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Dear Editor,The genus Mentha has considerable economic importance as a source of essential oils for the flavor,fragrance,and aromatherapy industries (Lange and Ahkami,2013).The history of commercial mint cultivars is complex and involves various polyploid hybrids.The most widely grown cultivars are essentially sterile,which has impeded the development of genetic resources.The work presented here aims to shed more light into the factors that determine oil composition,which is important for clonal selection strategies.Spearmint (Mentha spicata L.) most likely arose from a natural cross of diploid (2n =24) apple mint (Mentha suaveolens Ehrh.) and horse mint (Mentha Iongifolia L.) (Tucker and Naczi,2006) (Figure 1A).The signature component of the oil is the C6-oxygenated p-menthane monoterpene (-)-carvone (Figure 1B and Supplemental Information).The musty-odored oil of most accessions of horse mint is more complex,with abundant C3-oxygenated cispiperitone oxide,piperitenone oxide,and pulegone (Vining et al.,2005),while the oil of certain apple mint accessions is rich in (-)-carvone (El-Kashoury et al.,2012),indicating a dominant contribution of alleles from the apple mint parent to determine the oil characteristics of spearmint.Peppermint (Mentha x piperita L.) (2n =72;hexaploid) emanated from a natural hybrid cross between spearmint and octoploid (2n =96) watermint (Mentha aquatica L.) (Tucker and Naczi,2006) (Figure 1A).The oil distilled from peppermint contains predominantly C3-oxygenated monoterpenes;(-)-menthone and (-)-menthol are the most prominent metabolites.The principal constituent of watermint oil is (+)-menthofuran,which is also derived from C3-oxygenated precursors (Murray et al.,1972) (Figure 1B and Supplemental Information).The watermint parent contributes four of the six genomes in the peppermint hybrid,which is reflected in the predominance of C3-oxygenated monoterpenes in both species.