A Textbook Example of Recent Polyploidy in Cardamine: The True Story

Inter-species hybridization and polyploidy are playing a crucial role in speciation of land plants. Cardamine schulzii (Brassicaceae) is one of the few textbook examples of recent (<100 yrs) allopolyploid plant species. The presumably autoallohexaploid (2n=6x=48, RRRRAA) species originated at Alpine meadows of Urnerboden (Switzerland) from the triploid C. ×insueta (2n=3x=24, RRA), a spontaneous hybrid between two diploid (2n=2x=16) species C. rivularis (RR) and C. amara (AA). By cytogenetic and molecular tools, we revisited the origin and genome stability in C. schulzii and C. ×insueta. Within the hybrid C. ×insueta genome, both parental chromosome complements remained stable in all plants analyzed. Despite an extensive population screening, C. schulzii plants with 2n=48 have not been found at Urnerboden. Instead, all the presumably hexaploid plants from the locus classicus possessed 46 chromosomes, except for three individuals with 2n=38. Genomic in situ hybridization (GISH) and comparative chromosome painting (CCP) analyses revealed that C. schulzii includes in fact hybrids between the hypotetraploid C. pratensis (2n=30, PPPP_-2) and C. ×insueta: genome RRPP_-2A (2n=38) and RPPPP_-2A (2n=46). Taken together our data question the originally proposed scenario of the origin of C. schulzii, emphasize the evolutionary potential of the semifertile C. ×insueta, and finally demonstrate how important is to revisit familiar textbook examples using modern techniques.