Utility of rDNA ITS sequences in the systematics of Teucrium section Polium (Lamiaceae)

A phylogenetic study based on sequence data from the complete internal transcribed spacer region (ITS) of nuclear ribosomal DNA for sect. Polium of the genus Teucrium shows both intersectional congruence and intrasectional incongruence with traditional morphological classifications. We attribute this incongruence largely to problems related to the polyploid complex studied. Section Polium includes many poorly differentiated taxa of probable recent origin through hybridization followed by polyploidization. Both on the basis of parsimony and distance (Neighbor-Joining method) analyses, T. dunense and T. thymifolium are the species that diverge most from the sampled taxa. However, unlike T. thymifolium, the morphology of T. dunense is not much differentiated in relation to the other taxa. Both species are, nonetheless, the only sampled species to occupy isolated, exclusive environments and which may have undergone rapid evolution by a bottleneck effect. Teucrium dunense is found only on dunes along the Spanish and French coasts and T. thymifolium, a chasmophytic species with limited endemism, is found solely on limestone and dolomite cliffs in the low mountains in south-eastern Spain. A hypothesis is presented to explain the discrepancy between the observed comparatively large amount of ITS sequence divergence and the low morphological differentiation in T. dunense.