Interspecific gene flow and maintenance of species integrity in oaks

Oak species show a wide variation in morphological and physiological characters, and boundaries between closely related species are often not clear-cut. Still, despite frequent interspecific gene flow, oaks maintain distinct morphological and physiological adaptations. In sympatric stands, spatial distribution of species with different ecological requirements is not random but constrained by soil and other micro-environmental factors. Here, we discuss factors that may influence the maintenance of the integrity of oak species in the face of interspecific gene flow. Pre-zygotic isolation (e.g. cross incompatibilities, asynchrony in flowering, pollen competition) and post-zygotic isolation (divergent selection) contribute to the maintenance of species integrity in sympatric oak stands. The antagonistic effects of interspecific gene flow and divergent selection are reflected in the low genetic differentiation between hybridizing oak species at most genomic regions interspersed by regions with signatures of divergent selection (outlier regions). In the near future, the availability of high-density genetic linkage maps anchored to scaffolds of a sequenced Q. robur genome will allow to characterize the underlying genes in these outlier regions and their putative role in reproductive isolation between species. Reciprocal transplant experiments of seedlings between parental environments can be used to characterize selection on outlier genes. High transferability of gene-based markers will enable comparative outlier screens in different oak species.