Loss of function of the heterochronic gene
lin-42, the C. elegans homologue of the Period family of circadian rhythm genes, causes animals to execute certain developmental events too early. This phenotype is sensitive to environmental conditions.
lin-42(
ve11) mutants raised at 20 deg C exhibit precocious development and synthesize adult cuticle at the L3 molt; however, those grown at 25 deg C are often suppressed for this timing defect. The insulin-like and TGFb signaling pathways that mediate dauer formation in response to stress are not responsible for this suppression. Neither
daf-16(lf), nor Daf-d mutations in
daf-3 or
daf-5 alter the temperature-induced suppression of the
lin-42(lf) phenotype. In contrast, mutations in the nuclear hormone receptor
daf-12(0) eliminate the temperature sensitivity of
lin-42(lf). DAF-12 acts as a critical molecular switch that regulates stress response and developmental timing. Under favorable conditions, ligand bound DAF-12 activity is thought to negatively regulate the heterochronic gene
lin-28. Since the equilibrium between ligand bound and ligand free DAF-12 is controlled by environmental conditions, LIN-28 levels may also fluctuate depending on external stress. We are testing a model wherein a temperature-induced rise in LIN-28 compensates for loss of
lin-42 activity. In support of this model, we find that
lin-42 and
lin-28 act in parallel to time development:
lin-28(lf) mutants are precocious, with adult alae observed as early as the L2 molt, albeit at low frequency, and concomitant removal of
lin-42 activity greatly enhances this phenotype. Quantitative analysis of LIN-28 levels will provide a critical test of this model. Although the temperature suppression of
lin-42(lf) is not dependent on TGFb or insulin signaling pathways, these pathways can influence
lin-42 in other situations. For example, when
lin-42(
ve11) animals containing Daf-c mutations in either
daf-7 or
daf-2, components of TGFb and insulin signaling pathways respectively, are raised at the semi-permissive temperature of 20 deg C, the
lin-42 precocious alae phenotype is suppressed. This result shows that
lin-42(lf) is sensitive to environmental stress generally and not just temperature. It is clear that
lin-42 is a key component of a complex regulatory network that integrates environmental signals with the developmental clock. In the future, it will be critical to determine how temporal and environmental cues interact to regulate
lin-42 and to identify other factors that modulate
lin-42 activity and expression. Reagents are currently being prepared to investigate these issues.