Pheromones of the ciliate Euplotes octocarinatus not only induce conjugation but also function as chemoattractants

Cells of the ten mating types of the ciliate Euplotes octocarinatus communicate by pheromones before they enter conjugation. The pheromones induce homotypic pairing when applied to mating types that do not secrete the same pheromone(s). Heterotypic pairs (i.e., those between cells of different mating types) are formed only when both mating types in a mixture secrete a pheromone that the other does not. The genetics of mating types is based on four codominant mating type alleles, each allele determining production of a different pheromone. Here we report that the pheromones not only induce pair formation but also attract cells. This was shown by placing cells of various mating types in neighboring agar wells so that the pheromones could diffuse from one well to the next. We found that the cells accumulated on the side of the well where a pheromone entered by diffusion. This response was observed only if the pheromone had the capacity to induce the cells to conjugate. That the pheromones and not some other substances attract the cells was shown by placing pheromone 3, expressed in Escherichia coli, in wells next to tester strains. Mating types known to respond to pheromone 3 by pair formation also showed accumulation on the side of the well at which the pheromone entered by diffusion. Since the pattern of cell attraction corresponds with the pattern of conjugation induction, we suggest that not only conjugation induction but also cell attraction is governed by pheromone-specific receptors. In addition, we describe a succession of changes in the behavior of cells affected by the pheromones.