Genetic structure and gene flow patterns in Ulmus

The introduction into Europe of the exotic Ulmus pumila, highly tolerant to Dutch Elm Disease, has resulted in it widely replacing native U. minor populations. Bertolasi et al. investigate pollen dispersal patterns in a relic U. minor stand surrounded by U. minor and U. pumila trees scattered across approximately 80 km² in an agricultural area. They find high pollen immigration rates and extremely large dispersal distances, confirming that the pollen dispersal capabilities of the two species can maintain the genetic connectivity among isolated individuals and stands. The detection of a few inter-cluster pollinations and hybrid individuals suggests that hybridization, albeit rare, has occurred, supporting previous evidence for low mating barriers between U. minor and U. pumila.