Species Divergence Between Populus Euphratica And P. Pruinosa Based On Population Genetic Data

Populus euphratica and its sister species P. pruinosa are naturally distributed in the desert regions. Severe desertification has resulted in both species becoming endangered and all populations are seriously fragmented in northwest China. P. pruinosa grows in environments with a high-underground water level near ancient or extant rivers while P. euphratica occurs in the more deserted region with a low-underground water level. Both species can be easily identified based on their leaf traits. These two species together comprise a good model for studying speices divergence and the following fixture of the genetic variations between species.Firstly, genetic diversity and differentiation within and between populations of P. euphratica were examined using eight pairs of simple sequence repeat (SSR) markers. The results suggested that this species has the high genetic diversity with a mean of12.125alleles per locus. The expected heterozygosity (He) ranged from0.713to0.878. However, there was a high level of genetic diversity within the species (when compared within congeneric species) and low genetic differentiation between populations (average Fst=0.093). Analyses of molecular variance (AMOVA) suggested that5.21%of the total molecular variance was attributable to between-population diversity (p<0.001) while the remainder of the variance was associated with differences within populations. There was no distinct correlation between geographical distributions and genetic variation. There was no evidence to support the initial hypothesis of low genetic diversity within the species and high differentiation between populations separated by long distances. The fragmentation and endangered status of this species may result from the recent anthropologic disturbance and environmenta deterioration. The effective outcrossing of Populus and low species differentiation may result in the interspecific introgression and low genetic differentiation between it and sister species which are overlapped distributed.In order to further examine interspecific introgression and differention between Populus euphratica and its sister species, P. pruinosa, a total of290individuals from29allopatric and sympatric populations of two species were genotypes using chloroplast (cp) DNA, nuclear (nr) ITS sequences and eight SSR loci. Three major cpDNA haplotypes were widely shared between the two species and between-species cpDNA differentiation (FCT) was very low, even lower than among single species populations. The average SSR FCT values were higher. Bayesian clustering analysis of all loci allowed a clear delineation of the two species. Gene flow, determined by examining all SSR loci, was obvious but only slightly asymmetrical. However, the two species were almost fixed for two different nrITS genotypes that had the highest FCT, although a few introgressed individuals were detected both in allopatric and sympatric populations. The two species shared numerous ancestral polymorphisms at cpDNA and a few SSR loci. Both ITS and a combination of nuclear SSR data could be used to differentiate between the two species. Introgressions and gene flow were obvious between the two species either during or after their divergence. These findings further underscore the complex genetic differentiations between ecologically diverged species and highlight the importance of nuclear DNA (especially ITS) differentiation for delimiting closely related plant species.Finally, in order to model and test speciation process and estimate the timescales of the species divergence between P. euphratica and P. pruinosa, a total of11nuclear loci were sequenced for60individuals of two species. Divergence time between the two species was estimated to be0.66-1.28million years ago (Ma) based on population sequence genetic data of the sampled loci and it was estimated to be1.37Ma based on the nuclear ITS sequence variations. All these estimated timescales fell within the maximum glaciation occurred in China of the Quaternary stage, during which, deserts enlarged rapidly in central Asia. In addition, Isolation-with-Migration modeling also indicated that two species had diverged in the presence of gene flow. Two genes, GO and PhyB2, were found to show a sign of selection in P. euphratica. These findings suggested that habitat differentiation due to the Quaternary climatic changes may have leaded to the species of these two desert poplars. Their divergence undoubtedly provides a good model for plant speciation triggered by the Quaternary climatic change.Overall, all results together recovered the genetic diversity and differention within P. euphratica and clarified speciation, introgression and fixture of species-specific variations between it and sister species P. pruinosa. These findings not only provide the solid evidence for setting conservation strategies of genetic resources for two species, but also provide a good example for understanding plant speciation triggered by the Pleistocene climate oscillations.