Hybridization and evolution in Cardamine (Brassicaceae) at Urnerboden, central Switzerland: Biosystematic and molecular evidence

Abstract

Hybridization between two diploid (2n = 2x = 16) species of Brassicaceae, Cardamine rivalaris and C. amara, at Urnerboden, Central Switzerland, resulted in the rather unusual triploid hybrid C. insueta (2n = 3x = 24), and later on in the amphiploid C. schulzii (2n = 6x = 48). The hybrid and the neopolyploid species colonized successfully some manmade biotopes. Plants of C. insueta are mostly functional females with non-dehiscent anthers, but true hermaphrodite individuals with partly sterile pollen grains also occur within the population. Analyses of cpDNA and nuclear DNA permitted to establish the parentage of the hybrid: the maternal parent which contributed unreduced egg cells proved to be C. rivularis whereas the normally reduced pollen originated from C. amara. The pronounced genetic variability in C. insueta revealed by isozyme and RAPD analyses; at variance with the polarized segregation, heterogamy and strong vegetative reproduction of the hybrid, is possibly influenced by recurrent formation of C. insueta which party results from backcrosses between C. insueta and C. rivularis but may also proceed by other pathways. The amphiploid Cardamine schulzii has normally developed anthers but its pollen is sometimes highly sterile. The surprisingly uniform genetic make-up of the new amphiploid species might be related to its possible monotopic origin and/or young phylogenetic age but should be further assessed. Site management seems to be very important to a further development of hybridogenous populations and their parent species. In conclusion, the evolution at Urnerboden is discussed in the context of the traditional concept of multiple plant origins.