Bidirectional introgression between Betula tianshanica and Betula microphylla and its implications for conservation

Molecular markers can allow us to differentiate species that occupy a morphological continuum, and detect patterns of allele sharing that can help us understand the dynamics of geographic zones where they meet. Betula microphylla is a declining wetland species in NW China that forms a continuum of leaf morphology with its relative Betula tianshanica. We use ecological niche models (ENM) to predict the distribution of B. microphylla, B. tianshanica and the more commonly occurring B. platyphylla. We use restriction-site associated DNA sequencing and SSRs to resolve their genetic structure and patterns of allele sharing. ENM predicted an expansion of suitable range of B. tianshanica into B. microphylla since the Last Glacial Maximum and the contraction of B. microphylla9s range in the future. We resolved the species identification of some intermediate morphotypes. We found signatures of bidirectional introgression between B. microphylla and B. tianshanica with SNPs showing more admixture than SSRs. Introgression from B. microphylla into B. tianshanica was greater in the Tianshan Mountains where the two species have occurred in proximity. Unexpectedly, introgression from B. tianshanica into B. microphylla was widespread in the Altay Mountains where there are no records of B. tianshanica occurrence. This presence of B. tianshanica-derived alleles far beyond the species9 current range could be due to unexpectedly high pollen flow, undiscovered populations of B. tianshanica in the region, incomplete lineage sorting, or selection for adaptive introgression in B. microphylla. These different interpretations have contrasting implications for the conservation of B. microphylla.