Interfertile oaks in an island environment: I. High nuclear genetic differentiation and high degree of chloroplast DNA sharing between Q. alnifolia and Q. coccifera in Cyprus. A multipopulation study

The evergreen Quercus alnifolia and Q. coccifera form the only interfertile pair of oak species growing in Cyprus. Hybridization between the two species has already been observed and studied morphologically. However, little evidence exists about the extent of genetic introgression. In the present study, we aimed to study the effects of introgressive hybridization mutually on both chloroplast and nuclear genomes. We sampled both pure and mixed populations of Q. alnifolia and Q. coccifera from several locations across their distribution area in Cyprus. We analyzed the genetic variation within and between species by conducting analysis of molecular variance (AMOVA) based on nuclear microsatellites. Population genetic structure and levels of admixture were studied by means of a Bayesian analysis (STRUCTURE simulation analysis). Chloroplast DNA microsatellites were used for a spatial analysis of genetic barriers. The main part of the nuclear genetic variation was explained by partition into species groups. High interspecific differentiation and low admixture of nuclear genomes, both in pure and mixed populations, support limited genetic introgression between Q. alnifolia and Q. coccifera in Cyprus. On the contrary, chloroplast DNA haplotypes were shared between the species and were locally structured suggesting cytoplasmic introgression. Occasional hybridization events followed by backcrossings with both parental species might lead to this pattern of genetic differentiation.