Conflict and cooperation in the mutualism between ants and aphids.

Abstract

One of the best examples of a trophic mutualism in social insects is that between ants and aphids, whereby aphids secrete a sugary-rich food source termed honeydew, and the ants protect the aphids in return against natural enemies (Hölldobler & Wilson, 1990). Conflict, however, can occur when either the ants or the aphids exploit the mutualism by not returning their services. The aphids, for example, could gain a selfish advantage by secreting less or lower quality honeydew, but still take advantage of the protective services of the ants. Here we use the native Lasius niger-Aphis fabae ant-aphid system as a model to test whether such cheating occurs, and what processes could help to prevent it. One possibility is that aphid clones secreting little or low-quality honeydew are sanctioned by the ants relative to more cooperative clones. Evidence in this direction is provided by the fact that when Lasius niger ants have a choice of aphid species it preferentially tends those species that produce most honeydew, and kills the others (Fischer et al., 2001). In our study we plan to test whether such a preference also exists within one species (Aphis fabae fabae, black bean aphid) if the ants are presented with different aphid clones producing different amounts or qualities of honeydew. In order to do this, we collected A. fabae clones around Leuven and in Antwerp. Then, we chemically analyzed their honeydew using HPLC. Preliminary data show high variations in the quality and quantity of sugars secreted by the aphids. Therefore we will conduct more chemical analyses to confirm these results and average the plant influence on the aphids secretions. The second part of the study will consist in behavioural experiments where low- and high-producing clones will be presenting to the ants in binary choice experiments to test whether the ants will sanction uncooperative aphid clones. Another side of the study will be to evaluate clonal mixing in the field. Indeed, genetic data have highlighted the presence of clonal mixing in Aphis fabae. However the sample set was small and only from the beginning of the season. Therefore more sampling will be conduct during the next season to evaluate the importance of clonal mixing and its possible changes in time. These data on Aphis fabae fabae clonal mixing will be completed with data on Aphis fabae cirsiiacanthoides which can be found on thistle and Aphis rumicis which can be found on sorrel. Microsatellites from Aphis fabae have been used for these two species and have shown the presence of clonal mixing too. These species are easily found around Leuven and are ant-attended too, therefore it will permit to compare clonal mixing in the field between different species and subspecies of aphids. The laboratory of Entomology has a long working experience in social insects, and this project fits into the lines of research of behavioural ecology and evolutionnary biology developped by Dr Tom Wenseleers. One problem in evolutionary biology is to explain how competition and conflict is repressed in the evolution of more complex, cooperative groups. Previous and current work on this subject by Dr Tom Wenseleers and colleagues has focused on the case of conflicts occurring in animal societies. This project will explore another aspect of this problem using the example of interspecific mutualisms. Hölldobler B & Wilson EO (1990) The Ants Springer-Verlag, Berlin, Heidel berg. Fischer MK, Hoffmann KH, Völkl W (2001) Competition for mutualists in an ant homopteran interaction mediated by hierarchies of ant attendance. Oikos 92, 531-541.status: publishe