Genetic introgression and species boundary of two geographically overlapping pine species revealed by molecular markers.

Defang Zhang,Maomao Yan,Jin Xu,Shuxian Li,Xiaogang Dai,Tongming Yin,Tao Xia,Bryce Richardson

doi:10.1371/journal.pone.0101106

Abstract

Gene introgression and hybrid barriers have long been a major focus of studies of geographically overlapping species. Two pine species, Pinus massoniana and P. hwangshanensis, are frequently observed growing adjacent to each other, where they overlap in a narrow hybrid zone. As a consequence, these species constitute an ideal system for studying genetic introgression and reproductive barriers between naturally hybridizing, adjacently distributed species. In this study, we sampled 270 pine trees along an elevation gradient in Anhui Province, China and analyzed these samples using EST-SSR markers. The molecular data revealed that direct gene flow between the two species was fairly low, and that the majority of gene introgression was intermediated by backcrossing. On the basis of empirical observation, the on-site distribution of pines was divided into a P. massoniana zone, a hybrid zone, and a P. hwangshanensis zone. STRUCTURE analysis revealed the existence of a distinct species boundary between the two pine species. The genetic boundary of the hybrid zone, on the other hand, was indistinct owing to intensive backcrossing with parental species. Compared with P. massoniana, P. hwangshanensis was found to backcross with the hybrids more intensively, consistent with the observation that morphological and anatomical characteristics of trees in the contact zone were biased towards P. hwangshanensis. The introgression ability of amplified alleles varied across species, with some being completely blocked from interspecific introgression. Our study has provided a living example to help explain the persistence of adjacently distributed species coexisting with their interfertile hybrids.

Highlights

Hybridization, or the crossing of different species, subspecies or ‘races’, profoundly influences species evolution
Compare with P. massoniana, P. hwangshanensis was found to backcross with the hybrids more intensively, in agreement with the empirical observation that morphological and anatomical characteristics of trees in the contact zone are biased towards P. hwangshanensis [23]
Our findings were superficially in agreement with a previous conclusion that gene flow is more intense from P. massoniana to P. hwangshanensis, a more detailed examination revealed that gene introgression into P

Summary

Introduction

Hybridization, or the crossing of different species, subspecies or ‘races’, profoundly influences species evolution. Introgressive hybridization can promote gene flow between species, leading to the generation of new genotype combinations and thereby increasing species diversity and ecological adaptability [1,2,3]. A balance between gene flow and hybrid barriers is believed to maintain the hybrid zones [7,8] that develop when hybridization occurs between species with different environmental adaptations [9]. Environmental heterogeneity can lead to a ‘mosaic’ structure in the hybrid zone [10,11,12], as genotype selection often depends on habitat attributes

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Conclusion