Six Diana L, Klepzig Kier D
Department of Ecosystem and Conservation Science, The University of Montana, Missoula, MT, United States.
The Jones Center at Ichauway, Newton, GA, United States.
Front Microbiol. 2021 May 12;12:682187. doi: 10.3389/fmicb.2021.682187. eCollection 2021.
Context dependency occurs when biological interactions shift in sign or magnitude depending upon genetic, abiotic, and biotic context. Most models of mutualism address systems where interaction outcomes slide along a mutualism-antagonism continuum as environmental conditions vary altering cost-benefit relationships. However, these models do not apply to the many mutualisms that involve by-product benefits and others that do not have antagonistic alternate states. The ubiquity of such mutualisms indicates a need for different approaches and models to understand how environmental variability influences their strength, stability, and ecological roles. In this paper, we apply the concept of context dependency to mutualisms among bark beetles and fungi that span a variety of life strategies and exposures to environmental variability. Bark beetles and their mutualist fungi co-construct a niche based on by-product benefits that allows them to exist in a resource that is otherwise intractable or inaccessible. For the closest of these partnerships, this has resulted in some of the most influential agents of forest mortality in conifer forests worldwide. Understanding these symbioses is key to understanding their influence on forest structure and dynamics and responses to change. We found no evidence that bark beetle mutualisms change in sign as conditions vary, only in magnitude, and that the "closest" (and most environmentally influential) of these partnerships have evolved behaviors and mechanisms to reduce context-dependency and stabilize benefit delivery. The bark beetle-fungus symbioses most likely to slide along a mutualism-antagonism continuum are those involving loosely associated facultative symbionts that may provide benefits under some circumstances and that are horizontally transmitted by the beetle host. Additionally, some symbiotic fungi are never mutualists - these "third party" fungi are exploiters and may shift from commensalism to antagonism depending on environmental context. Our assessment indicates that a careful differentiation between bark beetle-fungus partnerships is crucial to understanding how they influence forests and respond to environmental variability.
当生物相互作用根据遗传、非生物和生物背景在正负或强度上发生变化时，就会出现背景依赖性。大多数互利共生模型所涉及的系统中，随着环境条件的变化改变成本效益关系，相互作用的结果会沿着互利共生 - 拮抗连续体滑动。然而，这些模型并不适用于许多涉及附带利益的互利共生关系以及其他不存在拮抗替代状态的关系。此类互利共生关系的普遍存在表明，需要不同的方法和模型来理解环境变异性如何影响它们的强度、稳定性和生态作用。在本文中，我们将背景依赖性的概念应用于树皮甲虫和真菌之间的互利共生关系，这些关系涵盖了各种生活策略以及对环境变异性的暴露情况。树皮甲虫及其互利共生真菌基于附带利益共同构建了一个生态位，使它们能够存在于原本难以处理或无法获取的资源中。对于这些最紧密的伙伴关系而言，这导致了全球针叶林中一些最具影响力的森林死亡因素。理解这些共生关系是理解它们对森林结构和动态以及对变化的响应的关键。我们没有发现证据表明树皮甲虫的互利共生关系会随着条件变化而在正负上发生改变，只是在强度上有所变化，并且这些伙伴关系中“最紧密的”（以及对环境影响最大的）已经进化出行为和机制来减少背景依赖性并稳定利益传递。最有可能沿着互利共生 - 拮抗连续体滑动的树皮甲虫 - 真菌共生关系是那些涉及松散关联的兼性共生体的关系，这些共生体在某些情况下可能提供益处，并且由甲虫宿主水平传播。此外，一些共生真菌从不处于互利共生状态——这些“第三方”真菌是剥削者，可能会根据环境背景从共生转变为拮抗。我们评估表明，仔细区分树皮甲虫 - 真菌伙伴关系对于理解它们如何影响森林以及对环境变异性的响应至关重要。
