In the GENOMINT project we have sampled 318 specimens from three Swedish herbaria. We have focused our sampling on Europe but have also included some specimens from North America, Asia, and Africa. We have then evaluated the morphology of all specimens and the genomes on a subset of the specimens. The combined data-set allowed us to study the evolutionary history and genetic connection between mint species.
Over 30 morphological traits have been characterized. By combining the morphological traits we were able to distinguish groups of specimens that roughly represent classical mint species and hybrids. But we also found a few cases of mismatches with the original taxonomic assignments. This suggests that there is room for improvement of the current taxonomic classification of mints and our work will be used as a basis for a partially revised taxonomy of mints.
The genomes of 195 of the specimens were fully sequenced which allowed us to study a number of things in detail. Firstly we found that the taxonomic assignments based on the morphological data were in most, but not all, cases genetically supported. In particular, we found that specimens that were interpreted as morphological intermediates also appeared to be genetically mixed. This is consistent with prevalent hybridization among mints. Secondly, we found that mint hybrids generally can be divided into two groups: 1) stable hybrid species that are sterile and 2) unstable hybrid populations that retain fertility.
Sterile hybrid species are reproductively isolated and can establish themselves as new species that, in the case of mints, propagate using runners. Hybridization can therefore facilitate increased species richness, an important aspect of biodiversity. In contrast, fertile hybrids are not completely reproductively isolated and can act as genetic bridges that facilitate the transfer of genetic material between species. This creates a network of populations with varying degrees of relatedness and morphological similarity. Eventually species boundaries can be erased and some species can disappear.
Many mint hybrids are widely cultivated for their desired aromatic compounds used as flavouring in cooking and as scents in hygiene products. Consistent with previous reports we show that many mint hybrids have escaped their cultivated lifestyle and are now established across large geographic areas. As expected we find no evidence of genetic exchanges between sterile mint hybrids, including peppermint (Mentha x piperita) and Scotchmint/Scotch spearmint (Mentha x gentilis) and the naive species. The escape of these sterile mint from their cultivated status can therefore be seen as a local increase in species richness. However, it needs to be established if these hybrids can outcompete native species.
On the other hand, the escape of some fertile mint hybrids from cultivation has caused increased genetic exchanges between native species. We have particularly focused on the widely cultivated native spearmint (Mentha spicata). Here we show that this hybrid lineage acts as a genetic bridge between the two native parent species horse mint (Mentha longifolia) and apple mint (Mentha suaveolens). This has resulted in a breakdown of reproductive barriers and the formation of a large network of populations where the species boundaries are no longer clear. We also show that there has been a transfer of human desirable traits, such as smooth leaves, into native plant populations. In evolutionary times we show that it is possible that hybridization can promote the loss of biodiversity.
In conclusion we show that when cultivated species escape gardens, they can indeed re-establish themselves in the wild. The effect on the local ecosystem depends on many factors, including the reproductive barriers between the alien and native taxa. When these are strong, newly formed hybrid species contribute to an increased species richness. In contrast, fertile hybrids have the ability to act as genetic bridges and in evolutionary time cause the merging of species into large complexes which eventually can decrease the local biodiversity. Human cultivation therefore has the ability to drastically as well as quickly change the make-up of local ecosystems.
