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

生态专门化是一种有机体比通才适应更窄范围条件的过程，这一过程对全球生物多样性的分布和维持至关重要。然而，支持生态专门化进化的理论未能充分考虑通才生物和专才生物与同一伴侣形成密切联系的情况，以及未能确定导致形成互利关系的生物专门化的生理机制。(i)中的情况在许多树根和土壤传播的“外生菌根”真菌之间发生的广泛和全球重要的互惠关系中很普遍。因此，我们对驱动外生菌根共生的专家和通才进化的因素的理解目前是不充分的。在本提案中，我们将验证植物和外生菌根真菌之间资源转移的差异（从植物到真菌的碳和从真菌到植物的氮和磷）是专业化的关键适应性结果的假设。我们预测，与通才真菌相比，专业外生菌根真菌（即那些通常只与一种寄主植物有关的真菌）从寄主植物接收的给定碳单位向寄主植物转移更多的矿物质营养（即它们具有更有利的“资源交换率”）。我们预测这种资源交换的效率将使它们成为更好的竞争者，然而，我们预计竞争优势只会在有限的利基宽度下出现。例如，当专门型真菌和通才型真菌从属于“专门型”寄主植物的凋落物中争夺资源时，就可能发生这种情况。我们还将验证一个假设，即通用型和专用性外生菌根真菌在单个根系上的空间分布影响植物对碳的获取，以及随后对外部菌丝体的分配，从而影响它们共存的能力。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，植物被寄主专家和通才外生菌根真菌定植，在这个微观世界中，真菌之间的空间分布和竞争范围被操纵。我们还将引入不同的凋落物类型，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伙伴之间的资源交换以及外生菌根真菌相对于矿质养分转移的碳利用效率。这些实验将辅以实地生物分析，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.