Disturbed landscapes are known to provide mosaics of habitats that may support natural hybridisation. We are exploring how large-scale disturbances as present in flooded retention areas, are promoting hybridisation, and possibly speciation, in a tree species. The grey poplar (Populus x canescens) is a known hybrid of P. alba and P. tremula. However, the detailed genetic structure of the hybrids was largely unknown until recently. We have used molecular genetic methods to assess the genetic status of trees in the hybrid zone along the river Danube in Austria. Hybrids were found to represent a continuum of types, from intermediate (F1-like) to highly backcrossed. However, in the initial hybrids, P. alba seems to be the preferred maternal parent and backcrosses are much more frequent to this species than to P. tremula. We are currently exploring the fate of individual genetic markers, representing chromosomal lineages, in backcross individuals. The introgression of markers of P. temula into the P. alba background does not seem to be random. We speculate that segments introgressing at a lower than expected rate may carry adaptationally malign genes, thus keeping up the species barrier.
