Dauer formation in C. elegans is strongly induced at 27°, a temperature just below the highest temperature that permits growth and reproduction. Induction of dauer formation at 27° occurs both in wild-type strains and in sensitized mutant backgrounds. For example,
unc-31 and
unc-64 mutants are not Daf-c at 25°, but are strongly Daf-c at 27°.
unc-31 and
unc-64 encode proteins involved in regulated secretion and the mutants have multiple behavioral phenotypes. However, it is not known how they affect dauer formation and fit into the previously characterized genetic and molecular pathways regulating dauer formation. We have shown that the Daf-c phenotypes of
unc-31 and
unc-64 mutants are suppressed by mutations in
daf-16 but not by mutations in
daf-5. These epistasis results are similar to those seen for the Daf-c genes
daf-2 and
age-1, which define an insulin-receptor/PI3 kinase signaling pathway that regulates both dauer formation and lifespan. Other components of this signaling pathway have not been identified genetically in C. elegans. Since
unc-31 and
unc-64 mutants appear to be new Daf-c genes in the
daf-2 pathway, we investigated whether these mutants also have extended lifespans. Both mutants do indeed live significantly longer than N2, though not as long as
daf-2, consistent with their weaker Daf-c phenotypes. Furthermore, the extended lifespans of both
unc-31 and
unc-64 mutants are suppressed by mutations in
daf-16. Thus, these two genes appear to define new components of the insulin signaling pathway that regulates both dauer formation and lifespan. Our model is that they are involved in regulated secretion of an as-yet-unidentified insulin ligand for the DAF-2 receptor. To examine what tissues might secrete this ligand, we determined the cellular focus of the
unc-64 Daf-c phenotype. UNC-64 protein is expressed in many types of secretory cells, including neurons and intestinal cells (M. Nonet, pers. communication). We examined whether nervous system or intestinal specific expression of an
unc-64 cDNA could rescue the Daf-c phenotype of an
unc-64 mutant. Expression of
unc-64 in neurons (using the
aex-3 promoter) rescued the Daf-c phenotype, but expression in the intestine (using the
elt-2 promoter) did not. This suggests that the ligand for DAF-2 is secreted from somewhere in the nervous system. We have conducted extensive screens for mutants that are Daf-c at 27° but not at 25°. Only a subset of this large mutant set has been well characterized, but we already have identified a number of new Daf-c genes. Several of these mutants are well suppressed by
daf-16 and have extended lifespans, suggesting that they may encode novel components of the insulin pathway.