The Phytogeographic History of Common Walnut in China

Abstract

Common walnut (Juglans regia L.) is an economically important hardwood tree species cultivated worldwide for its high quality wood and edible nuts. It is generally accepted that after the last glaciation J. regia survived and grew in almost completely isolated stands in Asia, and that ancient humans dispersed walnuts across Asia and into new habitats via trade and cultural expansion. The history of common walnut in China is a matter of debate, however. We estimated the genetic diversity and spatial genetic structure of 31 walnut populations sampled across its Chinese range using 22 microsatellite markers (13 neutral and 9 non-neutral). Using historical data and population genetic analysis, including approximate Bayesian analysis (ABC), we reconstructed the demographic history of J. regia in China. The genetic data indicated the likely presence of J. regia in glacial refugia in the Xinjiang province (Northwest China), Northeastern China (Beijing, Shandong, and Changbai Mountains), Central China (Qinling and Baishan Mountains and Xi’an), and Southwestern China (Tibet, Yunnan, Guizhou, and Sichuan provinces). Based on DIY-ABC analysis, we identified three ancient lineages of J. regia in China. Two lineages (subpopulation A and subpopulation B+C) diverged about 2.79 Mya, while Southwestern China, and Qinling and Baishan Mountains lineages diverged during the Quaternary glaciations (about 1.13 Mya). Remnants of these once-distinct genetic clusters of J. regia may warrant ecological management if they are to be retained as in situ resources. A population size expansion in Northeastern China was detected in the last five centuries. The present distribution of walnut in China resulted from the combined effects of expansion/contraction from multiple refugia after the Last Glacial Maximum and later human exploitation.

Population genetic structure and adaptive differentiation of iron walnut Juglans regia subsp. sigillata in southwestern China

“…Our SDM analyses also suggested that J . sigillata had experienced ranges shrinkage, retreated to the southern region about 25°N from LIG to LGM within refugia, which was similar to Juglans regia (Feng et al, ). Then, the species underwent a slight northeastward expansion, but the effective population size did not exceed LGM.…”

“…The genetic diversity parameters were computed by the GenALEx 6.5 (Peakall & Smouse, ). The Inverse Distance Weighted (IDW) of spatial interpolation analysis in the Geographic Information System (GIS) software ArcGIS 10.0 was implemented to display the geographic patterns of the number of alleles ( N A ), expected heterozygosity ( H E ), allelic richness ( R S ), genetic differentiation ( F ST ), and private allele richness ( P AR ) for 36 populations; then, we displayed the geographic patterns of the haplotypes based on cpSSR markers (Feng et al, ).…”

“…Transcriptome data generated by high‐throughput sequencing have been an excellent resource for SSR marker development (Dang et al, ; Hu et al, ). EST‐SSRs have a variety of uses; some sequences contain polymorphisms that are transferable to related taxa (Feng et al, ). Analysis of EST‐SSRs in populations where J. regia and J. sigillata are sympatric in SW China revealed that the J .…”

“…sigillata and J . regia populations were divided into two genetic clusters with frequent gene introgression (Yuan, Zhang, Peng, & Ge, ), but we found that iron walnut should be considered a subspecies or landrace of J. regia based on genotype by sequencing and EST‐SSRs data (Feng et al, ). Whatever the status of J. sigillata , the evolutionary and ecological changes that led to the current genetic patterns remain understudied.…”

Gene flow among wild and cultivated common walnut (Juglans regia) trees in the Qinling Mountains revealed by microsatellite markers

“…The common walnut (also called the English or Persian walnut, J. regia L.) is a monoecious wind-pollinated deciduous tree native to Eurasia but now grown in many countries with a temperate climate (Manning 1978;Gunn et al 2010;Aradhya et al 2017). It has been cultivated for its timber and nuts for at least 6800 years (Beer et al 2008;Feng et al 2018). In many parts of Asia it is difficult to know if autochthonous trees are wild or feral (Pollegioni et al 2015;Aradhya et al 2017;Bernard et al 2018) as there are no obvious phenotypic indicators of domestication.…”