Abstract: The obligate mutualism between leafcutter ants and their Attamyces fungi originated 8-12 million years ago in the tropics but extends today into temperate regions. The northernmost leafcutter ant Atta texana practices fungiculture during winter conditions that would harm the cold-sensitive Attamyces cultivars of tropical leafcutter ants. Cold-tolerance of Attamyces crops increases with winter harshness along a latitudinal temperature gradient across the range of A.texana, indicating selection for cold-tolerant Attamyces variants along the temperature cline. Ecological niche modeling identifies winter temperatures as a key range-limiting factor impeding northward expansion of leafcutter ants. A.texana is able to sustain fungiculture throughout winter because of their cold-adapted fungi and behaviors to regulate garden temperatures. Although the origin of leafcutter fungiculture was an evolutionary breakthrough that revolutionized the food niche of tropical fungus- growing ants, the dependence on cold-sensitive fungal symbionts eventually constrained the ecological success of leafcutter ants during their expansion into temperate habitats. Evolution of cold-tolerant fungi within the symbiosis relaxed constraints on winter fungiculture at the northern frontier of the leafcutter ant distribution, thereby expanding the ecological niche of a host-microbe symbiosis.