Natural isolates of C. elegans exhibit either social or solitary feeding behaviour. Social strains aggregate and feed together on a bacterial lawn; solitary strains show no aggregation and feed in isolation. This variation is due to a single amino acid change in NPR-1, a seven transmembrane receptor related to mammalian neuropeptide Y receptors. Null mutations in
npr-1 transform solitary wild strains into strongly social animals. We wish to elucidate the molecular and neuronal circuitry controlling social versus solitary feeding. An EMS mutagenesis screen has been performed starting with strongly social
npr-1(null) animals, and selecting for mutants that feed alone. Forty mutations have been isolated thus far, which show varying degrees of suppression of social behaviour. Twelve strong suppressors have been chosen and are being mapped and characterised further. We hope these suppressors will identify molecules that either regulate the choice between social and solitary feeding, or are essential signal transduction components of the sensory inputs leading to social behaviour. A survey of C. elegans mutants has shown that most do not disrupt social behaviour of
npr-1(null) animals. Alleles that do suppress social behaviour include loss-of-function mutations of
tax-2 and
tax-4, which encode subunits of a cyclic nucleotide-gated cation channel.
tax-4 expression has been reported in ten neurones, and null mutations in this gene disrupt thermosensation, chemotaxis to salts, and response to certain odorants. In which neurones is
tax-4 required for social behaviour? We are currently expressing
tax-4 cDNA (kindly provided by Ikue Mori) in subsets of
tax-4 expressing neurones, in a
tax-4;
npr-1 mutant background, and testing for rescue of social feeding behaviour. We are also taking a similar approach to identify the cellular focus of
npr-1 action in repressing social behaviour. What are the environmental signals regulating social versus solitary feeding? Social animals only aggregate in the presence of a food source, and a sufficiently high population density of worms. We are investigating the behaviour of worms on different types of soil bacteria, and on different growth media, to test for the presence of possible secreted or mechanical signals that induce social feeding.