The migration history of the Kamchatka rhododendron (Therorhodion camtschaticum s.l.) indicates two distinct dispersal routes towards Eastern Beringia

The Kamchatka rhododendron (Therorhodion camtschaticum s.l.) is a small, cold-resistant arcto-alpine shrub species with a controversial taxonomy and a wide Beringian distribution, from northern Eurasia to Arctic North America. Some authors regard T. camtschaticum and the closely related Therorhodion glandulosum as separate species while others suggest that they represent regionally differentiated subspecies. In order to resolve this relationship, we sampled 33 populations within the native species ranges, using three fragments of plastid DNA (ptDNA) and ten nuclear microsatellites (nSSR). We recovered two genetic lineages consistent with species rather than subspecies. The distribution of these genetic lineages are consistent with two different migration pathways of species from western Beringia to eastern Beringia: (i) for T. glandulosum a northern pathway from northern East Asia to the Seward Peninsula in northern Alaska; and (ii) for T. camtschaticum a southern pathway from the southern part of western Beringia to the Aleutian Islands and the seashore of south-east Alaska. The northern route extends from the Okhotsk seashore through the Kolyma region, Chukotka, and central and northern Kamchatka. The southern route begins at the Sikhote-Alin ridge, and goes through Sakhalin Island, the Kuril Islands, and southern Kamchatka. Demographic analysis using nSSR data inferred a divergence time between T. camtschaticum and T. glandulosum long before the Last Glacial Maximum.