| Wild isolates of Caenorhabditis elegans differ in their tendency to aggregate on food. Most quantitative variation in this behavior is explained by a polymorphism at a single amino acid in the G-protein coupled receptor NPR-1: gregarious strains carry the 215F allele, and solitary strains carry the 215V allele. The npr-1 gene regulates a behavioral syndrome with potential adaptive implications that includes: aggregation, bordering, speed of locomotion, burrowing, response to ambient oxygen and CO2, ethanol tolerance and pathogen sensitivity. The evolutionary causes and consequences of this natural polymorphism remain unclear. Here I present evidence that suggests that the npr-1 polymorphism can be maintained in C. elegans by balancing selection. First, npr-1 regulates C. elegans foraging pattern and dispersal propensity, two behaviors that can lead to the coexistence of the two alleles by promoting resource partitioning in both continuous and fragmented environments. Second, spatial variability in the environment differentially affects the fitness of npr-1 alleles. Third, the fitness of gregarious and solitary worms is frequency-dependent. Finally, the pattern of molecular variation in the npr-1 sequence is consistent with balancing selection. However, this evidence is not conclusive due to the overall low genetic variation of the C. elegans genome as a consequence of selfing and population structure. I conclude that the npr-1 polymorphism could be maintained by a trade-off between dispersal and competitive ability in a spatially heterogeneous environment, further reinforced by frequency and density-dependent mechanisms. |
