Evolutionary dynamics of mutualism between ants and aphids

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Aphid colonies on their host plants, in general, consist of several clones. Aphid clones on the same host plants could compete for the limited amount of resources or space. Interactions among clones on the same plant are crucial in determining the intensity of natural selection and the maintenance of genetic diversity. However, no studies have examined the effects of among-clone interactions. In particular, in aphids that are obligatorily attended by ants, aphid clones are likely to compete for ant attendance. This study censused the population of the aphid Macrosiphoniella yomogicola for a long time by rearing a single clone or two clones on one shoot of Artemisia montana. Under non ant-attendance conditions, using two aphid clones, a single clone treatment or a coexistence treatment was established on each shoot of the same clone of A.montana. The results of the experiments indicated that in the single clone treatment the aphids always propagated to the level of the caring capacity, whereas in the coexistence treatment one of the two clones was kept at a low population level. This result suggests that aphid clones differ in the ability of competition so that when two clones were forced to coexist, the population of one clone was suppressed to a lower level. However, when ants are allowed to attend aphid colonies of one clone or two clones, any aphid clone on each host shoot radically deceases in the population. As a result of ant attendance, no significant difference in the population was detected between the two clones in coexistence treatments. This result shows that attending ants function to reduce the competition between aphid clones on the same plant resulting in maintaining the genetic diversity of aphids.