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.