  Hybridization and introgression between Quercus alnifolia Poech and Q.
coccifera L. is studied by analyzing morphological traits, nuclear and
chloroplast DNA markers. The study site is a mixed stand on Troodos Mountains
(Cyprus) and the analyzed material includes both adult trees and progenies of
specific mother trees. Multivariate analysis of morphological traits shows that
the two species can be well distinguished using simple leaf morphometric
parameters. A lower genetic diversity in Q. alnifolia than in Q. coccifera and
a high interspecific differentiation between the two species are supported by
an analysis of nuclear and chloroplast microsatellites. The intermediacy of the
four designated hybrids is verified by both leaf morphometric and genetic data.
Analysis of progeny arrays provides evidence that interspecific crossings are
rare. This finding is further supported by limited introgression of chloroplast
genomes. Reproductive barriers (e.g. asynchronous phenology, post-zygotic
incompatibilities) might account for this result. A directionality of
interspecific gene flow is indicated by a genetic assignment analysis of
effective pollen clouds with Q. alnifolia acting as pollen donor. Differences
in flowering phenology and species distribution in the stand may have
influenced the direction of gene flow and the genetic differentiation among
effective pollen clouds of different mother trees within species.

Populations and Evolution