Abstract
Population size is a major ecological constraint on resources, which affects density-dependent selection over evolutionary time scales. Yet many species can also respond dynamically through phenotypic plasticity. In Caenorhabditis elegans and other nematodes, high population density drives juveniles into a dormant dauer stage. In Pristionchus pacificus high population density also induces the development of a predatory mouth form. However, it is unknown if such phenotypic switches respond simply to density, or to the density of specific age classes, as in density-dependent selection. Although nematodes have many experimental advantages, their small size has made investigating these and other experimental ecology questions challenging. In particular, no in vivo methodologies are currently available to label distinct populations without the need for transgenics. To study potential cross-generational signaling we developed a novel dye-based method to differentiate nematode populations on the same plate, allowing us to track adults with juveniles, or juveniles with juveniles. Surprisingly, only adult crowding induces the predatory morph, even though adult mouth-forms are no longer plastic. We then profiled secreted metabolites through development and traced this result to adult-specific pheromones. Thus, a novel dye-based method reveals age as a critical parameter controlling density-dependent plasticity.