Symbiont choice in a fungus-growing ant (Attini, Formicidae)

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Abstract
Cultivars of fungus-growing (attine) ants are vertically transmitted through inheritance from parent to offspring nest, but horizontal cultivar transfer between ant nests occurs occasionally, resulting in cultivar replacement within ant lineages. Two mechanisms could theoretically prevent the invasion of suboptimal cultivar strains and thus stabilize ant–cultivar coevolution: first, partner feedback inherent in vertical cultivar transmission and second, partner (symbiont) choice if the ants differentiate between productive and inferior cultivars during replacements. To elucidate the nature of symbiont choice, we presented workers of Cyphomyrmex muelleri with novel cultivars representing a phylogenetic cline of close and distant relatives of the native C. muelleri cultivar. Workers invariably preferred their native cultivar, discriminating against even very close relatives of the native cultivar. When given a choice between two non-native cultivar strains, workers accepted the strain most closely related to their native cultivar. Two conclusions emerge. First, colony switches to distantly related cultivars are behaviorally unlikely and may not be preference-based; rather, distant switches may occur under constrained choice, such as pathogen-related garden losses that force colonies to import novel cultivars. Second, the ability of attine ants to differentiate between closely related cultivar strains suggests that the ant–fungus mutualism is stabilized evolutionarily not only by partner feedback inherent in vertical cultivar transmission, but possibly also by symbiont choice through which the ants select against unwanted, presumably inferior, cultivars. The efficacy of symbiont choice now needs to be tested experimentally. Such research may benefit from application of theory and experimental paradigms that have been developed within the areas of mate choice and sexual selection.

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