Betula Mcallisteri Sp.Nov.(Sect.Acuminatae)and Its Hybridization With Betula Luminifera

Herbarium specimens play an important role in studying taxonomic,historical dynamics of a species,and species diversity within an area.However,some closely-related species are difficult to be distinguished based on specimens if they lack information on habitats or key morphological traits such as bark morphology and fragrance.Betula,consisting of about 65 species and subspecies,are widely distributed in various habitats in the Northern Hemisphere.Betula has a tough taxonomy due to extensive morphological variation,frequent hybridization and introgression and polyploidy.We found a Betula which grows sympatrically with B.luminifera during in the field.The species has similar fruits and leaves with B.luminifera.However,the species has peeled bark and B.luminifera has smooth bark;the species has no fragrance whereas B.luminifera has obvious fragrance in cambial tissues.We name the species as "unidentified sample" for easy use.We integrated morphology,RAD-seq data and ploidy(inferred from flow cytometry and RAD data)to address the evolutionary status of the "unidentified sample" and the extent of hybridization between the "unidentified sample" and B.luminifera in natural populations.Our main results are as follows :First,we assigned 38 samples as "unidentified sample" and 48 as B.luminifera based on morphological characters.The estimated genome size of the "unidentified sample" was 388-445 Mbp.Ploidy inferred from RAD data showed that the "unidentified sample" was diploid and B.luminifera was tetraploid.Second,PCA analysis based on 40,209 SNPs showed that the "unidentified sample" formed a cluster with Betula luminifera 19933472 from Royal Botanic Garden Edinburgh whereas B.luminifera formed another cluster.PC1 and PC2 explained 30.7 % and 3.2 % of the variation,respectively.Admixture results showed little introgression between the "unidentified sample" and B.luminifera.Phylogenomic analyses based on 2,309,898 SNPs showed that the "unidentified sample" and Betula luminifera 19933472 formed a monophyletic clade whereas B.cylindrostachya and B.luminifera formed another monophyletic clade.Third,Betula luminifera 19933472 peeled along lenticels and had no obvious fragrance.In addition,Betula luminifera 19933472 is a diploid.This suggests that the "unidentified sample" and Betula luminifera 19933472 refer to the same species.Finally,we named the "unidentified sample" as a new species,namely,Betula mcallisteri Nian Wang & Holstein,sp.nov.The species was named after Hugh A.Mc Allister,a botanist works on the genus Betula.Betula mcallisteri sp.nov mainly distributed in the Qinling-Daba Mountains,and may also distribute in Yunnan province.This study not only shows the importance of the integrated taxonomy in the discovery of new species but also highlights the existence of undiscovered species in botanical gardens.