Developmental elasticity is a ubiquitous property of developing systems, and is defined as the ability to accommodate varying lengths of developmental delays or pauses while maintaining proper cell fate programs. In C. elegans, developmental elasticity is exhibited in the context of making life history decisions in response to changing environmental conditions (food, temperature, population density): C. elegans may rapidly develop through four larval stages (L1-L4) or enter developmental arrest to form the dauer larvae. Importantly, the population density signals (ascarosides) that induce dauer formation can also cause an extended L2 stage (called 'L2d'), with or without full developmental arrest. In the context of this flexible L2 stage, the timing of L3 and L3 cell fates is robustly expressed.
hbl-1 promotes L2-specific cell division patterns, and down regulation of HBL-1 protein by the
let-7 family (
let-7-Fam) microRNAs controls progression from L2 to L3 cell fates.
let-7-Fam mutants reiterate the L2 stage divisions in later stages, which results in the extra seam cell phenotype. Ascarosides, while inducing an L2d, also impact
let-7-Fam levels, by regulating the level of the ligand (DA) of the DAF-12 nuclear hormone receptor. DAF-12 can either activate and repress the transcription of
let-7 family microRNAs, depending on the level of DA.
daf-12(
rh61) mutants express ligand-independent DAF-12, which constitutively represses the transcription of
let-7-fam microRNAs, causing an extra seam cell phenotype, just like that of
let-7-Fam mutants. Interestingly, the
daf-12(
rh61) extra seam cell phenotype is suppressed by passing through the L2d stage induced by ascarosides. This finding suggests that ascarosides can signal independently of DAF-12 ligand to control the temporal expression of HBL-1 during the L2d stage. Our data show that one of these factors is another
let-7 family microRNA,
mir-794. Moreover, we found that a TGF-beta signaling effector, DAF-3, is required for ascaroside-induced extended L2 stage, and for the suppression of the extra seam cell phenotype of the
daf-12(
rh61) mutant. We propose that a DAF-3-mediated L2d program intersects with the DAF-12 pathway so as to provide elasticity to developmental timing in the context of alternative L2 life history. We propose that these DAF-12/DAF-3 regulatory interactions modulate the levels and/or activities of
let-7-Fam microRNAs during the environmentally-extended L2 stage so as to confer precision to the temporal control of the
hbl-1 gene, and hence the proper timing of stage-specific cell fates.