Abstract
The influence of Quaternary climate fluctuation on the geographical structure and genetic diversity of species distributed in the regions of the Qinghai–Tibet Plateau (QTP) has been well established. However, the underlying role of the East Asian monsoon system (EAMS) in shaping the genetic structure of the population and the demography of plants located in the arid northwest of China has not been explored. In the present study, Nitraria tangutorum, a drought-tolerant desert shrub that is distributed in the EAMS zone and has substantial ecological and economic value, was profiled to better understand the influence of EAMS evolution on its biogeographical patterns and demographic history. Thus, the phylogeographical structure and historical dynamics of this plant species were elucidated using its five chloroplast DNA (cpDNA) fragments. Hierarchical structure analysis revealed three distinct, divergent lineages: West, East-A, and East-B. The molecular dating was carried out using a Bayesian approach to estimate the time of intraspecies divergence. Notably, the eastern region, which included East-A and East-B lineages, was revealed to be the original center of distribution and was characterized by a high level of genetic diversity, with the intraspecific divergence time dated to be around 2.53 million years ago (Ma). These findings, combined with the data obtained by ecological niche modeling analysis, indicated that the East lineages have undergone population expansion and differentiation, which were closely correlated with the development of the EAMS, especially the East Asian winter monsoon (EAWM). The West lineage appears to have originated from the migration of N. tangutorum across the Hexi corridor at around 1.85 Ma, and subsequent colonization of the western region. These results suggest that the EAWM accelerated the population expansion of N. tangutorum and subsequent intraspecific differentiation. These findings collectively provide new information on the impact of the evolution of the EAMS on intraspecific diversification and population demography of drought-tolerant plant species in northwest China.
Highlights
Geological events and climate fluctuations play a major role in shaping the distribution of plant species and their genetic diversity worldwide [1,2]
These haplotypes clustered into three lineages based on the use of the N. roborowskii as an outgroup haplotypes clustered into three lineages based on the use of the N. roborowskii as an outgroup according to their phylogenetic topology (Figure 2A)
Our analysis revealed that both East-A and East-B lineages located in the eastern region exhibited intraspecific divergence [ΦST (East-A versus East-B) = 0.388, p < 0.01], and that the East-A lineage possessed the highest level of genetic diversity (HT = 0.930, p < 0.05) relative to the other lineages
Summary
Geological events and climate fluctuations play a major role in shaping the distribution of plant species and their genetic diversity worldwide [1,2]. The glaciation cycles with arid/cold glacial and humid/warm inter-glacial episodes, might act as more predominant drivers for changes of phylogeographic patterns of the regional vegetation located in northwest China, which has been exemplified in recent studies [14,15,16,17]. This influence is partially plausible in a small time-scale, because multifaceted lines of evidence suggested that the QTP appeared to be barely uplifted since the mid-Eocene and has partly reached over 4000 m above the sea level [18]. Under extreme weather conditions during the glaciation periods, distributional ranges of most species in high mountainous or plateau regions were contracted when they migrated into the so-called glacial refugia, and experienced expansion again in postglacial time, thereby leading to the species differentiation or secondary contact evolution [2,10,17,19]
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