Trichome characters distinguish morphological groups
A PCA of all assessed morphological characters (Tables S2 and S3) separates the specimens rather well. The first component (PC1) separated the cultivated M. spicata from the other mints, while PC2 separates the two native mints (M. longifolia and M. suaveolens ) with the hybrid M. × rotundifolia (M. longifolia × suaveolens ) falling intermediate (Figure 1a). However, the separation between M. longifolia and M. ×rotundifolia is not very clear (Figure 1a). Five specimens were identified as M. × villosa (M. suaveolens ×spicata ), one as M. × villosa-nervata (M. longifolia × spicata ), and one as M. capensis , a South-African close relative of the EurasianM.longifolia /M.spicata (Figure 1). Although the clustering of specimens in the morphological space is overall consistent with current taxonomic assignment, the distinction of M. spicata is largely driven by trichome characters and when these are removed there is a much more cloud-like projection (Figure 1b). There is however, a tendency for the taxonomic units to roughly separate on the first PC with M. × rotundifolia intermediate to M. longifolia and M. suaveolens (Figure 1b). The importance of trichome morphology is particularly evident when only characters of indumentum are considered which results in a projection of specimens resembling that of the overall analysis and where the first PC explains as much as 53.3% of the variation among specimens (Figure 1c).
Distribution analyses of the indumentum characters show that the two wild taxa have large variations in characters of indumentum, withM. longifolia generally being more ‘hairy’ than M. suaveolens , and branched trichomes are almost exclusively found in the latter (Figure S1 and S2). However, while the hybrid M. × rotundifolia show large variation in indumentum with distributions mostly resembling that of M. longifolia , the widely cultivatedM. spicata shows less variation and is less hairy, mostly with distributions outside of the ranges of the native taxa (Figures S1 and S2).