Morphological characters associated with cultivation continues
to segregate despite genomic admixture
Despite low levels of genetic differentiation, there are clear
morphological separations between specimens identifiable as Mentha
longifolia , M. suaveolens , and M. spicata (Figures 1 and
2 and Table 2). In particular, M. spicata shows a distinct
morphology driven by a shift in indumentum toward fewer and shorter
trichomes (Figures 1, S1, and S2). Mentha spicata is widely
cultivated for culinary use and it is therefore possible that the
reduction in characters of indumentum is associated with human selection
for a more palatable plant (less hairy; Harley & Brighton, 1977;
Munguía-Rosas, Jácome-Flores, Bello-Bedoy, Solís-Montero, &
Ochoa-Estrada, 2019), but not all cultivated mints have smooth stems and
leaves. The glands associated with trichomes are however, the main
organs for production of the desired essential mint oils (Fahn, 1979;
Jia, Liu, Gao, & Xin, 2013; McCaskill & Croteau, 1995; Mishra, Lal,
Chanotiya, & Dhawan, 2017;Yu et al., 2018). It is therefore possible
that the shift in trichome characters is a byproduct of selection for
essential oil production in cultivation (Maffei, Gallino, & Sacco,
1986; Mishra et al., 2017; Jia et al., 2013, Szabó, Sárosi, Cserháti, &
Ferenczy, 2010). The lack of previously identified genes associated with
trichome function and morphology among F SToutlier scaffolds (Table S4) is likely a result of the polygenic nature
of these characters (Chalvin, Drevensek, Dron, Bendahmane, & Boualem,
2020; Chopra et al., 2019, Figueroa-Pérez et al., 2019; Mishra et al.,
2017) coupled with the large sharing of polymorphic sites betweenM. longifolia and M. spicata (Figure 3). Our sample size
is therefore too small to detect significant genome wide associations
(Hong & Park, 2012). However, three genes;
Phospho-2-dehydro-3-deoxyheptonate aldolase 2 (DAHP2; Langer et al.,
2014), Benzyl alcohol O-benzoyltransferase (BEBT1; Boatright et al.,
2004; Orlova et al., 2006), and Tetrahydrocannabinolic acid synthase
(THCAS; Sirikantaramas et al., 2004), involved in the production of
volatile organic compounds, were found among theF ST-outlier scaffold (Table S4), indicating that
there indeed is high sequence divergence between different mints for
some genes potentially involved in the production of their
characteristic aromatic oils.
Despite the overall genetic similarities between specimens of M.
spicata and M. longifolia only a few individuals morphologically
identified as one taxon genetically cluster with the opposite species
(Figure 2a). However, these few cases do suggest that genetic variants
associated with human desirable traits can be lost despite most of the
genome otherwise being of a M. spicata origin. More importantly,
mismatches between morphology and genomic clustering suggest that the
variants associated with human desirable traits can enter the genome of
other mints through hybridization similar to what has been observed in
other systems (e.g. Fuchs et al., 2016; Karlsson et al., 2016). In
addition, some specimens of M. × rotundifolia show
admixture profiles and genomic clustering that resemble M.
spicata (rot4 and spi1, Figure 2). The morphological clustering of
these specimens suggest that they likely represent cases where gene flow
between M. spicata and M. longifolia has resulted in the
loss of genetic variants associated with the cultivated morphology and
that their morphology resemble M. × rotundifolia due to a
retention of polymorphisms from M. suaveolens . Introgression
between M. spicata and M. longifolia has previously been
suggested (Harley & Brighton, 1977), but never been evaluated using
multiple specimens per species collected across large geographic ranges.
Here we show the genetic admixture between these two species indeed is
frequent. However, despite this it is clear that genetic variants
associated with the cultivated morphology continue to segregate in both
cultivated and wild populations.