A knowledge of cytotype diversity and distribution patterns is important in understanding evolutionary dynamics of diploid–polyploid contact zones. Castro et al. study large- and small-scale cytotype distribution patterns and reproductive variables in the only mixed-ploidy population so far detected of the diploid–hexaploid Aster amellus aggregate. They find a complex contact zone mostly composed of single-cytotype populations growing in parapatry in several areas. Both primary and secondary contacts are possible, supporting an autopolyploid origin of the aggregate. They observe no signs of cytotype hybridization, and suggest that minority cytotype exclusion may be one of the mechanisms driving the prevalence of the single-cytotype populations.