Natural Hybridization And Introgression Between Betula Albosinensis And Betula Platyphylla

Red birch(Betula albosinensis)and white birch(Betula platyphylla)are widely distributed in Yunnan,Sichuan,Shaanxi,Gansu,Henan and Shanxi provinces.In populations where the two species occur parapatrically or sympatrically,we detected some putative hybrids between the two species.In this study,we used twelve nuclear microsatellite markers to genotype 391 birch individuals from nine localities in China,and classified them into species via leaf morphology.Hybridization,genetic structure,and genetic mixture analysis were inferred using PCO analysis and STRUCTURE software.We compared the effects of determined “tetraploid” birch and undetermined “tetraploid” birch genotypes on population genetic analyses.We analysed the effect of ploidy,local species abundance,and latitude on levels of introgression between the species.The main results are as follows:1.Leaf morphology divided our samples into red birch and white birch,but the results of molecular markers indicated that red birch had tetraploid and diploid,and the diploid and tetraploid had produced obvious genetic differentiation.2.Phylogeny based on ITS sequences indicated that most of the “diploid” red birch clustered with “tetraploid” red birch despite low support values.3.STRUCTURE analysis based on microsatellite data indicated that low hybridization rate and gene introgression have occurred between white birch and “tetraploid” red birch and between “diploid” red birch and “tetraploid” red birch.Six individuals were identified as early-generation hybrids or backcrosses between “tetraploid” red birch and “diploid” red birch and five were identified between “tetraploid” red birch and white birch.4.The genetic mixture analysis indicated that levels of admixture from “diploid” red birch and white birch into “tetraploid” red birch were not significantly correlated with latitude.Relative species abundance and ploidy level differences do not impact the direction of introgression between red birch and white birch.Incomplete lineage sorting and ongoing gene flow explains the patterns of genetic admixture in the geographical distribution.5.Estimated genetic differentiation among species was not significantly different between determined “tetraploid” birch and undetermined “tetraploid” birch genotypes.The genetic structure is underestimated without distinguishing “tetraploid” red birch genotype.