Genetic legacy of ancient lands: Callitriche (Plantaginaceae) indicates Beringia and Paratethys Sea as diversification hotspots for aquatic vascular plants in Eurasia

Abstract

The taxonomic status of endemic Callitriche taxa from western Beringia (C. subanceps) and the Lower Volga (C. fimbriata, C. transvolgensis) has been poorly studied. To investigate this question we compared endemic taxa with close Callitriche species. To achieve this goal we studied nuclear (ITS) and plastid (trnS-trnG, petL-psbE, trnH-psbA) genetic variation of almost 200 Callitriche samples and incorporated data on fruit shape variation and chromosome numbers. We found two genetically well-differentiated evolutionary lineages within C. heterophylla s.l. with rounded, wingless fruits, which diversification was likely associated with Beringia. One lineage occurs in western Beringia (known as C. subanceps), Greenland and northern North America, while another is confined to North America. Callitriche palustris, bearing winged, ovate fruits, is well-differentiated genetically from both C. heterophylla s.l. lineages. We refrain from any taxonomic decisions in C. heterophylla s.l. unless revision of full spectrum of diagnostic characters coupled with genetic studies with inclusion of abundant material from the both sides of the Bering Strait will be done. We demonstrated that Lower Volga endemics (C. fimbriata and C. transvolgensis) are separate species and their divergence might have been associated with dynamics of the Mediterranean, Black and Caspian seas. We point out importance of northern cryptic refugia (e.g. Lena River Delta) for cold-tolerant aquatic plants diversification.