Hybridization as a mechanism of invasion in oaks

We review here our own research and related work on hybridization between two widespread and largely sympatric European oak species (Quercus petraea and Q. robur). There is a near total lack of local differentiation in chloroplast DNA markers between them. A model is proposed to account for this lack of differentiation: invasion by one species of the range occupied by the other through pollen swamping. In support of this model, ecological, palaeoecological and population genetic studies indicate that one species (Q. robur) disperses its seeds better than the other one, that pollen flow is much more efficient than seed flow in oaks and that hybridization and introgression are asymmetric, hence reinforcing the ecological dynamics by facilitating the dispersal of Q. petraea in regions already colonized by Q. robur. 'Resurrection' of Q. petraea following this wave of hybridization appears to be rapid. More generally, available evidence indicates that hybridization could constitute an important mechanism of dispersal in both natural and human-induced plant invasions. (C) New Phytologist (2003).