Evolutionary origin and hidden genetic diversity of paramecium (ciliophora: oligohymenophorea)

Abstract

Members of the genus Paramecium (Ciliophora, Oligohymenophorea) are widespread ciliates able to colonize many aquatic brackish and freshwater environments. They are currently divided in 5 subgenera encompassing 19 valid species, including Paramecium multimicronucleatum, Paramecium caudatum, Paramecium aurelia (complex) and Paramecium bursaria, which are considered cosmopolitan organisms. Despite that many Paramecium species have been historically and extensively used as model organisms in research fields ranging from genomics to conservation biology, their origin and populational structure remains elusive. Here, two datasets (18S rDNA and cytochrome oxidase subunit I [COI]) including sequences from all 5 subgenera and from most of the accepted species, we used the 18S rDNA to reconstruct a time-calibrated tree of Paramecium to elucidate the evolutionary origin of extant accepted Paramecium species and to evaluate intra-genus phylogenetic relationships. The COI dataset was used to evaluate, through species delimitation algorithms (PTP and mPTP) the existence of independent evolutionary unities (IEUs) within accepted Paramecium species. Our data rises more evidences supporting the idea that most accepted Paramecium species may be, in fact complexes of species, and that the diversity of cryptic species may be higher than previously though; the identified IEUs are not spatially structured, actually, many of than co-occurs over vast geographical areas, indicating that intercontinental distances are not barriers for IEUs` dispersion and gene flow, and other eco-physiological features may be responsible to their distribution.