Evolutionary dynamics of mutualism between ants and aphids

蚜虫在寄主植物上的菌落通常由几个无性系组成。同一寄主植物上的蚜虫克隆可以竞争有限的资源或空间。同一植物上无性系间的相互作用是决定自然选择强度和维持遗传多样性的关键。然而，还没有研究检验过克隆间相互作用的影响。特别是，在必须由蚂蚁照顾的蚜虫中，蚜虫的克隆可能会竞争蚂蚁的照顾。本研究通过在一株蒙大拿蒿（Artemisia montana）枝条上培育单株或两株蚜虫，对其种群进行了长期的种群普查。在非抗虫条件下，利用2个蚜虫无性系，分别对同一个蒙大拿无性系的每个芽进行单克隆处理或共存处理。实验结果表明，单克隆处理下蚜虫总能繁殖到照顾能力水平，而共存处理下，两个无性系中的一个保持在低种群水平。这说明蚜虫无性系的竞争能力存在差异，当两个无性系被迫共存时，一个无性系的种群数量会被抑制到较低水平。然而，当蚂蚁被允许进入一个或两个无性系的蚜虫群落时，每个寄主枝上的任何无性系蚜虫在种群中都急剧减少。由于蚂蚁的参与，两个无性系在共存处理中种群数量没有显著差异。这一结果表明，参加蚂蚁的作用是减少同一植物上蚜虫克隆之间的竞争，从而保持蚜虫的遗传多样性。

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.