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

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Abstract
A Caltech team has discovered that a cocktail of three small molecules at low concentrations acts as the sex pheromone in the roundworm Caenorhabditis elegans, whereas at higher concentration two of these compounds induce a dormant larval state known as the dauer stage. Both sexual reproduction and dauer formation, a population control mechanism that increases larval lifespan and resilience, are major life history traits. The discovery that they are regulated by largely overlapping families of small molecules suggests an intimate linkage between these aspects of C. elegans life cycle, and relates to the general observation that, for many organisms, changes in fecundity and lifespan are inversely correlated. In many organisms, population-density sensing and sexual attraction rely on small-molecule-based signalling systems1,2. 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 stage3,4. In addition, adult C. elegans males are attracted to hermaphrodites by a previously unidentified small-molecule signal5,6. 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 formation7. 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.