Abstract: Populus euphratica Olivier and P. pruinosa Schrenk are known for their tolerance to highly saline and arid habitats, and overlapping distribution. We examined interspecific differentiation and gene flow between these two species at six loci that encode vacuolar Na⁺/H⁺ exchanger genes. Interspecific divergence varied greatly between sampled loci and could collectively delimit the two species well. Simulations based on the isolation–migration model suggested gene flow primarily from P. euphratica into P. pruinosa. This asymmetrical gene flow may be related to the adaptive survival of the introgressed individuals. Our findings suggest that these species may have diverged in the presence of gene flow and that local adaptation may have played an important role in maintaining the distinct species lineages by restricting gene flow between them. Our results together indicate that interspecific divergence and gene flow differ greatly between members of the same gene family, possibly due to differential subfunctionalization and/or neofunctionalization during ongoing speciation of these two poplar species.

Key words: gene flow, interspecific divergence, Na⁺/H⁺ exchanger loci, Populus euphratica, Populus pruinosa.