Abstract
Forest trees can increase our understanding of how evolutionary processes drive the genomic landscape and understand speciation due to the majority of forest trees being distributed widely and able to adapt to different climates and environments. Populus davidiana and Populus tremula are among the most geographically widespread and ecologically important tree species in Northern Hemisphere. Whole-genome resequencing data of 41 individuals of P. davidiana and P. tremula throughout Eurasia was conducted, finding that genetic differentiation was evident between the two species, the FST values between P. davidiana and P. tremula was 0.3625. The ancestors of the two aspen diverged into P. davidiana and P. tremula species approximately 3.60 million years ago (Mya), which was in accordance with the rapid uplift of Qinghai–Tibet Plateau (QTP) around the Miocene/Pliocene boundary. The two species experienced a considerable long-term bottleneck after divergence, with population expansion beginning approximately 20,000 years ago after the end of the last glacial maximum. Although the majority of regions of genomic differentiation between the two species can be explained by neutral evolutionary processes, some outlier regions have also been tested that are significantly influenced by natural selection. We found that the highly differentiated regions of the two species exhibited significant positive selection characteristics, and also identified long-term balancing selection in the poorly differentiated regions in both species. Our results provide strong support for a role of linked selection in generating the heterogeneous genomic landscape of differentiation between P. davidiana and P. tremula. These results provide the detailed and comprehensive genomic insights into genetic diversity, demography, genetic burden, and adaptation in P. davidiana and P. tremula.
Highlights
Increasing our understanding of how evolutionary processes drive the genomic landscape of variation is fundamental to a better understanding of the genomic consequences of speciation
We found that the first two components explained 56.16 and 5.58% of total genetic variance according to a Tracy–Widom test, respectively (Figure 3)
The ancestors of the two aspen diverged into P. davidiana and P. tremula populations approximately 3.60 million years ago (Mya)
Summary
Increasing our understanding of how evolutionary processes drive the genomic landscape of variation is fundamental to a better understanding of the genomic consequences of speciation. A combination of evolutionary factors has an effect on the divergence during the process of speciation, such as demographic fluctuations, genetic drift, mutation, recombination rates, genetic hitchhiking, background selection and migration all play important roles to shape the heterogeneity of species divergence (Wang et al, 2016). Recombination and mutation rates that affect important functional architecture of the entire genome are essential evolutionary factors that determine the heterogeneity of genomic divergence (Noor and Bennett, 2009; Nachman and Payseur, 2012). A combination of evolutionary factors affects the patterns of overall genomic variation during the process of population differentiation, such as demographic fluctuations, genetic drift, mutation, recombination rates, genetic hitchhiking, background selection and migration, all performing important roles to shape the heterogeneity of genomic divergence (Wang et al, 2016).
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