Comparison of genetic structure of multiple Betula species across China reveals ploidy variation, latitudinal diversity patterns and interspecific hybridization and admixture

BACKGROUND AND AIMS: Hybridization and introgression play crucial roles in evolution and adaptation. Although the influence of ploidy differences on interspecific hybridization has been documented, most studies have focused on single pairs of hybridizing species, thereby limiting our understanding of broader patterns of interspecific gene flow. Here, we investigate hybridization among six partially sympatric Betula species to examine patterns of genetic admixture involving diploid, tetraploid and octoploid species occupying diverse microhabitats. METHODS: We sampled 148 populations across China, representing three diploid species (B. ashburneri and B. costata from section Costatae, and B. platyphylla from section Betula), two tetraploid species (B. ermanii and B. utilis from section Costatae) and one octoploid species (B. dahurica from section Dahuricae), and genotyped 2610 individuals at 15 microsatellite loci. We assessed population structure and compared levels of genetic admixture among the six Betula species using pairwise analyses. KEY RESULTS: Genetic diversity increased with ploidy level and was positively correlated with latitude. Analyses of population genetic structure revealed genetic admixture among Betula species with different ploidy levels. The proportion of hybrids among species within section Costatae was significantly higher than that among sections, although it was not significantly associated with phylogenetic distances between species. CONCLUSIONS: Higher genetic diversity in northern populations of the six Betula species suggests the existence of glacial refugia at higher latitude. The comprehensive dataset for six Betula species not only enables investigation of patterns of genetic admixture across ploidy levels but also fosters novel hypotheses regarding the origins of polyploid lineages and the factors influencing the dynamics of interspecific hybridization.