Evolution of hybrid taxa inNasturtium R.Br. (Brassicaceae)

Nasturtium officinale (2n=4x=32),N. microphyllum (2n=8x=64) andN. ×sterile (2n=6x=48) have been investigated by isozyme analyses to study evolutionary processes withinNasturtium. Four additional species assumed to be involved in the formation of the octoploidN. microphyllum (Rorippa amphibia, R. sylvestris, R. palustris, andCardamine 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 inN. officinale, twelve of them being fixed. All alleles fixed inN. officinale were also present and fixed inN. microphyllum. Seven additional fixed alleles were observed inN. microphyllum. The presence of these seven alleles inRorippa taxa provide evidence for an allopolyploid origin ofN. microphyllum withN. officinale and aRorippa taxon involved.C. amara is not a parent species ofN. microphyllum. N. ×sterile showed a fixed banding pattern which was identical to that ofN. microphyllum. It is argued thatN. ×sterile is a hybrid betweenN. officinale andN. microphyllum. Human impact has played a major role in the evolution ofN. ×sterile. The formation and persistence of the hybrid were influenced by introducingN. officinale into the natural distribution area ofN. microphyllum and by creating ditches and ponds where due to its vegetative capabilitiesN.×sterile could establish.