FOXO transcription factors regulate development, longevity, and stress-resistance across species. The C. elegans FOXO ortholog,
daf-16, has three major isoforms with distinct promoters and N-termini. Different combinations of isoforms regulate different processes. Adverse environments can induce dauer diapause after the second larval molt. During dauer,
daf-16 blocks specification of vulval precursor cells, including EGFR/Ras-mediated 1˚ fate specification and LIN-12/Notch-mediated 2˚ fate specification. Using isoform-specific mutants, we find that
daf-16a and
daf-16f are functionally redundant for the block to the expression of 1˚ fate markers. In contrast, all three isoforms contribute to blocking the expression of 2˚ fate markers.