A nearly 30-years living collection from the Royal Botanic Garden Edinburgh is a new species: a case study of Betula mcallisterii sp. nov. (sect. Acuminatae, Betulaceae) and its little hybridization with Betula luminifera

O_LISpecies description meets challenges arising from various species concepts. Integrating different sources of information and multiple lines of evidence are important for species recognition and discovery. C_LIO_LIHere we use morphology, restriction site-associated DNA sequencing and flow cytometry to study the species status of the unidentified Betula samples collected in this study and to assess the extent of hybridization between the unidentified Betula samples and B. luminifera in natural populations. C_LIO_LIMolecular analyses show the unidentified Betula samples as a distinct lineage and reveal very little genetic admixture between the unidentified samples and B. luminifera. Unexpectedly, the unidentified samples formed a well-supported monophyletic clade with the living collection of B. luminifera 19933472 in the Royal Botanic Garden Edinburgh which was introduced from Southwest China. Flow cytometry shows that the unidentified samples and B. luminifera 19933472 are diploid. C_LIO_LIOur data indicates that B. luminifera 19933472 and the unidentified Betula samples should be recognized as a new species, namely B. mcallisterii. The very little introgression between B. mcallisterii and B. luminifera indicates a strong reproductive barrier. Our research shows the importance of gathering information from wild populations and the value of an integrative approach in species discovery. C_LI Societal Impact StatementA comprehensive survey of plant species from natural populations can aid greatly in taxonomy and species delimitation. Here, we discovered a new birch species from the wild and unexpectedly found that this species has been introduced to the Royal Botanic Garden Edinburgh for nearly 30 years. We found very little introgression between the new species and its closely-related species. Our study highlighted the importance in integrating sources of information from natural populations and botanic gardens for species discovery.