Ciliate Pheromones: Primordial Self-/Nonself-Recognition Signals

As is common among multicellular life forms, single-cell organisms also use pheromones to communicate among members of the same species. In protist ciliates, pheromones have been identified in functional association with mating systems. Consistent with this association, they have been regarded only as nonself-signals committed to eliciting a mating response of cells to which they bind in paracrine-like (heterologous) fashion. However, their spectrum of activity has revealed wider borders. It also includes a self-activity, which promotes the vegetative growth of the same cells from which pheromones are secreted and to which they continuously bind in autocrine fashion. This double self- and nonself-activity is made possible by the pheromones’ ability to compete with one another in cell-binding reactions. In the Euplotes species, which synthesizes pheromone families under the control of multiple series of alleles at a single locus, this ability is ensured by the relationships of the structural homology that link these molecules into species-specific globular, disulfide-rich protein families having a common three-helix fold.