Pheromone evolution in the protozoan ciliate, Euplotes: The ability to synthesize diffusible forms is ancestral and secondarily lost

In relation to their self-nonself recognition systems (commonly referred to as mating type systems), ciliates synthesize signal molecules deputed at the control of the cell/organism switching between the asexual (mitogenic proliferation) and sexual (mating) stages of the life cycle. In the most cosmopolitan and ubiquitous ciliate, Euplotes, these molecules are represented by protein pheromones that some species constitutively diffuse into the extracellular environment, and others retain bound to the cell surface. We have determined (on the basis of SSU rRNA sequence comparisons) the phylogenetic relationships of a set of known and newly identified pheromone-secreting and pheromone-retaining Euplotes species, and found that the former species represent the earliest branches of the tree while the latter ones cluster together into the latest divergent clade. We thus concluded that pheromone secretion is ancestral in Euplotes evolution while pheromone retention represents a derived condition, and tentatively identified the cause of this evolutionary shift with a mutation that prevents pheromone genes from undergoing splicing.