Comparative phylogeography of Juglans regia and J. mandshurica combining organellar and nuclear DNA markers to assess genetic diversity and introgression in regions of sympatry

Key message By comparing the phylogeography of Juglans regia and J. mandshurica, we found that two walnut species, even when sympatric, rarely introgress, suggesting that strong hybridization barriers exist between these species. The biogeographic investigation of temperate walnut (Juglans) trees is of great interest because of their ecological and economical importance. Our goal was to perform an in-depth investigation of the genetic and phylogeographic history of J. regia and J. mandshurica, two walnut species that are sympatric in parts of their ranges, including tests for gene flow and incomplete lineage sorting. We employed a 234 bp locus of mitochondrial DNA, a 1.8 Kbp locus of chloroplast DNA, 3 nuclear loci totaling 1740 bp in length, and 17 EST-SSRs. We sampled 559 individuals, 332 of J. regia and 227 of J. mandshurica, from 69 locations. We found that J. regia and J. mandshurica, even when sympatric, rarely introgress, suggesting that strong barriers to hybridization exist between these species. Niche overlap analyses for the two species found that they occupy distinct ecological niches and that the sympatric populations may be the result of recent postglacial population expansion.

Automated summary

The study found that even when sympatric, walnut species rarely introgress, indicating strong hybridization barriers between them. The two species occupy distinct ecological niches, and sympatric populations may be the result of recent postglacial population expansion.