Species Boundaries Between Three Sympatric Oak Species: Quercus aliena, Q. dentata, and Q. variabilis at the Northern Edge of Their Distribution in China

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 (<i>Quercus aliena</i> Blume, <i>Quercus dentata</i> Thunb. ex Murray, and <i>Quercus variabilis</i> 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 <i>Q. variabilis</i> and either <i>Q. aliena</i> or <i>Q. dentata</i>. The sharing of chloroplast haplotype H1 between <i>Q. aliena</i> and <i>Q. dentata</i> 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 <i>Q. aliena</i> and <i>Q. dentata</i> and nearly no gene flow can be detected between <i>Q. aliena</i> and <i>Q. variabilis</i> and between <i>Q. dentata</i> and <i>Q. variabilis</i>. Prezygotic isolation may have contributed to the species boundaries of these three sympatric oak species.