MATING SYSTEM AND ASYMMETRIC HYBRIDIZATION IN A MIXED STAND OF EUROPEAN OAKS

The sessile (Quercus petraea [Matt.] Liebl.) and pedunculate (Quercus robur L.) oaks are two closely related species having a wide sympatric distribution over Europe. Under natural conditions, they frequently f orm mixed forests, where hybridization is suspected to occur. In this paper, two different approaches have been applied to the study of the mating system and the interspecific gene flow in a mixed stand formed by the two species. The mating systems of both species have been studi ed separately by means of the mixed-mating model. The relative contrib ution of the parental species to the progenies have been estimated wit h two different methods. The first uses the admixture model. The secon d is an extension of the mixed-mating model and subdivides the outcros sing rate into intra- and interspecific components. The two species we re almost completely outcrossing. This high level of outcrossing and i nterspecific gene flow could play an important role in the maintenance of the genetic diversity in these long-lived forest tree species. The contribution of the sessile oak to the pedunculate oak progenies vari ed from 17% to 48%. In contrast, ovules of sessile oak trees appear to be preferentially fertilized by other extreme sessile genotypes. We s uggest that interspecific and directional gene flow was responsible fo r such patterns. Pedunculate oak is considered as a pioneer species an d is progressively replaced by sessile oak. Our present findings add a further genetic component to this succession scheme, suggesting that unidirectional gene flow reinforces succession between the two species .