| Betula,consisting of 65 species and subspecies,is widely distributed in the Northern Hemisphere and has important ecological,economic and ornamental values.The classification of Betula is extremely difficult due to frequent hybridization and introgression,extensive morphological variation and polyploidy.Betula has ploidy levels ranging from diploid(2x = 28)to decaploid(2x = 168),and polyploid species accounts for more than 60%.In this study,we predicted the potential distribution of the "diploid" red birch and B.luminifera using ecological niche modeling(ENM),inferred the phylogenetic position of the "diploid" red birch and investigated potential gene flow between the "diploid" red birch and B.luminifera using RAD-seq.The results are as follows:1.ENM predicted that "diploid" red birch had suitable habitat in the Qinling-Daba Mountains,Western Sichuan and Northwest Yunnan province during the last glacial maximum(LGM).Suitable habitats remain suitable from LGM till the future.Betula luminifera had continuous suitable habitats in Chonging,Guizhou,Guangxi,Guangdong and Fujian provinces in the LGM and would shrink significantly in the abovementioned places.2.Population structure analyses of "diploid" red birch and B.luminifera showed that the two species formed distinct genetic clusters at the optimal value of K = 2.When K = 2,little introgression was revealed between the two species.The ABBA-BABA results also showed little introgression between "diploid" red birch and B.luminifera.The genetic differentiation(Fst)within "diploid" red birch and B.luminifera was low,suggesting weak genetic structure within species.3.The ploidy inferred from RAD-seq data showed that "diploid" red birch peaked around 0.5,while B.luminifera peaked around 0.25,0.50,and 0.75.This indicates that "diploid" red birch is a diploid species and B.luminifera is a tetraploid species.4.Phylogenomic analyses showed that "diploid" red birch formed a monophyletic clade with B.ashburneri,suggesting the two may refer to the same species. |
