A blend of small molecules regulates both mating and development in Caenorhabditis elegans

In many organisms, population- density sensing and sexual attraction rely on small- molecule- based signalling systems(1,2). In the nematode Caenorhabditis elegans, population density is monitored through specific glycosides of the dideoxysugar ascarylose ( the 'ascarosides') that promote entry into an alternative larval stage, the non- feeding and highly persistent dauer stage(3,4). In addition, adult C. elegans males are attracted to hermaphrodites by a previously unidentified small- molecule signal(5,6). Here we show, by means of combinatorial activity- guided fractionation of the C. elegans metabolome, that the mating signal consists of a synergistic blend of three dauer- inducing ascarosides, which we call ascr#2, ascr#3 and ascr#4. This blend of ascarosides acts as a potent male attractant at very low concentrations, whereas at the higher concentrations required for dauer formation the compounds no longer attract males and instead deter hermaphrodites. The ascarosides ascr#2 and ascr# 3 carry different, but overlapping, information, as ascr# 3 is more potent as a male attractant than ascr#2 , whereas ascr# 2 is slightly more potent than ascr# 3 in promoting dauer formation(7). We demonstrate that ascr# 2, ascr# 3 and ascr# 4 are strongly synergistic, and that two types of neuron, the amphid single- ciliated sensory neuron type K ( ASK) and the male- specific cephalic companion neuron ( CEM), are required for male attraction by ascr#3. On the basis of these results, male attraction and dauer formation in C. elegans appear as alternative behavioural responses to a common set of signalling molecules. The ascaroside signalling system thus connects reproductive and developmental pathways and represents a unique example of structure- and concentration-dependent differential activity of signalling molecules.