Taxonomic consequences of inter-specific gene flow
Taxonomy in the genus Mentha is solely based on morphology
(Harley & Brighton, 1977; Tucker et al., 1980; Tucker & Naczi, 2007;
Vining et al., 2020). However, here we show that the results of genomic
analyses are not fully congruent with a strictly morphology-based
taxonomy (Figures 1 and 2). However, although our genomic analyses
suggest that M. suaveolens , M. longifolia , M. ×rotundifolia , and M. spicata should be considered a
coalescent complex where no taxa are completely isolated genetically, we
also find that for most parts the classically recognized taxa are
morphologically supported (Figures 1 and 2). Hence, the classical
taxonomic units are still mostly useful although we suggest that an
in-depth taxonomic revision including more taxa is made. The observed
dissociations between the morphological and genomic clustering appear to
be partially due to the continued segregation of genetic variants
associated with characters of indumentum. Continued and on-going
cultivation and escape into the wild of various mint species and
varieties, including M. spicata , will likely continue to feed the
native gene pools with alleles from non-native taxa. It is therefore
likely that there will continue to be dissociations between
morphological and genomic clustering of specimens.
Here, we have focused on
mostly old herbarium specimens and hence we cannot be certain that we
have included any of the currently widely cultivated varieties ofM. spicata . These varieties could possibly be genomically
distinct and show a genomic admixture profile that is different from
what we present here. There are numerous studies on the evolutionary
origin of various mint hybrids and the phylogenetic relationships withinMentha (eg. Bunsawatt, Elliott, Hertweck, Sproles, & Alice,
2004; Gobert et al., 2002; Heylen et al., 2021; Panjeshahin,
Sharifi-Sirchi, & Samsampour, 2018). Contradictions in conclusions
between previous studies can partially be explained by biased inclusions
of particular specimens and/or inconsistent delimitation of taxa, but as
we, show here not including all sub-populations in such analyses can
also bias the results. Inclusion of multiple samples per taxonomic unit
is therefore of importance, especially for taxa, such as mints, where
frequent inter-fertility is reported (Heylen et al., 2021).