Polyploidy and hybridization in the Mediterranean and neighbouring areas towards the north: examples from the genus Cardamine (Brassicaceae)

Abstract

The genus Cardamine is distributed worldwide and is rich in polyploids. Here we review considerable amount of knowledge accumulated on polyploid species and related diploids during the last twenty years, particularly in the area of Mediterranean and neighbouring regions of Central Europe. The studies addressed taxonomic treatment of the polyploids, their origin and evolution, phylogenetic relationships to diploid and other polyploid relatives, as well as overall morphological, genetic, karyological and ecological patterns. The revealed parentage of polyploids such as C. asarifolia, C. flexuosa and C. ×schulzii shows the power of recently developed molecular and cytogenetic approaches that enable us to understand how the polyploid taxa originated, become established and further evolved. It is highlighted that diploid-polyploid species complexes, such as the C. amara and C. pratensis groups, represent true natural laboratories for polyploid studies. Particularly the species complex of C. pratensis that contains numerous diploid lineages and polyploid populations of all levels up to dodecaploids, including aneuploids and dysploids, provides excellent possibility for application of wide spectrum of methodical approaches to understand the processes underlying polyploid evolution. Hybridisation revealed in this genus, occurs both at homoploid and heteroploid levels and mainly in mountain regions, where parapatric and/or ecologically differentiated close relatives come into contacts. These studies documented that interspecific gene flow can result in various patterns, such as continuous variation in hybrid swarms (C. pratensis × C. raphanifolia), established hybrid species of significant evolutionary potential (C. ×schulzii, C. ×insueta), or largely sterile hybrids of first generations only and persistence through vegetative propagation (C. ×ferrarii, C. ×enriquei). All these cases also show how recent hybridization and introgression add to the complexity of reticulated patterns of evolution in Cardamine in the Mediterranean and adjacent areas.