Hybrid zone persistence in European Populus

Populus alba L. (white poplar) and P. tremula L. (European aspen), two native European tree species, have recently become interesting objects for evolutionary genetic studies. They are characterized by: (1) extensive hybridization in zones of contact, (2) the occurrence of both sexual and asexual (vegetative) reproduction, (3) clear morphological character differences, and (4) divergent habitat preferences. My project will focus on a large hybrid zone between these two species situated along the river Danube between Krems (Austria) and Bratislava (Slovakia). Recent molecular (proteins and DNA) analyses have indicated that the species barrier between the two parents is extremely `porous`. That is, many specific DNA segments can be transferred preferentially from P. tremula into P. alba. This natural `DNA transfer` is also visible in morphological characters. The morphology of hybrids, traditionally known as Populus canescens (grey poplar), varies from intermediate between the parental species to P. alba-like. The primary goals of my project are: (1) To determine the genomic composition of hybrids in the Danube hybrid zone using DNA (microsatellite) markers. (2) To study its relationship with a variety of abiotic and biotic habitat factors. This will allow me to test whether habitat- dependent (ecological) selection is acting on hybrid genotypes. (3) To study spatial genetic structure in the hybrid zone in order to assess the role of asexual versus sexual reproduction in hybrid persistence. Thus, this project concentrates on two extremely timely topics in plant evolutionary genetics: the role of ecological selection in maintaining species barriers, and the role of asexual reproduction in facilitating the persistence of hybrid populations.