Patterns of genetic variation and hybridization in a mixed oak (Quercus spp.) forest

Over the last decade the study of natural hybridization has become one of the most dynamic areas in molecular ecology and evolutionary genetics.
Amongst forest tree species, oaks (Quercus spp.) are well-known for their propensity to intercross. Many oak species grow in mixed forests where hybridization is suspected to occur.
The aim of the present work is to characterize hybridization in one of the most species-rich, temperate forests of oak in Europe, Bejan Oak Reserve, situated in west-central Romania. At this site five oak species cohabit naturally: Quercus robur L. – pedunculate oak, Q. petraea (Matt.) Liebl. – sessile oak, Q. pubescens Willd. – downy or pubescent oak, Q. frainetto Ten. – Italian or Hungarian oak and Q. cerris L. – Turkey oak.
A Bayesian admixture analysis without any a priori information about species showed that a genetic structure consisting of four clusters best fit the combined isozyme and microsatellite data (Q. cerris was not included). Moreover, there was a very good correspondence of each species with one of the inferred genetic clusters. The estimated introgression rate varied markedly between pairs of species, ranging from 1.7% between Q. robur and Q. frainetto to 16.2% between Q. pubescens and Q. frainetto.
Apparently asymmetrical gene flow and selection against hybrids contributed to the maintenance of species identity in these sympatric oak species.

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Curtu, A.L., Gailing, O. & R. Finkeldey. 2009. Patterns of contemporary hybridization inferred from paternity analysis in a four-oak-species forest. BMC Evolutionary Biology 9: 284.