Journal Article

Ancient and current gene flow between two distantly related Mediterranean oak species, <i>Quercus suber</i> and <i>Q. ilex</i>

Roselyne Lumaret and Roula Jabbour-Zahab

in Annals of Botany

Published on behalf of The Annals of Botany Company

Volume 104, issue 4, pages 725-736
Published in print September 2009 | ISSN: 0305-7364
Published online June 2009 | e-ISSN: 1095-8290 | DOI: http://dx.doi.org/10.1093/aob/mcp149
Ancient and current gene flow between two distantly related Mediterranean oak species, Quercus suber and Q. ilex

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  • Ecology and Conservation
  • Evolutionary Biology
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Background and Aims

Quercus suber and Q. ilex are distantly related and their distributions partially overlap. They hybridize occasionally, but the complete replacement of Q. suber chloroplast DNA (cpDNA) by that of Q. ilex was identified in two specific geographical areas. The objective of this study was to determine whether the contrasting situation reflected current or recent geographical interspecies gene flow variation or was the result of ancient introgression.

Methods

cpDNA PCR-RFLPs (restriction fragment length polymorphisms) and variation at ten nuclear microsatellite loci were analysed in populations of each species, in 16 morphologically intermediate individuals and the progeny of several of them. Interspecies nuclear introgression was based on individual admixture rates using a Bayesian approach with no a priori species assignment, and on a maximum-likelihood (ML) method, using allele frequencies in the allopatric populations of each species as controls. Gene flow was compared specifically between populations located within and outside the specific areas.

Key Results

High interspecies nuclear genetic differentiation was observed, with twice the number of alleles in Q. ilex than in Q. suber. According to Bayesian assignment, approx. 1 % of individuals had a high probability of being F1 hybrids, and bidirectional nuclear introgression affected approx. 4 % of individuals in each species. Hybrid and introgressed individuals were identified predominantly in mixed stands and may have a recent origin. Higher proportions including allospecific genes recovered from past hybridization were obtained using the ML method. Similar rates of hybridization and of nuclear introgression, partially independent of cpDNA interspecies transfer suggestive of gene filtering, were obtained in the populations located within and outside the areas of complete cpDNA replacement.

Conclusions

The results did not provide evidence for geographical variation in interspecies gene flow. In contrast, historical introgression is supported by palynological records and constitutes the more reliable origin of cpDNA replacement in specific regions.

Keywords: cpDNA PCR-RFLPs; nuclear microsatellite (nSSR) variation; hybridization; interspecies genetic introgression; Quercus suber; Quercus ilex

Journal Article.  8451 words.  Illustrated.

Subjects: Ecology and Conservation ; Evolutionary Biology ; Plant Sciences and Forestry

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