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Spatial genetic structure (SGS) holds the key to understanding the role of clonality in hybrid persistence, but multilocus SGS in hybrid zones has rarely been quantified. Here, the aim was to fill this gap for natural hybrids between two diploid, ecologically divergent European tree species with mixed sexual/asexual reproduction, Populus alba and P. tremula. Nuclear microsatellites were used to quantify clonality, SGS, and historical gene dispersal distances in up to 407 trees from an extensive Central European hybrid zone including three subpopulation replicates. The focus was on P. x canescens and its backcross parent P. alba, as these two genotypic classes co-occur and interact directly. Sexual recombination in both taxa was more prominent than previously thought, but P. x canescens hybrids tended to build larger clones extending over larger areas than P. alba. The 3.4 times stronger SGS in the P. x canescens genet population was best explained by a combination of interspecific gene flow, assortative mating, and increased clonality in hybrids. Clonality potentially contributes to the maintenance of hybrid zones of P. alba and P. tremula in time and space. Both clonality and SGS need to be taken into account explicitly when designing population genomics studies of locus-specific effects in hybrid zones.

The use of admixed human populations to scan the genome for chromosomal segments affecting complex phenotypic traits has proved a powerful analytical tool. However, its potential in other organisms has not yet been evaluated. Here, we use DNA microsatellites to assess the feasibility of this approach in hybrid zones between two members of the 'model tree' genus Populus: Populus alba (white poplar) and Populus tremula (European aspen). We analyzed samples of both species and a Central European hybrid zone (N=544 chromosomes) for a genome-wide set of 19 polymorphic DNA microsatellites. Our results indicate that allele frequency differentials between the two species are substantial (mean delta=0.619+/-0.067). Background linkage disequilibrium (LD) in samples of the parental gene pools is moderate and should respond to sampling schemes that minimize drift and account for rare alleles. LD in hybrids decays with increasing number of backcross generations as expected from theory and approaches background levels of the parental gene pools in advanced generation backcrosses. Introgression from P. tremula into P. alba varies strongly across marker loci. For several markers, alleles from P. tremula are slightly over-represented relative to neutral expectations, whereas a single locus exhibits evidence of selection against P. tremula genotypes. We interpret our results in terms of the potential for admixture mapping in these two ecologically divergent Populus species, and we validate a modified approach of studying genotypic clines in 'mosaic' hybrid zones.