Detailed knowledge of the geographic distribution of cytotypes is a prerequisite for any experimental or molecular study of ploidy-variable plant systems. The Arabidopsis arenosa group, an intricate di-tetraploid complex from the plant model genus Arabidopsis, has remained largely neglected regarding the distribution and habitat associations of its cytotypes. Using flow cytometry, we conducted a large population-level cytological screen across the A. arenosa group range, involving more than 2900 individuals from 194 populations. We characterized a largely parapatric distribution of the diploid (Southeast Europe) and tetraploid (Northwest Europe) cytotypes with two contact zones -a narrow contact zone in the Slovenian Forealps and a diffuse contact zone across the Carpathians. In addition, a previously unknown isolated diploid lineage with distinct ecology was revealed from sandy areas of the southeastern Baltic coast. We also recorded several adult triploid individuals for the first time in wild Arabidopsis arenosa. Particularly in the Western Carpathians, the diploid and tetraploid populations are largely intermingled, and both cytotypes are spread along the whole lowland-alpine gradient of habitats, exhibiting no signs of ploidy-linked habitat differentiation. In contrast with the complexity at the landscape scale, the within-population cytological homogeneity and the rare occurrence of triploids indicate that the contact zone is rather stable.

The observed spatial and genetic patterns likely reflect the evolutionary and colonization history of the two cytotypes and have been maintained by multiple factors such as ecological divergence, limited gene flow between the cytotypes, and the restricted dispersal capacity.

Methods: Two thousand four hundred and thirty-seven relev es from halophytic and sub-halophytic habitats were classified using a modified TWINSPAN. The crispness of classification was checked. DCA and CCA with climate data as explanatory variables were applied.