1. ¹Unidad de Anatomía, Fisiología y Genética, ETSIM, Ciudad Universidad s/n, Madrid 28040, Spain
2. ²Unidad de Genética Forestal, Centro de Investigación Forestal, CIFOR-INIA, Carretera de La Coruña km 7.5, Madrid 28040, Spain
3. ³Department of Ecology and Evolutionary Biology, University of California at Los Angeles (UCLA), CA 90095-1606, USA
4. ⁴Departamento de Biotecnología, ETSIM, Ciudad Universidad s/n, Madrid 28040, Spain
5. ⁵NEIKER A.B., Granja Modelo-Arkaute, Vitoria 01080, Spain

Correspondence: L Gil, Unidad de Anatomía, Fisiología y Genética, Universidad Politécnica de Madrid, ETSI Montes, Ciudad Universidad s/n, Madrid 28040, Spain. E-mail: luis.gil@upm.es

Received 4 March 2005; Accepted 26 July 2005; Published online 26 October 2005.

Abstract

Oaks are long-standing models for the study of gene flow and hybridisation. Temperate (Quercus petraea) and sub-Mediterranean (Quercus pyrenaica) oaks coexist in central Spain, showing remarkable differences in population size and structure. Q. petraea has a scattered distribution in central Spain, where it is at one of the southernmost limits of its range, and forms low-density stands; in contrast, Q. pyrenaica is widespread in the region. We selected a mixed population of the two species (13 ha, 176 adults and 96 saplings) to compare the patterns of gene flow within each species and the extent of introgression between them. Using five nuclear microsatellite markers, we performed a parentage analysis and found considerable immigration from outside the stand (38% for Q. petraea and 34% for Q. pyrenaica), and estimated average seed-dispersal distances of 42 and 14 m for Q. petraea and Q. pyrenaica, respectively. Introgression between species was also estimated using our microsatellite battery. First, we developed a multivariate discriminant approach and, second, we compared our results with a widely used clustering method (STRUCTURE). Both analyses were consistent with a low level of introgression between Q. petraea and Q. pyrenaica. Indeed, only 15 adult trees, 8.5%, were identified as putative hybrids when both methods of analysis were combined. Hybrids may be most common in contact zones due merely to physical proximity.