Practice question

Early C. elegans embryo distinguishes AB daughters ABa and ABp via Notch signaling integrating cell contact geometry and timing. At four-cell stage, P2 expresses APX-1 Delta ligand contacting ABp but not ABa due to spatial arrangement and spindle orientation governed by PAR polarity. APX-1 binds GLP-1 Notch receptor on ABp, gamma-secretase cleavage releases intracellular domain that activates lag-1 transcription factor and bHLH repressors ref-1 family. This induces distinct transcriptomes; ABa generates anterior pharynx and neurons while ABp generates dorsal posterior structures. Without GLP-1-APX-1 both adopt ABa-like fate.