Pheromone signalling in ciliates: insights into the evolution of the pheromone molecular structure and pheromone-gene organization in Euplotes

As is common among many life forms, also ciliates communicate via diffusible pheromones that act as cell signals for a self/non-self recognition mechanism. In Euplotes, pheromone secretion appears to be a pervasive phenomenon, which provides a unique opportunity to characterize the structures of pheromones and pheromone genes from different species, and compare these structures in an evolutionary context. For long time, knowledge of the pheromone and pheromone-gene structures has been limited to only three species, namely E. raikovi, E. nobilii and E. octocarinatus, that are proxies of only two of the six, or seven clades that are usually recognized in the Euplotes phylogenetic tree. We recently determined the pheromone and pheromone-gene structures also from E. petzi which forms the earliest branch of the tree, and from E. crassus and E. focardii that cluster inside the latest branching clade of the tree. In the light of these new determinations, the following two overall comparative pictures come forth. (i) The evolution of the pheromone-gene structure essentially involves lengthening of both the coding region and the leader 5’ non-coding region with a parallel insertion of intron sequences and, in E. crassus, gene duplication. (ii) The evolution of the pheromone molecular structure primarily involves an addition of random-coil domains that cause substantial modifications of the basic pheromone architecture based on a three-helix bundle core.