Signal pheromones of Euplotes: structure, activity and evolution.

Past research on the multiple mating-type systems of three species of Euplotes, E. raikovi, E. nobilii and E. octocarinatus, has decisively contributed to resolve the structure of the water-borne signalling molecules (pheromones) that each mating-type synthesizes to control the switching between the growth and sexual stages of its life cycle. In E. raikovi and E. nobilii in particular, it is now clear that these pheromones form species-specific families of structurally homologous proteins of 40-60 amino acids, whose basic architecture is provided by a common core of three anti-parallel alpha helices that are tightly secured together by three strictly conserved disulfide bridges. Because of their structural homology, Euplotes pheromones can thus bind to their cell-surface receptors in competition with one another and elicit a growth, or a mating cell response according to whether this binding occurs in autocrine (autologous), or paracrine (heterologous) fashion. More recently, research has been extended to characterize the pheromones of the multiple mating-type systems of E. crassus and its closely allied species, which branch much later than E. raikovi and E. nobilii in Euplotes evolution. In contradiction to the traditionally held concept that E. crassus pheromones are represented by insoluble cell-bound proteins, we succeeded in isolating these pheromones from supernatant preparations of a set of interbreeding strains and determined their primary structures by integrating chemical and genetic approaches. From this determination it comes out that the mat (mating-type) gene locus, which appears to be unique in E. raikovi and E. nobilii, underwent a phenomenon of duplication in E. crassus. As a consequence, E. crassus in addition to synthesizing mating-type-specific pheromones, like E. raikovi and E. nobilii, synthesizes also pheromones that appear to be population-specific (i. e., shared in common by a plurality of interbreeding mating-types).