Whole genome sequencing of mints for population gneomic analyses

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. Here, we explore the phenotypic and genomic effects of increased inter-specific gene flow following the introduction of the cultivated Mentha spicata (spearmint) into the ranges of the related wild species M. longifolia and M. suaveolens. We find that the cultivated M. spicata has altered trichome characters that is possibly 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 show that there is extensive genetic admixture between mints that is to some extent mediated by the cultivated M. spicata. This has, at least partially, resulted in a breakdown of the species barriers between native mints. 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 the likely outcome is a hybrid swarm and a breakdown of species barriers.