Theoretical Biology, Centre for Ecological and Evolutionary Studies, University of Groningen, Groningen, The Netherlands.
BMC Evol Biol. 2012 Jul 2;12:106. doi: 10.1186/1471-2148-12-106.
Mutualistic interactions are wide-spread but the mechanisms underlying their evolutionary stability and ecological dynamics remain poorly understood. Cultivation mutualisms in which hosts consume symbionts occur in phylogenetically diverse groups, but often have symbiont monocultures for each host. This is consistent with the prediction that symbionts should avoid coexistence with other strains so that host services continue to benefit relatives, but it is less clear whether hosts should always favor monocultures and what mechanisms they might have to manipulate symbiont diversity. Few mutualisms have been studied in sufficient genetic detail to address these issues, so we decided to characterize symbiont diversity in the complex mutualism between multiple root aphid species and Lasius flavus ants. After showing elsewhere that three of these aphid species have low dispersal and mostly if not exclusively asexual reproduction, we here investigate aphid diversity within and between ant nest mounds.
The three focal species (Geoica utricularia, Forda marginata and Tetraneura ulmi) had considerable clonal diversity at the population level. Yet more than half of the ant mounds contained just a single aphid species, a significantly higher percentage than expected from a random distribution. Over 60% of these single-species mounds had a single aphid clone, and clones tended to persist across subsequent years. Whenever multiple species/clones co-occurred in the same mound, they were spatially separated with more than 95% of the aphid chambers containing individuals of a single clone.
L. flavus "husbandry" is characterized by low aphid "livestock" diversity per colony, especially at the nest-chamber level, but it lacks the exclusive monocultures known from other cultivation mutualisms. The ants appear to eat most of the early instar aphids, so that adult aphids are unlikely to face limited phloem resources and scramble competition with other aphids. We suggest that such culling of carbohydrate-providing symbionts for protein ingestion may maintain maximal host yield per aphid while also benefitting the domesticated aphids as long as their clone-mates reproduce successfully. The cost-benefit logic of this type of polyculture husbandry has striking analogies with human farming practices based on slaughtering young animals for meat to maximize milk-production by a carefully regulated adult livestock population.
互利共生关系广泛存在，但它们的进化稳定性和生态动态的机制仍知之甚少。在宿主消耗共生体的共生关系中，宿主消耗共生体的现象存在于不同的进化群体中，但通常每个宿主都只有单一的共生体培养。这与共生体应避免与其他菌株共存的预测一致，以便宿主服务继续使亲属受益，但尚不清楚宿主是否应该始终青睐单培养物，以及它们可能拥有哪些机制来操纵共生体多样性。很少有共生关系在遗传细节上得到充分研究来解决这些问题，因此我们决定研究多种根蚜虫物种与拉氏黄蚁之间复杂共生关系中的共生体多样性。在其他地方表明，其中三种蚜虫物种的扩散能力较低，且主要为无性繁殖后，我们在此研究了蚁巢丘内和蚁巢丘之间的蚜虫多样性。
三种焦点物种（Geicoa utricularia、Ford marginata 和 Tetraneura ulmi）在种群水平上具有相当大的克隆多样性。然而，超过一半的蚁巢丘只包含一种蚜虫物种，这一比例明显高于随机分布的预期。这些单种蚁巢丘中有超过 60%只有一个蚜虫克隆，而且克隆倾向于在随后的年份中持续存在。只要多种物种/克隆同时出现在同一蚁巢丘中，它们就会在空间上分开，超过 95%的蚜虫室都含有单个克隆的个体。
L. flavus 的“养殖”以每巢较低的蚜虫“牲畜”多样性为特征，特别是在巢室水平上，但缺乏其他养殖共生关系中已知的排他性单培养物。蚂蚁似乎会吃掉大部分早期的蚜虫，因此成年蚜虫不太可能面临有限的韧皮部资源和与其他蚜虫的争夺竞争。我们认为，这种对提供碳水化合物的共生体进行清除以摄取蛋白质的行为可能会在维持每只蚜虫最大宿主产量的同时，使被驯化的蚜虫受益，只要它们的克隆伴侣成功繁殖。这种多培养殖养殖的成本效益逻辑与基于屠宰幼年来最大限度地提高由精心调控的成年牲畜种群产生的牛奶产量的人类养殖实践具有惊人的相似之处。
