Experimental hybridization of species in the genus Rorippa.

Experimentally produced interspecific hybrids between four Central European species of Rorippa (Brassicaceae), which are wide-spread in the Czech and Slovak Republics (allogamic R. amphibia, R. austriaca, R. sylvestris and autogamic R. palustris), were studied. The hybrid between the allogamic tetraploid species R. amphibia and R. sylvestris can produce hybrid swarms when they occur sympatrically with the parental species. The most plausible mode of formation of the tetraploid hybrid swarms introgressed by diploid R. austrica in nature was confirmed: The chromosome numbers of the offspring resulted from the controlled pollination of the triploid experimental hybrid R. austriaca × R. sylvestris mostly tended to the tetraploid level. Even healthy tetraploid plants, with high quality pollen, developed in the second generation after open pollination of the experimental triploid R. amphibia × R. austriaca. Plants with nearly tetraploid or tetraploid chromosome numbers and sufficiently fertile pollen gave rise to fully fertile tetraploid hybrid swarms, even without the presence of tetraploid R. austriaca. Failure of most experimental crosses of the autogamous tetraploid R. palustris with allogamous species (totally sterile F1 acquired only in combination R. austriaca × R. palustris) indicated that this species is unlikely to have participated in the formation of hybrid swarms in nature.