What makes a specialist special? The physiology of ecological specialization in plant-fungal mutualisms

生态专业化是一个生物体比通才适应范围更窄的条件的过程，这一过程对全球生物多样性的分布和维持至关重要。然而，支持生态专业化进化的理论未能充分考虑多面手和专业生物与同一伙伴形成密切联系的情况，以及确定导致生物形成互利关系的专业化的生理机制。（i）中的情况普遍存在于许多树根和土传“外生菌根”真菌之间广泛和全球重要的互惠共生中。因此，我们的理解的因素，驱动专家和通才外生菌根共生体的演变是目前不够的。在这个提议中，我们将测试的假设，即植物和外生菌根真菌之间的资源转移（碳从植物到真菌，氮和磷从真菌到植物）的差异是专业化的关键健身结果。我们预测，专家外生菌根真菌，即那些往往与一个属的宿主植物）转移更多的矿物营养物质的宿主植物为一个给定单位的碳从宿主植物相比，通才（即他们有一个更有利的“资源交换率”）。我们预测这种资源交换的效率使他们成为更好的竞争对手，然而，我们预计竞争优势只会在有限的利基范围内出现。例如，这种情况可能会发生时，专家和通才真菌竞争资源，从垃圾属于'专家'宿主植物。我们还将测试的假设，即空间分布的通才和专业外生菌根真菌对个人根系影响收购的碳从植物，其随后分配到外部菌丝体，从而它们的共存能力。为了验证这些假设，我们将建立一个微观世界，在这个微观世界中，植物被宿主专家和通才外生菌根真菌所殖民，在这个微观世界中，真菌之间的竞争的空间分布和范围被操纵。我们还将引入不同的垫料类型，以提供适合专家的不同基质。稳定和放射性同位素示踪剂将用于量化合作伙伴之间的资源交换和外生菌根真菌相对于矿物营养转移的碳利用效率。这些实验将通过基于实地的生物测定进行补充，使用接种了专家和通才外生菌根真菌的幼苗，将其移植到被认为对专家和通才有利或不利的环境中。

Ecological specialization is the process whereby an organism adapts to a narrower range of conditions than a generalist, and the process is central to the distribution and maintenance of global biodiversity. Yet, theory underpinning the evolution of ecological specialization has failed to i) fully consider situations where both generalist and specialist organisms form close associations with the same partner, and ii) identify the physiological mechanisms that lead to specialization in organisms that form mutually-beneficial relationships. The situation in (i) is prevalent in the widespread and globally-important mutualism that occurs between many tree roots and soil-borne 'ectomycorrhizal' fungi. Thus our understanding of the factors that drive the evolution of specialists and generalists in ectomycorrhizal symbioses is presently inadequate. In this proposal, we will test the hypothesis that differences in resource transfer between plants and ectomycorrhizal fungi (carbon from plants to fungi, and nitrogen and phosphorus from fungi to plants) are key fitness consequence of specialization. We predict that specialist ectomycorrhizal fungi, i.e. those that often associate with just one genus of host plant) transfer more mineral nutrients to host plants for a given unit of carbon received from the host plant compared to generalists (i.e. they have a more favourable 'resource exchange rate'). We predict this efficiency in resource exchange makes them better competitors, however, we expect competitive advantage to be seen only under a limited breadth of niches. For example, such a situation may occur when specialist and generalist fungi are competing for resources from litter belonging to the 'specialist' host plant. We will also test the hypothesis that the spatial distribution of generalist and specialist ectomycorrhizal fungi on individual root systems affects acquisition of carbon from plants, and its subsequent allocation to external mycelium, and thus their ability to coexist. To test these hypotheses, we will establish microcosms in which plants are colonised by host specialist and generalist ectomycorrhizal fungi in which the spatial distribution and scope for competition between the fungi is manipulated. We will also introduce different litter types to provide substrates that differ in suitability for the specialist. Stable and radioisotope tracers will be used to quantify resource exchange between partners and the efficiency of carbon use by ectomycorrhizal fungi relative to mineral nutrient transfer. These experiments will be complemented by field-based bioassays using seedlings inoculated with specialist and generalist ectomycorrhizal fungi, which are transplanted to environments that are deemed either favourable or unfavourable for specialists and generalists.