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Evolution of cold-tolerant fungal symbionts permits winter fungiculture by leafcutter ants at the northern frontier of a tropical ant-fungus symbiosis

Ulrich G. Mueller, Alexander S. Mikheyev, Eunki Hong, Ruchira Sen, Dan L. Warren, Scott E. Solomon, Heather D. Ishak, Mike Cooper, Jessica L. Miller, Kimberly A. Shaffer, Thomas E. Juenger and Nancy A. Moran
Proceedings of the National Academy of Sciences of the United States of America
Vol. 108, No. 10 (March 8, 2011), pp. 4053-4056
Stable URL: http://www.jstor.org/stable/41061059
Page Count: 4
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Evolution of cold-tolerant fungal symbionts permits winter fungiculture by leafcutter ants at the northern frontier of a tropical ant-fungus symbiosis
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Abstract

The obligate mutualism between leafcutter ants and their Attamyces fungi originated 8 to 12 million years ago in the tropics, but extends today also into temperate regions in South and North America. The northernmost leafcutter ant Atta texana sustains fungiculture during winter temperatures that would harm the coldsensitive Attamyces cultivars of tropical leafcutter ants. Cold-tolerance of Attamyces cultivars increases with winter harshness along a south-to-north temperature gradient across the range of A. texana, indicating selection for cold-tolerant Attamyces variants along the temperature dine. Ecological niche modeling corroborates winter temperature as a key range-limiting factor impeding northward expansion of A. texana. The northernmost A. texana populations are able to sustain fungiculture throughout winter because of their cold-adapted fungi and because of seasonal, vertical garden relocation (maintaining gardens deep in the ground in winter to protect them from extreme cold, then moving gardens to warmer, shallow depths in spring). Although the origin of leafcutter fungiculture was an evolutionary breakthrough that revolutionized the food niche of tropical fungus-growing ants, the original adaptations of this host-microbe symbiosis to tropical temperatures and the dependence on cold-sensitive fungal symbionts eventually constrained expansion into temperate habitats. Evolution of coldtolerant fungi within the symbiosis relaxed constraints on winter fungiculture at the northern frontier of the leafcutter ant distribution, thereby expanding the ecological niche of an obligate hostmicrobe symbiosis.

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