, the P-2 blastomer e provides a signal that allows two initially equivalent sister blasto meres, called ABa and ABp, to adopt different fates. Preventing P-2 si gnalling in wild-type embryos results in defects in ABp development th at are similar to those caused by mutations in the glp-1 and apx-1 gen es, which are homologs of the Drosophila genes Notch and Delta, respec tively. Previous studies have shown that GLP-1 protein is expressed in 4-cell stage embryos in both ABa and ABp. In this report, we show tha t APX-1 protein is expressed in the P-2 blastomere and that a temperat ure-sensitive apx-1 mutant has a temperature-sensitive period between the 4-cell and 8-cell stages. We propose that APX-1 is part or all of the P-2 signal that induces ABp to adopt a fate different than ABa.