Symbiont recognition of mutualistic bacteria by Acromyrmex leaf-cutting ants.

Department of Bacteriology, University of Wisconsin-Madison, Madison, WI 53706, USA.
The ISME Journal (Impact Factor: 8.95). 09/2007; 1(4):313-20. DOI: 10.1038/ismej.2007.41
Source: PubMed

ABSTRACT Symbiont choice has been proposed to play an important role in shaping many symbiotic relationships, including the fungus-growing ant-microbe mutualism. Over millions of years, fungus-growing ants have defended their fungus gardens from specialized parasites with antibiotics produced by an actinomycete bacterial mutualist (genus Pseudonocardia). Despite the potential of being infected by phylogenetically diverse strains of parasites, each ant colony maintains only a single Pseudonocardia symbiont strain, which is primarily vertically transmitted between colonies by the founding queens. In this study, we show that Acromyrmex leaf-cutter ants are able to differentiate between their native actinomycete strain and a variety of foreign strains isolated from sympatric and allopatric Acromyrmex species, in addition to strains originating from other fungus-growing ant genera. The recognition mechanism is sufficiently sensitive for the ants to discriminate between closely related symbiont strains. Our findings suggest that symbiont recognition may play a crucial role in the fungus-growing ant-bacterium mutualism, likely allowing the ants to retain ecological flexibility necessary for defending their garden from diverse parasites and, at the same time, resolve potential conflict that can arise from rearing competing symbiont strains.

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    Journal of Chemical Ecology 02/2014; · 2.46 Impact Factor
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    ABSTRACT: Mutualisms are commonly threatened by parasites and cheaters: species that exploit the host-derived resources without providing an adequate service. Here, we summarize mechanisms for the stabilization of obligate defensive ant–plant mutualisms, a typical element of tropical lowland forests. Host plants exert partner choice and can sanction non-defending ants by shedding the domatia that serve as nesting space or ceasing the production of ant rewards. Hosts can also restrict the exploitation of the ant rewards by means of specific biochemical traits that decrease their quality for non-adapted generalist exploiters and, thus, convert them into exclusive rewards. Reward provisioning can even shift the competitive balance between mutualists and exploiters in favor of the mutualists. In turn, plant-ants show adaptations in their colony structure and changes in their digestive capacities that enhance their efficiency in the use of the host-derived resources. Founding queens use plant odors for host choice behavior, and ants not supplied with adequate amounts of EFN decrease their defensive service and thereby exert partner sanctions. Theoretical models and empirical research into mutualisms usually focus on actions that are taken by the host. Using ant-plants as model systems, we are now discovering the importance of contributions that come from the symbiont. This discovery indicates the potential for multiple reciprocal interactions between phenotypically plastic hosts and symbionts, which contribute significantly to what is still considered a miracle: the stability of mutualisms in the presence of exploiters.
    Biotropica 05/2013; · 2.35 Impact Factor
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    ABSTRACT: Mutualisms require protection from non-reciprocating exploiters. Pseudomyrmex workers that engage in an obligate defensive mutualism with Acacia hosts feed exclusively on the sucrose-free extrafloral nectar (EFN) that is secreted by their hosts, a behaviour linking ant energy supply directly to host performance and thus favouring reciprocating behaviour. We tested the hypothesis that Acacia hosts manipulate this digestive specialisation of their ant mutualists. Invertase (sucrose hydrolytic) activity in the ant midguts was inhibited by chitinase, a dominant EFN protein. The inhibition occurred quickly in cell-free gut liquids and in native gels and thus likely results from an enzyme-enzyme interaction. Once a freshly eclosed worker ingests EFN as the first diet available, her invertase becomes inhibited and she, thus, continues feeding on host-derived EFN. Partner manipulation acts at the phenotypic level and means that one partner actively controls the phenotype of the other partner to enhance its dependency on host-derived rewards.
    Ecology Letters 11/2014; 17:185–192. · 17.95 Impact Factor

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