Evolution of the genetic mechanism of self/non-self recognition in the protozoan ciliate Euplotes. REPORT of the XIIth scientific meeting of the Italian Association of Developmental and Comparative Immunobiology (IADCI), Padova

In the phylogenetic trees (mostly based on comparisons of SSU-rRNA gene sequences) of the nearly 100 morpho-species of the most cosmopolitan and ubiquitous ciliate, Euplotes, the marine complex of species E. crassus-E. minuta-E. vannus systematically appears as the latest, deeply divergent branch. In relation to the mating type systems that represent the genetic mechanism by which Euplotes, and ciliates in general, control the switching from the growth (asexual) stage to the mating (sexual) stage of the life cycle, this species complex has historically been credited by the evolution of multiple mating type systems characterized by: (i) a genetic control of the mating types provided by multiple alleles inherited at the single genetic locus mat and mutually regulated by relationships of hierarchical dominance (e. g. mat-1>mat-2>mat-3 ad so forth), and (ii) the synthesis of mating type-factors (pheromones) represented by water-insoluble, membrane-bound proteins. Working on E. crassus, we have first isolated and structurally characterized soluble, water-borne pheromones, thus demonstrating that (contrary to the historically held concept) this species and, most likely, the whole E. crassus-E. minuta-E. vannus species complex constitutively secrete their pheromones into the extracellular environment as it occurs in other, earlier branching (more ancient) Euplotes species. Then, based on the knowledge of pheromone amino acid sequences, it was possible to clone and structurally characterize a set of different pheromone coding genes. It was found that each cell type synthesizes one pheromone that is shared in common with other mutually mating compatible cell types, plus one (if mat-homozygous) or two (if mat-heterozygous) other pheromones which are cell type-specific. The implication that arises from these observations is that, in Euplotes species, the mat-locus underwent evolutionary duplication. While early branching (more ancient) Euplotes species (such as E. raikovi and E. nobilii) show this locus as single copy, E. crassus and its late branching related species would carry it replicated into two distinct copies. One copy would retain the ancestral state as multi-allelic locus deputed to control the synthesis of pheromones which are specific for each cell type, while the new copy would be manifested as mono-allelic locus deputed to control the synthesis of a pheromone which is shared in common by a group of different, but mutually interbreeding cell types. As such, this novel pheromone might function as a self-recognition signal not for a single cell type, but for a population of different and interbreeding cell types, and ensure that mating pairs are formed only between different cell types of the same population.