Protists thrive in polar oceans, where they\ud
represent a major driving force for globally important\ud
biogeochemical cycles and a key food-web component.\ud
Their biogeography is frequently associated to\ud
bipolar patterns of distribution. Although conceptually\ud
well supported by apparently unrestricted migration\ud
rates, the experimental certification of these patterns\ud
copes with the protist paucity of morphological\ud
characters with taxonomic value and difficulties in\ud
applying conventional species concepts. We studied\ud
three marine species of the ciliate Euplotes, E.\ud
euryhalinus, E. nobilii, and E. petzi, for their bipolar\ud
distribution by comparing the SSU-rRNA gene sequences\ud
and mating interactions of Antarctic,\ud
Patagonian, and Arctic strains. Each species was\ud
analogously found not to carry significantly varied\ud
SSU-rRNA gene sequences, implying a common\ud
occurrence of trans-equatorial genetic mixing. However,\ud
mating analyses revealed significant interspecies\ud
differences. Scarce Antarctic 9 Arctic strain\ud
mating compatibility distinguished E. petzi from E.\ud
euryhalinus and E. nobilii, in which mating pairs\ud
between Antarctic and Arctic strains were successfully\ud
induced. Yet, E. nobilii was the only one of the two\ud
species to show cross-fertilizing and fertile mating\ud
pairs. Taking the biological concept of species as\ud
discriminatory, it was thus concluded that only E.\ud
nobilii warrants the definition of genuine bipolar\ud
species