Ecological segregation does not drive the intricate parapatric distribution of diploid and tetraploid cytotypes of the A rabidopsis arenosa group ( B rassicaceae)

Detailed knowledge of the geographic distribution of cytotypes is a prerequisite for any experimental or molecular study of ploidy‐variable plant systems. The A rabidopsis 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 ( S outheast Europe) and tetraploid ( N orthwest E urope) cytotypes with two contact zones – a narrow contact zone in the S lovenian F orealps and a diffuse contact zone across the C arpathians. In addition, a previously unknown isolated diploid lineage with distinct ecology was revealed from sandy areas of the southeastern B altic coast. We also recorded several adult triploid individuals for the first time in wild A rabidopsis arenosa . Particularly in the W estern C arpathians, 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. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society , 2015, ●● , ●●–●●.