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

Nasturtium officinale (2n=4x=32), N. microphyllum (2n=8x=64) and N. xsteril e (2n=6x=48) have been investigated by isozyme analyses to study evolutiona ry processes within Nasturtium. Four additional species assumed to be invol ved 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 dehydrog enase, aspartate aminotransferase, glutamate dehydrogenase, leucine aminope ptidase, malate dehydrogenase, and phosphoglucoisomerase). Eleven gene comp lexes coding for 43 allozymes were detected. Fifteen alleles were observed in N. officinale, twelve of them being fixed. All alleles fixed in N. offic inale were also present and fixed in N. microphyllum. Seven additional fixe d alleles were observed in N. microphyllum. The presence of these seven all eles in Rorippa taxa provide evidence for an allopolyploid origin of N. mic rophyllum with N. officinale and a Rorippa taxon involved. C. amara is not a parent species of N. microphyllum. N. xsterile showed a fixed banding pat tern which was identical to that of N. microphyllum. It is argued that N. x sterile 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 int o the natural distribution area of N. microphyllum and by creating ditches and ponds where due to its vegetative capabilities N. xsterile could establ ish.