Mutations in heterochronic genes alter the relative timing of stagespecific developmental events. One event altered in heterochronic mutants is the timing of cell divisions in P3.p-P8.p, the vulva precursor cells (VPCs). In precocious
lin-14 and
lin-28 mutants the VPCs divide approximately one stage earlier (L2 vs. L3) than in wild type. Results from epistasis analysis indicate that
lin-4,
lin-14, and
lin-28 control the timing of the VPC divisions and
lin-14 and
lin-28 are the most direct known regulators. These results suggest a model in which wild-type
lin-14 and
lin-28 are active early in development to inhibit VPC divisions. Temperature shift experiments with
lin-14 ts alleles define a temperature sensitive period (tsp) for the timing of the VPC divisions between approximately 3 hours after hatching to the Ll molt. Since this tsp begins during the previous cell cycle (P cell), it appears that
lin-14 may regulate the length of the VPC cell cycle indirectly. Since in precocious
lin-14 and
lin-28 mutants the VPC cell cycle is shortened while the previous and subsequent cell cycle lengths are unaffected, we were interested in determining whether lin- 14 and/or
lin-28 wild-type gene products are involved in the proper timing of a discrete cell cycle phase. The time of S phase was delineated in wild type using propidium iodide staining and microflourimetry of VPC nuclei. Using neighboring neurons as 2N value controls, our results indicate that Gl phase extends from the mid-Ll to a point shortly after the L2 molt (~ 20 hours of the 24 hour cell cycle) in wild-type VPC nuclei. This result implies that Gl is shortened in VPC nuclei of precocious
lin-14 and
lin-28 animals. Dauer larva developmental arrest occurs at a stage comparable to the L2 molt in continuous development. We were interested in determining whether
lin-14 and
lin-28 gene products act to inhibit the cell division cycle in animals entering the dauer larvae stage. We have observed
lin-14 and
lin-28 L2 dauers and found a reduction in the penetrance of the precocious VPC division phenotype relative to mutants in continuous development. This suggests that although
lin-14 and
lin-28 are required to inhibit early VPC divisions at the time of dauer entry, the requirement is less stringent than during continuous development.