| Summary: | The Antarctic convergence together with the west-to-east circumpolar current provide a powerful barrier to the movement of marine life into, or out of the region. Despite this physical isolation, Antarctic coastal sea waters are extremely rich in biodiversity. Among unicellular eukaryotes, diatoms and dinoflagellates are the dominant autotrophic forms that bloom into huge biomasses in the water column, while filter-feeding ciliates are a dominant heterotrophic group that links the benthic and pelagic food webs by eating bacteria decomposers on the seabed and being, in turn, a relevant food source for sub-adult stages of benthonic and planktonic animals. Through many years of sampling coastal sites of Terra Nova Bay (Ross Sea) for species of the most speciose ciliate, Euplotes, we established a numerous laboratory collection of strains representing five well-distinct morphospecies, E. euryhalinus, E. focardii, E. nobilii, E. petzi and E. rariseta, which manifest sex in the form of conjugation under the genetic control of high-multiple mating-type systems that, by greatly favoring outbreeding, generate an ample intraspecific genetic diversity instrumental to establish reliable inter-population phylogenetic links and phylogeographic patterns. By determining and comparing the small-subunit ribosomal RNA (SSU-rRNA) gene sequences of these strains with homologous gene sequences from strains of non-Antarctic congeneric populations, evidence was obtained that E. euryhalinus, E. focardii, E. nobilii, E. petzi and E. rariseta split into distinct clades of the Euplotes phylogenetic tree. This phylogenetic split implies that the evolutionary history of Euplotes entails multiple, independent events of colonization of the Antarctic waters and suggests, in a general perspective of microbial biogeography, that the ecological barriers to move into, or out of Antarctic waters are largely ineffective to disrupt a bipolar, cosmopolitan dispersal of eukaryotic microorganisms. Which reinforces the concept of “metapopulation” (i.e., a ...
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