Frequent cytoplasmic exchanges between oak species that are not closely related: Quercus suber and Q-ilex in Morocco

Chloroplast (cp) and mitochondrial (mt) DNA variation were studied in 97 po pulations of cork oak (Quercus suber) in Morocco; in 31 of these population s, holm oak (Quercus ilex), a clearly distinct species, also occurred and w as compared with Q. suber. Three cpDNA and one mtDNA primer pairs were used in the survey, each in combination with one restriction enzyme. Six haplot ypes belonging to two very divergent lineages were detected; one lineage pr edominates in each species, and is probably ancestral, as inferred from com parisons with other oak species. In the mixed-species populations, cytoplas mic genomes were frequently shared across species, as indicated by an intro gression ratio of 0.63. This index is a new measure of the propensity of sp ecies to share locally genetic markers, varying from zero (complete differe ntiation) to one (no differentiation). By contrast, more closely related de ciduous oak species (Q. robur, Q. petraea and Q. pubescens) have introgress ion ratios varying from 0.82 to 0.97. The introgression events appear to ha ve been more frequent in the direction Q. ilex (female) x Q. suber (male), a finding which seems attributable to the flowering phenology of these two species. This asymmetry may have favoured immigration of Q. suber beyond it s main range, in regions already colonized by Q. ilex. There, rare hybridiz ation and further introgression through long distance pollen flow have esta blished populations that are morphologically indistinguishable from Q. sube r but that have cytoplasmic genomes originating from the local Q. ilex popu lations.