Two distinct foraging behaviors have been recognized among wild isolates of C. elegans: solitary worms forage individually and spread on the bacterial lawn, while social worms ("clumpers") gather in groups and spend most of their time at the border of the lawn. Most quantitative variation in this behavior can be explained by a single amino acid polymorphism in the
npr-1 gene (de Bono and Bargman 1998 Cell 94: 679-689). NPR-1 has been implicated in important biological processes, such as oxygen avoidance and ethanol tolerance. Nevertheless, there is no obvious geographical pattern to the distribution of this polymorphism and the evolutionary causes for its maintenance in nature remain unclear. Here we show that worms carrying different
npr-1 alleles differ in their propensity to disperse away from food on a patchy environment and in their male mating success. Hermaphrodites from three social strains (CB4856, CB4857 and RC301) were approximately six times more likely to disperse away from food per unit time than were hermaphrodites from three solitary strains (CB4555, N2 and TR389). A strain containing the social
npr-1 allele from RC301 in a solitary N2 background (DA650), dispersed at the same rate as the social strains. One possible evolutionary cause of dispersal is limitation of resources. In agreement with this hypothesis, we found that social worms were more active foragers and ran out of food more quickly than solitary worms. In addition, social strains were capable of maintaining a higher frequency of males than solitary ones. In RC301, the increase in the rate of outcrossing is explained by a
npr-1-dependent improvement of male mating ability. Preliminary results suggest that social males disperse faster than solitary ones. Thus, male dispersal propensity might be causally related to male mating success via a mate-searching behavior regulated by the serotonin, insulin and sex-determination pathways (Lipton et al. 2004 J. Neurosci. 24: 7427-34).We propose that the polymorphism in
npr-1 is maintained by spatio-temporal variation in resource availability and/or quality: favorable, stable conditions would select for solitary behavior, low dispersal ability and a low rate of outcrossing, whereas poor, unpredictable conditions would select for aggregation behavior, high dispersal ability and a high rate of outcrossing.