Evolutionary response to Quaternary climate aridification and oscillations in north-western China revealed by chloroplast phylogeography of the desert shrubNitraria sphaerocarpa(Nitrariaceae)

Abstract

To investigate the influence of climate aridification and oscillations on the genetic diversity and evolutionary processes of organisms in the Quaternary in north-western China, we selected Nitraria sphaerocarpa and examined the phylogeographical structure and response to historical and environmental factors in populations of this species across most of its covered range. We found twelve haplotypes on the basis of two chloroplast DNA sequences (trnH-psbA and rpl32-trnL). The drying climate during the Quaternary is proposed to have been a driver for significant genetic isolation and divergence among populations in N. sphaerocarpa. Except for the sharing of haplotype D between the Hami Basin and Hexi Corridor, as well as of haplotype F between the Hexi Corridor and Alxa Desert, network analysis showed haplotypes to be almost completely different from region to region. Analysis of molecular variance indicated that genetic variation primarily occurred among populations and among nine geographical groups that were distinguished by spatial analysis of molecular variance, and a Mantel test showed that the correlation between genetic and geographical distances was significant. On the other hand, there was evidence for the occurrence of an episode of more favourable conditions in some regions. Geographical range expansion of two groups of N. sphaerocarpa populations was supported by significant values for Fu's FS and unimodel mismatch distributions. During the last interglacial period, a warmer and wetter climate contributed to range expansion within portions of the Hexi Corridor. By contrast, based on ecological niche modelling, N. sphaerocarpa was indicated to have had a shrunken and more fragmented range during the Last Glacial Maximum.