Evolution of hybrid taxa in Nasturtium R.Br. (Brassicaceae)

Nasturtium officinale (2n=4x=32), N. microphyllum (2n=8x=64) and N. xsterile (2n=6x=48) have been investigated by isozyme analyses to study evolutionary processes within Nasturtium. Four additional species assumed to be involved in the formation of the octoploid N. microphyllum (Rorippa amphibia, R. sylvestris, R. palustris, and Cardamine amara) were also examined. A total of 641 individuals were analyzed for six isozyme systems (alcohol dehydrogenase, aspartate aminotransferase, glutamate dehydrogenase, leucine aminopeptidase, malate dehydrogenase, and phosphoglucoisomerase). Eleven gene complexes coding for 43 allozymes were detected. Fifteen alleles were observed in N. officinale, twelve of them being fixed. All alleles fixed in N. officinale were also present and fixed in N. microphyllum. Seven additional fixed alleles were observed in N. microphyllum. The presence of these seven alleles in Rorippa taxa provide evidence for an allopolyploid origin of N. microphyllum with N. officinale and a Rorippa taxon involved. C. amara is not a parent species of N. microphyllum. N. xsterile showed a fixed banding pattern which was identical to that of N. microphyllum. It is argued that N. xsterile is a hybrid between N. officinale and N. microphyllum. Human impact has played a major role in the evolution of N. xsterile. The formation and persistence of the hybrid were influenced by introducing N. officinale into the natural distribution area of N. microphyllum and by creating ditches and ponds where due to its vegetative capabilities N. xsterile could establish.