TIME-DEPENDENT RESPONSES TO GLP-1-MEDIATED INDUCTIONS IN EARLY C-ELEGANS EMBRYOS

In an embryo of the nematode Caenorhabditis elegans, two blastomeres a t the 4-cell stage, ABa and ABp, are born with equivalent developmenta l potential. Subsequently, interactions with the P-2 blastomere at the 4-cell stage and the MS blastomere at the 12-cell stage generate diff erences in developmental fate among descendants of ABa and ABp. We hav e reproduced these inductions in vitro using embryonic blastomeres iso lated in cell-culture medium. We show that during these inductions onl y the responding AB descendants require the activity of the glp-1 gene , which is similar in sequence to Drosophila Notch, supporting models in which GLP-1 protein acts as a receptor for both the P-2 and MS sign als. We also show that P-2 signaling requires the activity of the apx- 1 gene, similar in sequence to Drosophila Delta, and that MS signaling requires the putative transcription factor SKN-1. We present evidence that the primary factor determining the different responses to these two signals is the age of the AB descendants, not the identity of the signaling cell or ligand. Therefore, we suggest that time-dependent ch anges in factors within AB descendants are responsible for their diffe rent responses to inductive signals that use a common receptor.