Genetic structure and evolution of diploid Cochlearia in Iceland

In northern European Cochlearia (Brassicaceae), considerable chromosome variation has taken place without corresponding morphological differentiation, resulting in an intricate species complex including two base chromosome numbers and several ploidies. Here, we investigate the situation in Iceland. The distribution, genetic structure, taxonomy and origin of the two Cochlearia cytotypes (2n = 12 and 2n = 14) present in Iceland are discussed. Chromosome counts indicate that both cytotypes occur along the coast, but 2n = 12 populations dominate (eight 2n = 12 vs. two 2n = 14 among the investigated populations), whereas 2n = 14 was reported for the two inland alpine populations investigated here. RADseq data support geographically structured genetic variation along the Icelandic coast and environmentally structured genetic differentiation between coastal and alpine populations. The alpine populations show genetic and morphological affiliation with C. groenlandica (2n = 14), which is widely distributed in the Arctic, but more comprehensive sampling is needed to draw conclusions concerning the taxonomic status of the Icelandic coastal plants. To uncover the origin of and phylogenomic relationships between the two chromosome variants, comparative whole-genome sequencing should be performed.