DNA barcoding to analyse taxonomically complex groups in plants: The case of Thymus (Lamiaceae)

We evaluated the utility of the core barcode regions (matK and rbcL) and the plastid intergenic spacer trnH-psbA to distinguish between Thymus spp. This is a taxonomically complex group that has been investigated so far mainly using morphological approaches. Thirty-six samples representing nine different morphospecies were collected and used for molecular analysis. The three markers showed clear amplification and sequencing. However, the genetic variation and the resulting haplotype networks showed that only Thymus capitatus forms a well-defined ‘barcoding gap’ compared with the other taxa. The identification problems observed in the other Thymus spp. may be related to reduced gene flow among populations, resulting in high intraspecific and low interspecific genetic variation. This situation does not permit the definition of species-specific barcodes. A second hypothesis suggests that morphological traits used for the delimitation of Thymus spp. do not reflect real biological and molecular species boundaries. If this is the case, the taxonomy of Thymus should be revised through extensive sampling and analyses with different tools (i.e. molecular variability, morphology, geographical distribution, etc.) to define the natural units at the species level.