1. ¹Jodrell Laboratory, Royal Botanic Gardens, Richmond, Surrey, UK
2. ²Department of Botany, University of Wyoming, Laramie, WY, USA
3. ³Department of Genetics, Federal Research Centre for Forests, Hauptstrasse, Vienna, Austria

Correspondence: Dr C Lexer, Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, UK. E-mail: c.lexer@kew.org

Received 17 September 2005; Revised 28 July 2006; Accepted 14 August 2006; Published online 20 September 2006.

Abstract

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 =0.6190.067). 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.