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.