It had previously been proposed that fungus-growing ants and their fungal cultivars are diffusely associated — that is, the two partners are not exclusively associated with each other. However, this study suggests that this is not the case. By examining the relationship between fungal cultivars and four ant species in North, Central and South America, the authors found that each ant species has been exclusively associated with one fungal cultivar lineage over millions of years. Moreover, analysis of these relationships suggested that rare shifts in cultivar association are linked to ant speciation.