New insights into the adaptive molecular evolution of a bipolar ciliate, Euplotes nobilii.

The biological concept of “metapopulation”(i.e, a group of populations of the same species separated by space but linked by dispersal and migration) applies properly also to protists’biogeography, as originally fostered by Finlay and Fenchel (2004) with particular reference to ciliates. By sampling ciliates over many years from coastal marine waters of Antarctic, Patagonian and Arctic sites, we realized a wide collection of strains representing heterospecific psychrotrophic and psychrophilic populations of Euplotes, an extremely diversified genus that stands at the forefront of experimental ciliatology. Among these strains, those representing E. nobilii have more profitably been used to improve our knowledge of the molecular adaptation of single-celled organisms to the extreme environmental conditions of polar waters. Following evidence from genetic and breeding analysis that they effectively represent geographically separated populations of the same panmictic metapopulation, a dozen of them (growing better at 4 ℃ at a stable rate of one fission every other day) were chosen to determine the structures of their water-borne pheromones (cell type-specific proteins that preside self/not-self cell-cell interactions) and get insights on the molecular basis of the cell defense against UV radiation and protein oxidation. In comparison with its sister species, E. raikovi, that is widespread in temperate sea waters, E. nobilii (i) synthesizes less thermostable pheromones characterized by a marked reduction of regular helical structures and a consequent extension of random coil regions (functional to improve structural flexibility), and (ii) reinforces its enzymatic barriers to UV and oxidative damages also utilizing chaperons and antioxidant enzymes (including methionine sulfoxide reductases responsible for reducing methionine sulfoxide back to methionine) which are continuously released into the cytoplasm by endosymbiotic Parafrancisella bacteria that appear to be pervasive colonizers of E. nobilii cells.