¹Department of Systematic Botany, University of Osnabrück, Barbarastr. 11, 49076 Osnabrück, Germany

Correspondence: W Bleeker, Department of Systematic Botany, University of Osnabrück, Barbarastr. 11, 49076 Osnabrück, Germany. E-mail: bleeker@biologie.uni-osnabrueck.de

Received 4 October 2004; Accepted 10 March 2005; Published online 20 April 2005.

Abstract

Hybrid zones may serve as natural laboratories for evolutionary studies. One common viewpoint is that hybrids may always be less fit than their parents due to genetic discontinuities. An alternative idea is that genotype–environment interactions influence the outcome of natural hybridization. Our comparative study of two different natural hybrid zones between the invasive diploid Rorippa austriaca and the native polyploid R. sylvestris in Germany identified the ploidy level as a major determinant of hybrid fitness. Different ploidy levels and patterns of fitness were detected in different hybrid zones. In one hybrid zone (Mülheim, Ruhr valley) hybrids were pentaploid and showed a relatively high seed set, whereas in the second hybrid zone (Randersacker, Main valley) hybrids were triploid and displayed extremely low fitness values. Analyses of fitness values in different natural hybrid zones between the same two species may lead to very different conclusions about the evolutionary significance of natural hybridization.