PT Unknown
AU Gonzalez-Teuber, M
TI Mechanisms enabling specific plant-ant mutualisms: Acacia-Pseudomyrmex as a model system
PD 04
PY 2010
LA en
AB Mutualisms are interactions among different species that lead to net fitness benefits for all partners involved. In plant-ant mutualisms, plants provide to ants an array of rewards, such as extrafloral nectar (EFN), food bodies, or nesting space. Ants are attracted, or completely nourished, by plant-derived food rewards and serve as a means of indirect defence of plants against herbivores. Although these mutualisms can become very specific, the rewards traded among mutualist partners may also be attractive for non-mutualist organisms, i.e., exploiters that make use of the host-derived rewards without reciprocating. Thus, the goal of this study was to investigate mechanisms that drive the specificity of plant-ant interactions, and that stabilize it from exploitation. The mutualism of Acacia plants with Pseudomyrmex ants was used as a model system, in which we can find different kinds of plant-ant interactions that vary in their specificity: facultative and obligate. Whereas Acacia obligate plants (myrmecophytes) secrete EFN at high quantities and constituvely, to house and nourish symbiotic ants of P. ferrugineus, facultative ones (non-myrmecophytes) secrete it only in response to damage, attracting generalist ants. These differences in plant-ant interactions make this genus Acacia highly suitable to study mechanisms that may determine species-specific interaction. Specifically, I focused my study on the chemistry of EFN (amino acids and proteins) and on the ant behaviour in terms of defence against nectar robbers, herbivores and leaf pathogens.
Amino acid composition of obligate Acacia was highly specialized and adapted to the preferences and nutritive requirements of the specialised mutualist ant P. ferrugineus. Mutualist ants preferred EFN solutions that contained exactly those amino acids that were quantitatively dominating in myrmecophyte EFN. By contrast, generalist ants preferred sugar solutions with amino acids over mere sugar solutions but were not able to discriminate among different numbers or concentrations of specific amino acids, suggesting, thus, that amino acids of non-myrmecophyte EFN play an important role in the attraction but less so in the nutrition of ants. On the other hand, EFN of obligate Acacia species appeared (bio)chemically protected from microbe infestation. Bioassays demonstrated that fungal growth was inhibited in EFN of myrmecophytes. The identification of proteins in myrmecophyte nectar revealed an abundant presence of PR-proteins, such as glucanases, chitinases and thaumatin- and osmotin like proteins, of which activities were also detected in EFNs. Furthermore, the total amount of proteins was significantly higher in myrmecophyte EFN than in the EFN of non-myrmecophytes. These data, together with the observations that the protein-fraction of myrmecophyte EFN significantly inhibited the growth of various fungi, suggests that nectar proteins are associated with the protection of EFN from microbes.
In parallel to these chemical adaptations on the side of the plant, symbiotic ants of P. ferrugineus, unlike the parasite P. gracilis, exhibited relevant ecological and chemical adaptations, which contribute to the specificity of the mutualism. P. ferrugineus effectively defended their host plants against herbivores and leaf bacteria and protected the EFN from nectar robbers. Nevertheless, the defensive efficiency provided by P. ferrugineus was associated with the amounts of rewards provided by the host plant: the host species that invest less in ant rewards received less defence by the symbiotic ant. Thus, P. ferrugineus tended to diminish its defensive service when it did not receive the respective pay-off from the host. On the other hand, P. ferrugineus had the capacity to induce EFN secretion by myrmecophytes, demonstrating that the host plant also can cease reward production when it does not receive the expected biotic defence. The results of the present study illustrate different chemical and ecological mechanisms that drive the specificity of the Acacia-Pseudomyrmex mutualism, thus, helping 1) to prevent the mutualism from exploitation and, 2) to stabilize the mutualist interaction.
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