Contrasting Population Differentiation And Asymmetric Gene Flows Between Populus Euphratica And P. Pruinosa Based On Vacuolar Na~+/H~+ Exchanger Genes

Popuhts euphratica and P. pruinosa, the only members of section. Turanga of Populus, are noted for their high salinity tolerance. Both species are widely but discontinuously distributed in northwest China and central Asia. These two species are mainly differentiated in leaf morphology and flowering phenology. It is interesting to explore the genetic divergences underlying these characters, since both species together provide a good model to study ecological speciation.We sampled 48 plants from12 populations covering the entire geographic range of the two species. Then, we sequenced six vacuolar Na+/H+exchangers genes (NHX1, NHX2, NHX3, NHX-like4, NHX5, NHX6), which are all critical for the salt resistance. Genetic differentiation between species was compared and their divergence was inferred under isolation-migration model.The total of nucleotide diversity in average varied greatly, from 0.00379 in P. euphratica to 0.00757 in P. prutinosa. Two species showed a high interspecific differentiation based on the AMOVA analysis (average FST=0.671). STRUCTURE analysis suggested that two species clustered into distinct groups. However, NETWORK analyses that the genetic divergences between alleles of two species vary depending on the sampled loci. In addition, our simulation using IM models demonstrated bidirec';onal gene flow between two species, particularly from P. euphratica into P. pruinosa.These results suggest that genetic divergences between two species vary significantly among sampled loci and these two species diverged probably in the presenece of gene flow. Their ecological speciation may be associated with salt and drought related traits and habitat preference. Incomplete lineage sorting and introgressions because of gene flow may have resulted in the shared allele polymorphisms in a few loci.