Laboratory evolution from social to solitary behavior in the N2 reference strain is unnecessary for its fitness advantages

Abstract

The standard reference Caenorhabditis elegans strain, N2, has evolved marked behavioral changes since its isolation from the wild 67 years ago. Laboratory-derived variation in two genes, npr-1 and glb-5, suppress aerotaxis behaviors on food, resulting in N2 animals evolving from social to solitary feeding strategies. We show here that the derived alleles of npr-1 and glb-5 can confer large fitness advantages in standard laboratory conditions, suggesting that the changes in feeding strategies were beneficial to the N2 strain. However, by using environmental manipulations that suppress social behaviors, we showed the fitness advantages of the derived alleles remained unchanged, suggesting selection on these alleles acted through biological traits unrelated to solitary behavior. Transcriptomics analysis, developmental timing assays, and feeding assays showed that N2 animals mature faster, produce more sperm, and eat more food than a strain containing ancestral alleles of these genes (CX12311) regardless of the behavioral strategies. The O2-sensing neurons URX, AQR, and PQR and the pheromone biosynthesis and lipid regulating enzyme encoded by daf-22 are necessary for the full fitness advantages. We suggest that changes to social/solitary behavior in N2 were a pleiotropic consequence of npr-1 and glb-5s ability to modify integrated O2 and pheromone neural circuits that regulate feeding rate and reproductive development. Together, our results demonstrate how laboratory evolution can lead to profound changes in a strain used as a model by for understanding a variety of fundamental biological processes.