Interspecific gene flow in sympatric oaks.

Variation of chloroplast DNA and nuclear ribosomal DNA (DNA encoding ribosomal RNA) was studied for five species of white oak native to the eastern United States. Although these species differ in many morphological characters and have different (though overlapping) geographical ranges and ecological tolerances, they are interfertile and often grow in mixed stands, and hybrids are occasionally found in nature. All individuals studied were morphologically typical members of their respective species-i.e., showed no evidence of recent hybrid ancestry. Restriction site markers in the chloroplast DNA reveal several clear cases of localized gene exchange between species, showing that there is appreciable gene flow between sympatric species in this group. One length variant of the nuclear ribosomal DNA, however, is species specific. The sharp morphological and ecological differences between the species, together with the one ribosomal DNA variant, suggest that nuclear genes may be exchanged less freely between species than are chloroplast genotypes. IMAGES:

本研究针对美国东部原产的五种白栎物种，开展了叶绿体DNA（chloroplast DNA）与核糖体DNA（ribosomal DNA，即编码核糖体RNA的DNA）的变异分析。尽管这些物种在诸多形态性状上存在差异，且地理分布范围与生态耐受度虽存在重叠但彼此不同，但它们之间可相互杂交，且常形成混交林，自然界中也偶有杂交个体被发现。本次研究所涉及的所有个体，均为对应物种的形态典型个体——即未表现出近期杂交祖先的相关特征。叶绿体DNA的限制性酶切位点标记揭示了多起明确的物种间局部基因交流事件，表明该类群的同域分布物种之间存在可观的基因流。然而，细胞核核糖体DNA的一种长度变异类型具有物种特异性。物种间显著的形态与生态差异，结合该核糖体DNA变异类型，提示核基因在物种间的交流或许不如叶绿体基因组那般顺畅。图像：