Genetic variation and self-incompatibility within and outside a Rorippa hybrid zone (Brassicaceae)

Rorippa amphibia and R. sylvestris are both self-incompatible, perennial species. In northern Germany they show a geographic pattern of interspecific gene flow: R. amphibia and R. sylvestris form a hybrid zone at the river Elbe but do not hybridise elsewhere in northern Germany. In the present paper we analyse the relationship between molecular diversity, seed set and seed germination within and outside the hybrid zone. In both species we observed high genetic variation and high seed set in the hybrid zone, and low genetic variation and low seed set outside the hybrid zone. In R. amphibia, high genetic variation is correlated with high seed set, but with low germination rates of the seeds. The results of an isolation experiment show that self-incompatibility is maintained in the parental species and their hybrids. The impact of genetic self-incompatibility on hybrid zone formation and hybrid fitness is discussed.