Mentha (Lamiaceae) Phylogenetic Analysis Using Chloroplast TRNL-TRNF and Nuclear Ribosomal DNA ITS Sequences

Mentha (Nepetoideae, Lamiaceae) is a taxonomically complex genus that includes economically important members such as spearmint and peppermint and species of global conservation interest. Mentha is considered challenging systematically due to its high incidence of polyploidy, diverse morphology, variation in base chromosome number, and frequent interspecific hybridization. Our objectives were to test the monophyly of Mentha and each of its traditionally recognized sections, assess phylogenetic relationships of the Mentha species, test hypotheses of hybridization for the putative stabilized allopolyploids (M spicata and M. canadensis), and determine the ancestral base chromosome number using DNA sequence data from the chloroplast trnL-trnF and nuclear ribosomal internal transcribed spacer (ITS) regions. Based on trnL-trnF data, Mentha appears monophyletic. However, ITS data place the Mentha species into two distinct clades that include 12 other Mentheae genera. None of the sections with more than one species sampled form monophyletic groups based on either data set, and are therefore inconsistent with traditional classification. Cloned ITS sequences of M. canadensis and M. spicata support the hypothesis of hybridization as evidenced by nucleotide site polymorphism in ITS direct sequences and divergent clones cluster with different species. Moreover, our data indicate that M. spicata, rather than M. longifolia, may be a parent of M. canadensis. Character optimization of base chromosome number on to the molecular phylogenies shows that x = 12 may be ancestral based on trnL-trnF data or possibly x = 9 or x = 10 based on ITS data