Four species in the ELEGANS group of the subgenus Caenorhabditis are distinguished by two very different mating systems: androdioecy in C. elegans and Caenorhabditis briggsae with males and self-fertilizing hermaphrodites and dioecy in Caenorhabditis remanei and Caenorhabditis sp. strain CB5161 with males and females. Using chemotaxis assays, we demonstrate that females secrete a potent sex pheromone that attracts males from a distance, whereas hermaphrodites do not. The female sex pheromone is not species-specific, with males of all four species attracted to both the C. remanei and Caenorhabditis sp. female sex pheromones. The pheromone is, however, sex-specific, with only females secreting the pheromone and attracting only males. Furthermore, the sex pheromone is stage-specific, with female secretion and male detection of the pheromone beginning near adulthood. Females lose their attractiveness immediately after mating but regain it several hours after mating ceases. Finally, the female somatic gonad is required for sex-pheromone production, and the male-specific cephalic neurons (CEM) are required for male response. chemotaxis ͉ cephalic neuron ͉ sex attractant ͉ nematode mating D irect observations show that the mating efficiency of the androdioecious species Caenorhabditis elegans is poor compared with the related dioecious species Caenorhabditis remanei and that both C. remanei and C. elegans males find C. remanei females more attractive than C. elegans hermaphrodites (1). Perhaps during evolution C. elegans hermaphrodites lost the ability to secrete a sex pheromone that is still secreted by C. remanei females and this pheromone still attracts C. elegans males. The focus of the present study is to definitively determine the presence of a sex pheromone in dioecious Caenorhabditis that is absent in related hermaphroditic species.Although C. elegans is a well-studied organism, it is yet debatable whether there is a true sex pheromone in this species, although apparently males can sense a hermaphrodite-derived chemical (2, 3). If hermaphrodites do indeed secrete a true sex pheromone, it is surprising that males do not rapidly and consistently chemotax to hermaphrodites, because C. elegans is capable of chemotaxing to both water-soluble and volatile chemicals (4). In fact, detection of sex pheromones secreted by other nematode species has been fairly straightforward (5), with the secreting female commonly establishing a gradient of attractant readily followed by conspecific males.Females, in contrast to self-fertilizing hermaphrodites, must mate with males to reproduce, and natural selection should favor those females that do so rapidly and consistently after reaching adulthood. Here, we demonstrate that females from the dioecious species C. remanei and Caenorhabditis sp. strain CB5161 attract conspecific males from a distance, whereas hermaphrodites from the androdioecious species C. elegans and Caenorhabditis briggsae do not. We then obtain, by soaking females, a supernatant solution that can attract males fr...

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