Molecular evolution of Euplotes pheromones and pheromone-coding genes.

In the ciliate Euplotes, species-specific families of water-borne protein pheromones regulate self/not-self recognition phenomena which are responsible for the cell decision to switch between vegetative (mitotic) growth and sexual mating. The knowledge of the pheromone and pheromone-gene structures has recently been widened to a number of species that localize in different positions of the Euplotesphylogenetic tree and thrive in different environments, making it possible to seek into how the structures of these molecules evolve in relation to speciation. The development of one or more (usually Gly-rich) random-coil segments is a major trait of the evolution of the pheromone structure, which is basically determined by a common tightly conserved, disulfide-rich helical fold. By determining sites of local flexibility of the molecular backbone,these segments come to serve the double function of greatly improving the pheromone adaptive plasticity and capability to interact with other proteins. In parallel, the pheromone-gene structural evolution primarily involves the inclusion of multiple intron sequences within the 5’-leader region or, more rarely, within the coding region. By determininga mechanism of alternative splicing, these sequencesmake each pheromone gene (which is expressed in the somatic genome of the cell macronucleus) capable of synthesizing multiple mRNAs in addition to the pheromone-specific transcript.