Abstract
Oaks are important timber trees with wide distributions in China, but few genetic studies have been conducted on a fine scale. In this study, we seek to investigate the genetic diversity and differentiation of three sympatric oak species (Quercus aliena Blume, Quercus dentata Thunb. ex Murray, and Quercus variabilis Blume) in their northern distribution in China using 17 bi-parentally inherited nSSRs markers and five maternally inherited chloroplast DNA (cpDNA) fragments. Both the cpDNA and the nSSRs show a high level of genetic differentiation between different oak sections. The chloroplast haplotypes are clustered into two lineages. Clear species boundaries are detected between Q. variabilis and either Q. aliena or Q. dentata. The sharing of chloroplast haplotype H1 between Q. aliena and Q. dentata suggests very recent speciation and incomplete lineage sorting or introgression of H1 from one species to another. The nSSRs data indicate a complete fixation of variation within sites for all three oak species, and that extensive gene flow occurs within species whereas only limited gene flow is detected between Q. aliena and Q. dentata and nearly no gene flow can be detected between Q. aliena and Q. variabilis and between Q. dentata and Q. variabilis. Prezygotic isolation may have contributed to the species boundaries of these three sympatric oak species.
Highlights
Transfer of genetic material across closely related species is common, especially in coexisting taxa (Rieseberg and Wendel, 1993; Arnold, 1997; Rieseberg et al, 2006; Kim et al, 2008)
Three sympatric oak species: Q. aliena, Q. dentata, and Q. variabilis were studied in their northern distribution, at Shangfang Mountain located in the southwest of Beijing Municipality, P
The inconsistencies between the Analysis of molecular variance (AMOVA) results for chloroplast DNA (cpDNA) and nuclear microsatellite markers or Simple Sequence Repeats (nSSRs) markers for all species, Q. aliena & Q. variabilis and Q. dentata & Q. variabilis agree with those reported by Petit et al (2005), which can be explained by the different modes of inheritance of cpDNA and nuclear DNA. cpDNA is maternally inherited dispersing limited gene flow through seeds while nuclear DNA is bi-parentally inherited and can mediate extensive gene flow via both seeds and pollen
Summary
Transfer of genetic material across closely related species is common, especially in coexisting taxa (Rieseberg and Wendel, 1993; Arnold, 1997; Rieseberg et al, 2006; Kim et al, 2008). In recent years DNA sequence analysis has been widely employed to examine the relationship between genetic variation and traditional morphological boundaries among closely related species (e.g., Jaramillo-Correa et al, 2008; Du et al, 2011 for gymnosperm-conifers; Arnold et al, 2012; Eaton et al, 2015 for angiosperms). These studies show that molecular polymorphisms for neutral markers are often widely shared between closely related species, and that fixed genetic differences may vary because of the genomes or genes studied (Petit and Excoffier, 2009). Large scale studies, and studies on a fine scale have verified the occurrence of interspecific gene flow and introgression among different oak species (Salvini et al, 2009; Antonecchia et al, 2015)
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