ARE CHLOROPLAST AND MITOCHONDRIAL DNA VARIATION SPECIES INDEPENDENT IN OAKS?

Extensive introgression of cytoplasmic genomes across oak species is now a well-established fact. To distinguish between ancient hybridization events and ongoing introgression, a direct test for the existence of local exchanges is proposed. Such local exchanges must be comparatively recent, that is, contemporaneous with or later than the last postglacial recolonization. The test is applied to an extensive set of data comprising 377 pure or mixed populations (1744 individuals) of four white oak species in southern France. After demonstrating that local exchanges have occurred frequently between all species pairs, another test is performed to check if species status does nevertheless play some role in restricting cytoplasmic gene flow. The results vary according to the species pairs considered, and the observed pattern may be related to the ecology and/or compatibility of interspecific crosses. It is also shown that, for some of these oak species, the presence of related species in a population significantly influences the intraspecific diversity. Altogether, the results demonstrate that (1) intraspecific cytoplasmic gene flow varies according to the species, (2) interspecific cytoplasmic gene flow varies according to the species pair, and (3) both components of gene flow are at least partly related. © 1999 The Society for the Study of Evolution.