The subcosmopolitan genus Callitriche is comprised of 50–75 species that can usually be reliably distinguished only by their reproductive characters, a problem that leads to frequent misindentifications. Barcoding of the Callitriche samples, stored in the herbarium of Lomonosov Moscow State University (MW), allowed discovery C. lenisulca on the north-eastern coast of the Black sea in Russia. It is a new species for flora of Russia that was previously known only from the Mediterranean. The revealed locality belongs to the relic ecosystem of arid sub-Mediterranean forests. The observed genetic difference between the Russian sample and the Mediterranean samples suggests independent evolution of Russian population after fragmentation of a more or less continuous ancient Mediterranean vegetation. We also report a new locality for C. lenisulca in Lebanon where the species is a novelty to the Lebanon flora too.
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