Periodic Reporting for period 1 - GENOMINT (Biogeography and natural hybridization in the genus Mentha)
Reporting period: 2019-09-01 to 2021-08-31
Saving and promoting biodiversity is one of the greatest challenges humans currently face. Human activities, including plant cultivation, have the potential to drastically change native plant biodiversity. In particular, the release of cultivated taxa into the wild will allow previously isolated taxa to come into contact with each other. This can promote reproduction between closely related cultivated and local species (hybridization) which ultimately can affect the native biodiversity.
The evolutionary outcome of hybridization between previously isolated taxa varies greatly. One possibility is that hybridization results in the formation of a new species that is reproductively isolated from its parents. Another possibility is that hybridization results in fertile hybrids. If the reproductive barriers between these hybrids and the parents are weak it is possible that hybridization will continue and that the original species barriers are erased. Hybridization between previously isolated species therefore has a great potential to change plant biodiversity. It is therefore important to study cases where cultivated taxa have escaped into the wild and come into contact with native plants.
The overall objective of the GENOMINT project is to understand how hybridization between mint species affects the evolutionary ability and biogeographic history of mints as well as how it contributes to biodiversity. In addition, we aim to understand if hybridization between cultivated and native mints can transfer genetic material associated with human desired characters into local populations. In order to do so we take advantage of the vast plant biodiversity stored in herbarium collections. Technological advances in the past decades have also made it possible to access the genetic information stored in these collections.
The evolutionary outcome of hybridization between previously isolated taxa varies greatly. One possibility is that hybridization results in the formation of a new species that is reproductively isolated from its parents. Another possibility is that hybridization results in fertile hybrids. If the reproductive barriers between these hybrids and the parents are weak it is possible that hybridization will continue and that the original species barriers are erased. Hybridization between previously isolated species therefore has a great potential to change plant biodiversity. It is therefore important to study cases where cultivated taxa have escaped into the wild and come into contact with native plants.
The overall objective of the GENOMINT project is to understand how hybridization between mint species affects the evolutionary ability and biogeographic history of mints as well as how it contributes to biodiversity. In addition, we aim to understand if hybridization between cultivated and native mints can transfer genetic material associated with human desired characters into local populations. In order to do so we take advantage of the vast plant biodiversity stored in herbarium collections. Technological advances in the past decades have also made it possible to access the genetic information stored in these collections.
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.
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.
The main mode of communication of the results of the GENOMINT project is scientific publications. However, the results are also of interest to the wider community and therefore participation in public communication has also been a main aim. Therefore, as COVID-19 restriction eases, we will actively communicate our results at public events including Science and Culture nights. An article aimed at the popular science magazine “The Conversation” will also be prepared once the final results of the project have been published.
The genetic data produced, and the interpretation of it, in the project is also of interest to the mint breeding community. Genetic information on human desirable traits, including the production of aromatic compounds, is also highly relevant to breeders of other aromatic herbs, including sage and thyme. Targeted announcements, similar to a press-release, to relevant breeding communities will therefore be prepared. The genetic data can for example be used for target breeding efforts or efforts to genetically improve cultivars using new technologies, including the Nobel prize winning CRISPER/Cas9 technology. The results of such efforts will have a large socio-economic impact as it will provide firsthand knowledge on how crops and cultivated plants can be modified to meet the growing food demands.
The genetic data produced, and the interpretation of it, in the project is also of interest to the mint breeding community. Genetic information on human desirable traits, including the production of aromatic compounds, is also highly relevant to breeders of other aromatic herbs, including sage and thyme. Targeted announcements, similar to a press-release, to relevant breeding communities will therefore be prepared. The genetic data can for example be used for target breeding efforts or efforts to genetically improve cultivars using new technologies, including the Nobel prize winning CRISPER/Cas9 technology. The results of such efforts will have a large socio-economic impact as it will provide firsthand knowledge on how crops and cultivated plants can be modified to meet the growing food demands.