Angiosperms are believed to have emerged initially in the tropics and expanded their distribution range polewards through diverse mechanisms, for example polyploidization-driven cold-tolerance evolution. Reversed expansion from temperate to pantropic climate through a polyploidization-driven shift in heat-tolerance remains largely unknown. Here, we found autopolyploidy in relation to the global expansion of Solidago canadensis from its temperate-climate native range in North American to hot-summer climate in an introduced range. Our cytogeographical study of 2062 accessions from...