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

生态专门化是指有机体比通才适应更窄范围的条件的过程，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：1)充分考虑通才和专家有机体与同一伙伴形成密切联系的情况；2)确定导致有机体专业化形成互惠关系的生理机制。(I)中的情况普遍存在于许多树根和土壤传播的“外生菌根”真菌之间的广泛和全球重要的互惠共生中。因此，我们目前对推动外生菌根共生中专家和多面手进化的因素的了解还不够充分。在这个提案中，我们将检验这样一个假设，即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，以及从真菌到植物的氮和磷)是专业化的关键适宜性结果。我们预测，对于从寄主植物获得的特定单位碳，专业外生菌根真菌，即那些通常只与一种寄主植物相关的真菌，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使它们成为更好的竞争对手，然而，我们预计竞争优势只能在有限的利基市场下才能看到。例如，当专门性真菌和通用性真菌从属于“专门性”寄主植物的枯枝落叶中争夺资源时，可能会出现这种情况。我们还将测试这一假设，即通用和专业外生菌根真菌在单个根系上的空间分布影响从植物获得碳，并随后将其分配给外部菌丝体，从而影响它们共存的能力。为了验证这些假设，我们将建立微型宇宙，在其中植物被寄主专家和通才的外生菌根真菌殖民，在其中真菌之间的空间分布和竞争范围受到操纵。我们还将介绍不同的猫砂类型，以提供不同的适合专家的基质。将使用稳定的和放射性同位素示踪剂来量化伙伴之间的资源交换和外生菌根真菌相对于矿质养分转移的碳利用效率。除了这些实验，还将利用接种了专科和通用型外生菌根真菌的幼苗进行实地生物检测，将这些菌根真菌移植到对专业型和通用型菌根真菌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.