Lumaret Roselyne, Jabbour-Zahab Roula
UMR5175, Centre d'Ecologie Fonctionnelle et Evolutive, Centre National de la Recherche Scientifique, 1919 route de Mende, F-34293 Montpellier Cedex 05, France.
Ann Bot. 2009 Sep;104(4):725-36. doi: 10.1093/aob/mcp149. Epub 2009 Jun 25.
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.
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.
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 F(1) 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.
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.
栓皮栎和冬青栎亲缘关系较远，其分布区域部分重叠。它们偶尔杂交，但在两个特定地理区域发现冬青栎的叶绿体DNA（cpDNA）完全取代了栓皮栎的cpDNA。本研究的目的是确定这种截然不同的情况是反映了当前或近期的地理种间基因流变异，还是古代基因渗入的结果。
对每个物种的种群、16个形态学上介于两者之间的个体及其部分后代，分析了cpDNA的PCR-RFLP（限制性片段长度多态性）以及10个核微卫星位点的变异。种间核基因渗入基于个体混合率，采用无先验物种分配的贝叶斯方法，并基于最大似然（ML）方法，以每个物种异域种群中的等位基因频率作为对照。特别比较了特定区域内外种群之间的基因流。
观察到种间核遗传分化程度较高，冬青栎的等位基因数量是栓皮栎的两倍。根据贝叶斯分配，约1%的个体极有可能是F(1)杂种，双向核基因渗入影响了每个物种中约4%的个体。杂种和基因渗入个体主要在混交林中被识别，且可能起源较近。使用ML方法获得了更高比例的包含过去杂交中回收的异源特异性基因。在cpDNA完全取代区域内外的种群中，获得了相似的杂交率和核基因渗入率，部分独立于暗示基因过滤的cpDNA种间转移。
结果未提供种间基因流存在地理变异的证据。相反，孢粉学记录支持历史基因渗入，且它构成特定区域cpDNA取代更可靠的起源。
