Increased opportunities for hybridization can merge species into
a coalescent complex
Inter-fertility of the taxa in Mentha subgen. Mentha is
well known and species barriers are possibly retained by the formation
of sterile hybrids and/or usage of distinct ecological niches (Harley &
Brighton, 1977; Gobert et al., 2002). However, the emergence of the
fertile allopolyploid M. spicata followed by its re-establishment
following cultivation in the native range of M. longifolia andM. suaveolens has resulted in frequent gene flow between the
classically recognized taxa (Figure 2; Harley & Brighton, 1977).
Indeed, all morphologically and genomically defined groups of specimens
show very low levels of genetic distances not consistent with divergent
species (Table 2; Meirmans & Hedrick, 2011; Roux et al., 2016). In
particular, the genomic differentiation between M. longifolia andM. spicata is elusive (Figure 2 and Table 2). The extensive
genetic admixture points to a breakdown of reproductive barriers between
taxonomic units and morphological groups. However, given the frequent
hybridization of M. longifolia and M. suaveolens in areas
of sympatry (Harley & Brighton, 1977) it is unlikely that these taxa
have ever been completely reproductively isolated. The emergence of the
allopolyploid M. spicata has further promoted the introgression
between M. longifolia and M. suaveolens by acting as a
genomic bridge (McDonald, Parchman, Bower, Hubert, & Rahel, 2008;
Sigel, 2016). The preferential crossing of M. spicata to M.
longifolia has resulted in low levels of genetic differentiation
between them (Table 2) and has transferred M. suaveolens genes
into a mostly M. longifolia genomic background (Figure 2).
Overall, our genomic analyses point to genetic swamping and an on-going
merging of M. longifolia , M. suaveolens , M.
spicata , and their hybrids into a coalescent complex (Beninde et al.,
2018; Ellstrand & Schierenbeck, 2000; Pinto et al., 2005; Todesco et
al., 2016; Quilodrán et al., 2020a). However, we do find that someF ST-outlier scaffolds contain genes with a
function in reproduction, and in particular, pollen recognition (Table
S4), which do suggest that there might be some genetic barriers to
cross-fertilization between the analyzed mint species, especially
between the native mints.
Human assisted movements of species, including cultivation and rearing
of species outside their native ranges have undoubtedly changed many
ecosystems. On the one hand the addition of species to new ranges
increases biodiversity by adding to the total number of species recorded
in any one location (Schlaepfer, Sax, & Olden, 2011; Hallman, Olsson,
& Tyler, 2022). However, although increased biodiversity is often
regarded as positive, shifts in ecosystem compositions can have
detrimental effects in the long term. For example, biodiversity might
drastically decrease due to alien taxa out-competing native species
(Pyšek et al., 2012). In addition, hybridization between native and
introduced taxa can also change the local biodiversity (Wolf,
Takebayashi, & Rieseberg, 2001). The exact effect of hybridization on
local biodiversity is largely controlled by the strength of reproductive
barriers between hybrids and parents and here we show that when the
reproductive barriers are weak genetic swamping is a likely outcome
(Lowry, 2008). Climate changes associated with anthropogenic activities
will cause shifts in geographic ranges and ecological niches (Kelly &
Goulden, 2008; Chen, Hill, Ohlemüller, & Thomas, 2011) and similar to
naturalizations of cultivated taxa this will bring previously isolated
taxa into contact with each other and increase the opportunities for
hybridization and inter-specific gene flow. As we show here this can
indeed promote gene flow between species and speed up the formation of
new coalescent complexes.