The symbiosis between fungus-growing ants and their cultivated fungi has been shaped by over 50 million years of coevolution. Phylogenetic analyses indicate that this long coevolutionary history includes a third symbiont lineage: specialized microfungal parasites (Escovopsis “weeds”) of the ants’ fungus gardens. At ancient phylogenetic levels, the phylogenies of the three symbionts are perfectly congruent, revealing that the ant-microbe symbiosis is the product of tripartite coevolution between the farming ants, their cultivars, and Escovopsis garden parasites. At recent phylogenetic levels, ant-fungus coevolution has been punctuated by occasional host-switching by ants to novel cultivars, thus intensifying continuous coadaptation between symbionts in a tripartite 'arms-race'. Population-genetic and phylogenetic analyses further indicate that the Escovopsis parasites are specialized on the cultivars, not the ants, such that switches by ant lineages to novel cultivar lineages requires not only the evolution of novel fungicultural practices by the ants, but also the evolution of novel defense strategies against the parasites associated with a newly acquired cultivar. These dual complications of cultivar switching should constrain switches to novel cultivars, possibly explaining why single attine ant species are generally specialized on a phylogenetically narrow group of cultivars.
Species 1: Hymenoptera Formicidae Atta (leafcutter ant)
Species 2: Hymenoptera Formicidae Cyphomyrmex muelleri (fungus-growing ant)
Species 3: Hymenoptera Formicidae Cyphomyrmex longiscapus (fungus-growing ant)
Keywords: phylogeny, mutualism
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