A long-standing problem in the study of mutualism is to understand the effects of non-mutualistic community members that exploit the benefits of mutualism without offering commodities in exchange (i.e., ‘exploiters’). Mutualisms are continually challenged by exploiters and their persistence may depend on the costliness of exploitation or on adaptations that allow mutualists to avoid the negative effects of exploiters. Coevolution could lead to changes in mutualists and exploiters that allow mutualisms to persist. Although coevolution is considered essential for mutualism persistence and resistance to disturbance, we have yet to obtain direct experimental evidence of the role of coevolution in resistance to exploitation. Additionally, resistance to exploitation via coevolutionary processes might vary with the degree of dependency between mutualistic partners, as facultative mutualisms are thought to be under weaker coevolutionary selection than obligate mutualisms. Here, we conducted an experimental evolution study using a synthetic yeast mutualism to test how coevolution in facultative and obligate mutualisms affects their resistance to exploitation. We found that naïve facultative mutualisms were more likely to breakdown under exploitation than naïve obligate mutualisms. After 15 weeks of coevolution, both facultative and obligate evolved mutualists were more likely to survive exploitation than naïve mutualists when we reassembled mutualist communities. Additionally, coevolved exploiters were more likely to survive with mutualists, whereas naïve exploiters frequently went extinct. These results suggest that coevolution between mutualists and exploiters can lead to mutualism persistence, potentially explaining why exploitation is ubiquitous but rarely associated with mutualism breakdown.

中文翻译：

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共同进化和依赖性影响共生者对剥削的抵抗

互利共生研究中一个长期存在的问题是理解那些利用互利共生利益而不提供商品交换的非互利社区成员（即“剥削者”）的影响。互利共生不断受到剥削者的挑战，其持久性可能取决于剥削的成本或允许互利共生避免剥削者负面影响的适应。共同进化可能会导致互利共生和剥削者发生变化，从而使互利共生得以持续存在。尽管共同进化被认为对于互利共存和抵抗干扰至关重要，但我们尚未获得共同进化在抵抗剥削中的作用的直接实验证据。此外，通过共同进化过程对剥削的抵制可能会随着互利共生伙伴之间的依赖程度而变化，因为兼性互利共生被认为比强制互利共生处于更弱的共同进化选择之下。在这里，我们使用合成酵母互利共生进行了实验进化研究，以测试兼性和专性互利共生中的共同进化如何影响它们对剥削的抵抗力。我们发现，幼稚的兼性互利共生比幼稚的义务互利共生更容易在剥削下崩溃。经过 15 周的共同进化后，当我们重新组建互利共生群体时，兼性和专性进化的互利共生体比幼稚的互利共生体更有可能在剥削中幸存下来。此外，共同进化的剥削者更有可能与互利共生者一起生存，而幼稚的剥削者经常灭绝。这些结果表明，互利共生者和剥削者之间的共同进化可以导致互利共生的持续存在，这可能解释了为什么剥削无处不在，但很少与互利共生崩溃相关。