Geographic variation in the structure of oak hybrid zones provides insights into the dynamics of speciation

Authors

  • YAN-FEI ZENG,

    1. Key Laboratory of Silviculture of the State Forestry Administration, Research Institute of Forestry, Chinese Academy of Forestry, Beijing 100091, China
    2. State Key Laboratory of Earth Surface Processes and Resource Ecology & MOE Key Laboratory for Biodiversity Science and Ecological Engineering, Beijing Normal University, Beijing 100875, China
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  • WAN-JIN LIAO,

    1. State Key Laboratory of Earth Surface Processes and Resource Ecology & MOE Key Laboratory for Biodiversity Science and Ecological Engineering, Beijing Normal University, Beijing 100875, China
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  • RÉMY J. PETIT,

    1. INRA, UMR Biodiversity, Genes and Ecosystems, 69 route d’Arcachon, F-33610 Cestas, France
    2. University of Bordeaux, UMR Biodiversity, Genes and Ecosystems, 69 route d’Arcachon, F-33610 Cestas, France
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  • DA-YONG ZHANG

    1. State Key Laboratory of Earth Surface Processes and Resource Ecology & MOE Key Laboratory for Biodiversity Science and Ecological Engineering, Beijing Normal University, Beijing 100875, China
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Da-Yong Zhang, Fax: +86 10 58807721; E-mail: zhangdy@bnu.edu.cn

Abstract

Studying geographic variation in the rate of hybridization between closely related species could provide a useful window on the evolution of reproductive isolation. Reinforcement theory predicts greater prezygotic isolation in areas of prolonged contact between recently diverged species than in areas of recent contact, which implies that old contact zones would be dominated by parental phenotypes with few hybrids (bimodal hybrid zones), whereas recent contact zones would be characterized by hybrid swarms (unimodal hybrid zones). Here, we investigate how the hybrid zones of two closely related Chinese oaks, Quercus mongolica and Q. liaotungensis, are structured geographically using both nuclear and chloroplast markers. We found that populations of Q. liaotungensis located around the Changbai Mountains in Northeast China, an inferred glacial refugium, were introgressed by genes from Q. mongolica, suggesting historical contact between the two species in this region. However, these introgressed populations form sharp bimodal hybrid zones with Q. mongolica. In contrast, populations of Q. liaotungensis located in North China, which show no sign of ancient introgression with Q. mongolica, form unimodal hybrid zones with Q. mongolica. These results are consistent with the hypothesis that selection against hybrids has had sufficient time to reinforce the reproductive barriers between Q. liaotungensis and Q. mongolica in Northeast China but not in North China.

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