Confusing boundaries of the Labrador tea species: dispersal history explains the lack of clear species structure

The Labrador teas (genus Rhododendron, subsection Ledum) are a complex of species widely distributed in the Northern Hemisphere. They occupy cold-resistant plant communities from highlands to forest understorey and wetland habitats almost circumboreally and they are especially abundant in Northeast Asia and northern North America, yet there are no clear species boundaries in this group. The genetic structure of species of subsect. Ledum from Eurasia and North America as well as the dispersal history of the group require clarification.

The phylogeny and biogeography of subsect. Ledum of the genus Rhododendron were assessed using phylogenetic trees constructed based on the analysis of variation in chloroplast petB-petD, trnV-ndhC, trnH-psbA, K2R-K707, atpB oligo2 - rbcL oligo5 and nuclear (ITS1) markers of four Eurasian and one American species (65 populations, 408 individuals). The data were evaluated with maximum parsimony and Bayesian analysis. Molecular dating and reconstruction of ancestral areas were performed.

Dense sampling revealed widespread presence of shared haplotypes and ribotypes among Ledum populations and species. Two American, three mixed and one Eurasian lineage diversified during the Neogene climate cooling and then rapidly dispersed during the Pleistocene. The ability to accumulate high genetic diversity and to preserve it across distribution ranges and generations prevented Ledum from lineage sorting. As a result, a species complex with a reserve of genetic variability appeared.

Although no clear phylogenetic inference can be obtained at present, the plastid genealogy is consistent with the nuclear genealogy and demonstrates the processes involved in speciation in the Ledum species complex.