Inter-specific gene flow following the naturalization of a cultivated
mint promotes the formation of a coalescent complex
Abstract
Cultivation and naturalization of plants beyond their natural range can
bring previously geographically isolated taxa together, thereby
increasing the opportunity for hybridization and inter-specific gene
flow, the outcomes of which are not predictable. These anthropogenic
events therefore allow us to study how hybridization and inter-specific
gene flow affect genetic and phenotypic diversity. Here, we explore the
phenotypic and genomic effects of increased inter-specific gene flow
following the re-introduction of the cultivated Mentha spicata
(spearmint) into the ranges of the native mints M. longifolia and M.
suaveolens. Using morphological analyses, we show that the cultivated M.
spicata has altered trichome characters, which is likely a product of
human imposed selection for a more palatable plant or a byproduct of
selection on essential oil production. Using whole genome sequencing, we
then show that there is extensive genetic admixture between the
morphologically defined mint taxa that to some extent is mediated by the
cultivated M. spicata. This has, at least partially, resulted in a
breakdown of the species barriers. However, despite this breakdown, we
find that genetic variants associated with the cultivated trichome
morphology continue to segregate in cultivated, naturalized, and wild
populations and we identify three genes that may function in the
production of the characteristic aromatic oils of mints. Although
hybridization can increase species richness by forming new hybrid taxa,
we here show that unless reproductive barriers are strong it can also
merge species into population/coalescent complexes over evolutionary
time.