Geographical and environmental range expansion through polyploidy in wild potatoes (Solanum section Petota)

Aim To assess evidence for geographical and environmental range expansion through polyploidy in wild potatoes (Solanum sect. Petota). There are diploids, triploids, tetraploids, pentaploids and hexaploids in this group. Location Wild potatoes occur from the south-western USA (Utah and Colorado), throughout the tropical highlands of Mexico, Central America and the Andes, to Argentina, Chile and Uruguay. Methods We compiled 5447 reports of ploidy determination, covering 185 of the 187 species, of which 702 determinations are presented here for the first time. We assessed the frequency of cytotypes within species, and analysed the geographical and climatic distribution of ploidy levels. Results Thirty-six per cent of the species are entirely or partly polyploid. Multiple cytotypes exist in 21 species, mostly as diploid and triploid, but many more may await discovery. We report the first chromosome count (2n = 24) for Solanum hintonii. Diploids occupy a larger area than polyploids, but diploid and tetraploid species have similar range sizes, and the two species with by far the largest range sizes are tetraploids. The fraction of the plants that are polyploids is much higher from Mexico to Ecuador than farther south. Compared with diploids, triploids tend to occur in warmer and drier areas, whereas higher-level polyploids tend to occur in relatively cold areas. Diploids are absent from Costa Rica to southern Colombia, the wettest part of the group's range. Main conclusions These results suggest that polyploidy played an important role in this group's environmental differentiation and range expansion.