Abstract
Interspecies or intraspecies cooperation can be stabilized evolutionarily if choosing partners favor beneficial partners and discriminate against non-beneficial partners. We quantified such partner choice (symbiont choice) in the leafcutter ant Atta texana (Attini, Formicidae) by presenting the ants in a cafeteria-style preference assay with genotypically distinct fungal cultivars from A. texana and Acromyrmex versicolor. Symbiont choice was measured as the ants' tendency to choose one or more cultivar(s) from several pure (axenic) cultivar fragments and convert a given fungal fragment into a garden. Microsatellite DNA fingerprinting enabled us to identify the cultivars chosen by the ants for their gardens. In 91% of the choice tests, A. texana workers combined multiple cultivars into a single intercropped, chimaeric garden, and the cultivars coexisted in such chimaeric gardens for as long as 4 months. Coexistence of distinct fungal genotypes in chimaeric gardens appears to contradict a recent model of cultivar competition postulating that each cultivar secretes incompatibility compounds harming other cultivars, which presumably would preclude the intercropped polyculture observed in our experiments. Although we found no clear evidence of novel, recombinant genotypes in the experimental chimaeric gardens, the intercropping of cultivar genotypes may occasionally lead under natural conditions to exchange of genetic material between coexisting cultivars, thus introducing novel cultivar genotypes into the leafcutter symbiosis. Symbiont choice by ants and any competition between coexisting cultivar strains in chimaeric gardens do not appear to operate fast enough in our laboratory assay to convert chimaeric gardens into the monocultures observed for A. texana under natural conditions.
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Acknowledgements
We thank K. Fabri and G. Chao for help with the maintenance of the ant colonies. The work was supported by NSF Grant DEB-0639879. We thank Peter Nonacs for his advice on adapting skew statistics to test cultivar choice. We thank our associate editor Lotta Sundström and two anonymous reviewers for exceptionally constructive comments.
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The authors declare that the experiments comply with the current laws of the United States, where they were performed. The authors declare that they have no conflict of interest.
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Communicated by L. Sundström
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Sen, R., Ishak, H.D., Kniffin, T.R. et al. Construction of chimaeric gardens through fungal intercropping: a symbiont choice experiment in the leafcutter ant Atta texana (Attini, Formicidae). Behav Ecol Sociobiol 64, 1125–1133 (2010). https://doi.org/10.1007/s00265-010-0928-x
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DOI: https://doi.org/10.1007/s00265-010-0928-x