Blow to the Northeast? Intraspecific differentiation of Populus davidiana suggests a northeastward skew of a phylogeographic break boundary in East Asia

Aim: There is increasing interest in the role that biological traits, and historical and biogeographic processes, play in the formation of phylogeographic patterns. An arid belt that once existed in northern China might have affected many plants, but this has yet to be untested in an arid-tolerant, wind-dispersed species. Here we tested how intrinsic and extrinsic factors have affected the phylogeography of Populus davidiana.

Location: East Asia

Methods: Genetic variation was surveyed across 40 populations (555 individuals) covering the Chinese range of P. davidiana, using 16 nuclear microsatellite loci (nSSRs) and four chloroplast fragments (cpDNA). Demographic and migration hypotheses were tested using coalescent-based approaches, and the present and past potential distributions were predicted using species distribution modelling.

Results: Molecular data divides P. davidiana into two lineages, northeastern China (NECR), and central and northern China (CNCR); however, the dividing line is around 118ºE for nSSRs, but 122ºE for cpDNA. The range and habitat of the two lineages barely overlap at present, and their ecological separation may have initiated around the Pliocene-Quaternary boundary, when major infraspecific cpDNA clades diverged. NECR and CNCR experienced postglacial northeastward and northward range shifts, respectively. Bi-directional historical gene flow was detected between NECR and CNCR for both bi-parentally inherited nSSRs and maternally inherited cpDNA. Demographic inferences suggest a severe bottleneck for CNCR and especially NECR, around the latest Pleistocene.

Main conclusions: The phylogeographic break within P. davidiana reflects the impacts of biogeographic history, climate and biological traits. Its plumed, wind-dispersed seeds might be especially significant, because prevailing southwestern spring winds may have moved the NECR-CNCR boundary further east than similar phylogenetic breaks in other species, and also moved the cpDNA boundary relative to that for nuclear markers. Biological traits, therefore, should also be considered when examining the genetic and ecological differentiation between closely related taxa.