Molecular changes that underlie evolutionary changes in behavior and physiology are not well understood. Dauer formation in C. elegans is a temperature-sensitive process controlled through a network of signaling pathways associated with sensory neurons, and is potentially an excellent system in which to investigate molecular changes in neuronal function during evolution. To begin to investigate the evolution of dauer formation in the genus Caenorhabditis at the molecular level, we isolated dauer-formation mutations in Caenorhabditis briggsae, a species closely related to the model organism C. elegans. We identified mutations in orthologs of C. elegans genes
daf-2 (insulin receptor),
daf-3 (Smad) and
daf-4 (TGF-beta type 2 receptor), as well as genes required for formation of sensory cilia. Phenotypic analyses revealed that functions of these genes are conserved between C. elegans and C. briggsae. Analysis of C. briggsae mutations also revealed a significant difference between the two species in their responses to high temperatures (>26 degrees ). C. elegans is strongly induced to form dauers at temperatures above 26 degrees , near the upper limit for growth of C. elegans. In contrast, C. briggsae, which is capable of growth at higher temperatures than C. elegans, lacks this response.