Behavioral Ecology Advance Access published online on November 26, 2008
Behavioral Ecology, doi:10.1093/beheco/arn150
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Caste-specific symbiont policing by workers of Acromyrmex fungus-growing ants
a Department of Biology, Centre for Social Evolution, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark b Theoretical Biology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, PO Box 14, NL-9750 AA Haren, the Netherlands c Department of Bacteriology, University of Wisconsin—Madison, 4325 Microbial Sciences Building, 1550 Linden Dr, Madison WI 53706, USA
Address correspondence to A.B.F. Ivens. E-mail: a.b.f.ivens{at}rug.nl.
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Abstract |
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The interaction between leaf-cutting ants and their fungus garden mutualists is ideal for studying the evolutionary stability of interspecific cooperation. Although the mutualism has a long history of diffuse coevolution, there is ample potential for conflicts between the partners over the mixing and transmission of symbionts. Symbiont transmission is vertical by default, and both the ants and resident fungus actively protect the fungal monoculture growing in their nest against secondary introductions of genetically dissimilar symbionts from other colonies. An earlier study showed that mixtures of major and minor Acromyrmex workers eliminate alien fungus fragments even in subcolonies where their resident symbiont is not present. We hypothesize that the different tasks and behaviors performed by majors and minors are likely to select for differential responses to alien fungi. Major workers forage and cut new leaves and masticate them after delivery in the upper parts of the fungus garden and so are likely to more frequently encounter alien fungus than minor workers maintaining the established fungus garden and caring for the brood. We show that major workers of Acromyrmex echinatior indeed express stronger incompatibility reactions toward alien fungus garden fragments than minor workers. This implies that only the major workers, through recognition and exclusion of foreign fungus clones at their point of entry to the nest, have a realistic possibility to eliminate alien fungal tissue before it gets incorporated in the fungus garden and starts competing with the resident fungal symbiont.
Key words: evolutionary stability, leaf-cutting ants, mutualism, symbiont policing.
Received 31 July 2008; revised 27 October 2008; accepted 30 October 2008.