Functional Ecology ( IF 4.6 ) Pub Date : 2022-09-07 , DOI: 10.1111/1365-2435.14177 Guillaume Minard 1, 2, 3 , Aapo Kahilainen 1, 4 , Arjen Biere 5 , Hannu Pakkanen 6 , Johanna Mappes 1, 7 , Marjo Saastamoinen 1, 8
|
1 INTRODUCTION
In most ecosystems, communities are regulated through a network of trophic interactions (Fretwell, 1987). In these networks, herbivore–plant interactions play a central role and often drastically impact the dynamics of the rest of the community (e.g. bottom-up interactions with predators, parasites or top-down interactions with detritivores), as well as matter fluxes and nutrient cycles (Fretwell, 1987; Metcalfe et al., 2014). Both partners of this interaction are involved in a coevolutionary arms race, in which the plant evolves new traits against herbivores, while in turn, herbivores adapt to these traits to be able to consume the plant (Ehrlich & Raven, 1964). Coevolutionary theory predicts that such forces lead to the diversification of both partners, and symmetric diversification patterns have been observed between plants and herbivores across many empirical data (Futuyma & Agrawal, 2009).
Being sessile, plants are easily exposed to herbivory. Instead, plants have developed an array of chemical and physical traits as defence mechanisms against herbivores that act either constitutively or after induction [reviewed by Aljbory & Chen (2018)]. Many plants produce secondary metabolites that aim to reduce the performance of the herbivores through direct (repellent, toxins) or indirect defence (attract another trophic level such as a predator or a parasitoid of the herbivore). Among compounds involved in direct defence, terpenoids are by far the most diverse and ubiquitous class with more than 30,000 different molecules (Mithöfer & Boland, 2012). They are followed by alkaloids (~12,000 molecules) and phenolic compounds (~9000 molecules). The functions of defensive metabolites are diverse and often poorly characterized. As an example, some of them alter the digestion, induce oxidative stress and cytotoxicity (Bhonwong et al., 2009; Treutter, 2005).
Due to their common dual antiherbivory and antimicrobial activity, recent studies have suggested that in certain cases, the main target of defensive compounds may not be the herbivore itself but instead its gut microbiota (Hammer & Bowers, 2015). Modification of the herbivore's microbiota could then negatively impact the physiology and consequently the performance of the herbivore feeding on the defended host plant (Hammer & Bowers, 2015; Mithöfer & Boland, 2012). Conversely, the gut microbiota may also be involved in the detoxification of defensive compounds, and thus contribute to the adaptation of specialist herbivores to highly defended host plants (Hammer & Bowers, 2015). For example, the microbiota of pine weevils and bark beetles help their host to degrade pine tissues rich in terpenoid resins or phenolic compounds (Berasategui et al., 2017; Cheng et al., 2018). These coleopterans can, thus, only interact efficiently with their host plant when they form a holobiont composed of the insect host and specialized microorganisms.
Similar patterns may also be evident in Lepidoptera. Recent studies have, however, suggested that Lepidoptera harbour a transient and highly variable microbiota with limited impact on host performance and primary metabolism (Duplouy et al., 2020; Hammer et al., 2017; Whitaker et al., 2016). Conversely, few studies have indicated that this relatively transient microbiota can contribute to lepidopteran immunity (Duplouy et al., 2020; Galarza et al., 2021; Mason et al., 2019; Yoon et al., 2019), that some microbes might lead to transgenerational responses and facilitate host plant shifts (Voirol et al., 2020), or in contrast elicit host plant defences (Wang et al., 2017). Interactions regarding potential detoxification of plant defensive compounds, however, remain poorly studied.
In order to further understand the potential relevance of host plant defence–herbivore–microbiota interactions, we used a well-characterized plant–herbivore study system represented by the host plant ribwort plantain (Plantago lanceolata) and its specialist herbivore, the Glanville fritillary (Melitaea cinxia) butterfly. The bioactive metabolites of Plantago lanceolata have previously been investigated (Tamura & Nishibe, 2002) and P. lanceolata is known to synthesize aucubin and catalpol, two monoterpenoids that belong to the class of iridoid glycosides (IGs) that carry antibacterial, antifungal and antipredatory properties (Baden & Dobler, 2009; Davini et al., 1986; Marak et al., 2002; Reudler et al., 2011).
The molecules are not active while formed in the plant and are compartmentalized within the vacuole. Whenever a herbivore or a pathogen degrades the plant cell, the plant will release the IGs that could then react with the β-glucosidases of either the plant, the herbivore or the pathogen to form reactive aglycones (Kim et al., 2000; Pankoke et al., 2013). In another system, it has been demonstrated that the aglycone links proteins through an activated dialdehyde that forms protein adducts and reacts with the side chain of lysine, an essential amino acid (Mander & Liu, 2010). This reaction decreases the amount of lysine available, denatures digestive enzymes and decreases the nutritional value of the food to herbivores and pathogens. The same dialdehyde structure appears after activation of aucubin by β-glucosidases and consistently results in decreased levels of lysine (Kim et al., 2000; Konno et al., 1999).
Some specialist herbivores have, however, adapted to cope with iridoid glycosides, and even store them for their own defence against parasites and predators (Bowers & Puttick, 1986). Previous studies in M. cinxia have shown that pre-diapause larvae contain between 0.2% and 2% of IGs, mostly catalpol (Nieminen et al., 2003; Suomi et al., 2001, 2003). All studies so far have been assessing an entire individual, and thus, it remains unclear whether larvae actively transport and store the IG compounds in specific tissues or whether the compounds are maintained within the gut. However, as 1-day starved larvae as well as adult butterflies (which do not feed anymore on the host plant) contain equivalent levels of IGs, it has been suggested that the compounds are actively sequestered (Nieminen et al., 2003; Suomi et al., 2003). This may contribute to the higher tolerance of larvae to their natural enemies when they are feeding on highly defended plants (Laurentz et al., 2012; Nieminen et al., 2003; Reudler et al., 2011). Studies assessing the impacts of high IG levels in host plants on larval performance vary between no effects to a small increase in weight and development rate (Laurentz et al., 2012; Reudler et al., 2011; Saastamoinen et al., 2007). In addition to IGs, P. lanceolata also produce the high levels of a phenolic compound, the phenylethanoid glycoside verbascoside (alternatively named acteoside) (Tamura & Nishibe, 2002), which antimicrobial and antiherbivory properties remain controversial (Fazly Bazzaz et al., 2018; Holeski et al., 2013; Pardo et al., 1993; Reichardt et al., 1988).
In this study, our first objective was to investigate whether the microbiota of M. cinxia affect the performance of larvae. We selected prediapause larvae since they live as gregarious families on a single host plant (contrarily to postdiapausing larvae) and their condition partially explains their survival probability during diapause (Duplouy et al., 2018; Kahilainen et al., 2018; Kuussaari & Singer, 2017; Tack et al., 2015). Our second objective was to evaluate whether any differences in performance are mediated by effects of the microbiota on the ability of the host larvae to cope with the defences of their host plants. Our third objective was to assess whether the composition of the larval gut microbiome and its effect on larval performance differ among larvae whose parents originated from populations with a different history of host plant use.
In order to investigate the impact of larval microbiota on larval performance and the ability of larvae to cope with IGs, we addressed the following three questions: (1) Do microbiota confer an advantage to their host larvae? For this, we used leaf antibiotic treatments to manipulate the larval microbiome and tested whether larvae feeding on untreated leaves had higher performance (survival, development rate, biomass production) than larvae feeding on antibiotics-treated leaves. (2) Can any advantages conferred by microbiota be related to mitigation of the impact of ingested host IGs? For this, we used selection lines of P. lanceolata, in which plants had been selected for high or low constitutive levels of leaf IGs (high-IG and low-IG lines) (Marak et al., 2000). We assessed the metabolomes (including levels of IGs and lysine) of larvae fed on antibiotics-treated and untreated leaves of high-IG and low-IG plants to test (i) whether larvae fed high-IG lines had higher levels of IGs and reduced levels of lysine, (ii) whether microbiota mitigated a reduction in lysine levels and (iii) whether larvae feeding on untreated high-IG plants had higher performance than larvae of antibiotics-treated high-IG plants. (3) Does the parental origin of the larvae affect the microbiome composition of the larvae and its effect on larval performance and ability to cope with IGs? For this, we used M. cinxia larvae whose parents originated from populations in two distinct regions (Eckerö and Sund) within the Åland archipelago in Finland that differ in their evolutionary history of host use. Populations from Eckerö have a history of encountering lower defence levels due to the presence of two host plant species, Veronica spicata and Plantago lanceolata, that generally contain low and high levels of IGs respectively. By contrast, larvae originating from Sund have a history of encountering only the more highly defended host plant P. lanceolata. We studied the composition of their microbiota and performance after being exposed to contrasted IGs concentrations. We thus specifically tested (i) whether the assembled gut bacterial and fungal microbiome of larvae differs between parental population origin and (ii) whether larvae originating from regions with a stronger evolutionary history with high-IG plants (Sund) benefit more from their associated microbiome when feeding on high-IG plants than larvae from regions with a weaker evolutionary history with high-IG plants (Eckerö).
中文翻译：
复杂的植物质量——微生物群——种群相互作用调节专业草食动物对其宿主植物的防御的反应
1 简介
在大多数生态系统中，群落通过营养相互作用网络进行调节（Fretwell， 1987 ）。在这些网络中，食草动物与植物的相互作用发挥着核心作用，通常会极大地影响群落其他部分的动态（例如与捕食者、寄生虫的自下而上的相互作用或与食腐动物的自上而下的相互作用），以及物质通量和养分循环（Fretwell， 1987 ；Metcalfe 等人， 2014 ）。这种相互作用的双方都参与了共同进化的军备竞赛，其中植物进化出针对食草动物的新特征，而食草动物反过来适应这些特征以便能够食用植物（Ehrlich＆Raven， 1964 ）。共同进化理论预测，这种力量会导致双方的多样化，并且在许多经验数据中观察到植物和食草动物之间的对称多样化模式（Futuyma＆Agrawal， 2009 ）。
由于植物是无柄的，因此很容易受到食草动物的侵害。相反，植物已经形成了一系列化学和物理特征，作为针对草食动物的防御机制，这些机制可以持续发挥作用，也可以在诱导后发挥作用[Aljbory & Chen ( 2018 ) 审查]。许多植物产生次生代谢物，旨在通过直接（驱避剂、毒素）或间接防御（吸引另一个营养级，例如食草动物的捕食者或寄生生物）来降低食草动物的性能。在参与直接防御的化合物中，萜类化合物是迄今为止最多样化、最普遍的一类，拥有超过 30,000 种不同的分子（Mithöfer & Boland， 2012 ）。其次是生物碱（约 12,000 个分子）和酚类化合物（约 9000 个分子）。防御性代谢物的功能多种多样，而且通常很难表征。例如，其中一些会改变消化，诱导氧化应激和细胞毒性（Bhonwong 等， 2009 ；Treutter， 2005 ）。
由于它们常见的双重抗草食和抗菌活性，最近的研究表明，在某些情况下，防御化合物的主要目标可能不是食草动物本身，而是其肠道微生物群（Hammer & Bowers， 2015 ）。食草动物微生物群的改变可能会对生理机能产生负面影响，从而对食草动物以防御宿主植物为食的性能产生负面影响（Hammer & Bowers， 2015 ；Mithöfer & Boland， 2012 ）。相反，肠道微生物群也可能参与防御化合物的解毒，从而有助于专业食草动物适应高度防御的宿主植物（Hammer & Bowers， 2015 ）。例如，松树象鼻虫和树皮甲虫的微生物群帮助宿主降解富含萜类树脂或酚类化合物的松树组织（Berasategui 等， 2017 ；Cheng 等， 2018 ）。因此，这些鞘翅目动物只有在形成由昆虫宿主和特殊微生物组成的全生物体时才能与其宿主植物有效地相互作用。
类似的模式在鳞翅目中也可能很明显。然而，最近的研究表明，鳞翅目动物拥有短暂且高度可变的微生物群，对宿主性能和初级代谢的影响有限（Duplouy 等人， 2020 ；Hammer 等人， 2017 ；Whitaker 等人， 2016 ）。相反，很少有研究表明这种相对短暂的微生物群有助于鳞翅目免疫（Duplouy et al., 2020 ；Galarza et al., 2021 ；Mason et al., 2019 ；Yoon et al., 2019 ），一些微生物可能导致跨代反应并促进宿主植物转变（Voirol 等， 2020 ），或者相反，引发宿主植物防御（Wang 等， 2017 ）。然而，有关植物防御化合物潜在解毒作用的相互作用的研究仍然很少。
为了进一步了解宿主植物防御-草食动物-微生物群相互作用的潜在相关性，我们使用了一个特征良好的植物-草食动物研究系统，以宿主植物车前草（ Plantago lanceolata ）及其专门的食草动物格兰维尔贝母（ Melitaea ）为代表。辛夏）蝴蝶。先前已对车前草的生物活性代谢物进行了研究（Tamura & Nishibe， 2002 ），已知车前子可以合成桃叶珊瑚苷和梓醇，这两种单萜属于环烯醚萜苷 (IG) 类，具有抗菌、抗真菌和抗捕食特性（Baden & Dobler， 2009 ；Davini 等人， 1986 ；Marak 等人， 2002 ；Reudler 等人， 2011 ）。
这些分子在植物中形成时不活跃，并在液泡内划分。每当食草动物或病原体降解植物细胞时，植物就会释放 IG，然后这些 IG 可以与植物、食草动物或病原体的 β-葡萄糖苷酶反应，形成反应性糖苷配基（Kim 等， 2000 ；Pankoke 等）等， 2013 ）。在另一个系统中，已经证明糖苷配基通过活化的二醛连接蛋白质，形成蛋白质加合物并与赖氨酸（一种必需氨基酸）的侧链反应（Mander & Liu， 2010 ）。这种反应会减少可用赖氨酸的量，使消化酶变性，并降低食物对草食动物和病原体的营养价值。桃叶珊瑚苷被 β-葡萄糖苷酶激活后，会出现相同的二醛结构，并持续导致赖氨酸水平下降（Kim 等人， 2000 ；Konno 等人， 1999 ）。
然而，一些专门的食草动物已经适应了环烯醚萜苷，甚至储存它们以防御寄生虫和捕食者（Bowers & Puttick， 1986 ）。先前对M. cinxia的研究表明，滞育前幼虫含有0.2%至2%的IG，主要是梓醇（Nieminen等， 2003 ；Suomi等， 2001，2003 ）。迄今为止，所有研究都是对整个个体进行评估，因此，尚不清楚幼虫是否主动转运并储存 IG 化合物在特定组织中，或者这些化合物是否维持在肠道内。然而，由于 1 天饥饿的幼虫以及成年蝴蝶（不再以寄主植物为食）含有相同水平的 IG，因此有人建议这些化合物被积极隔离（Nieminen 等人， 2003 年；Suomi 等人）等， 2003 ）。这可能有助于幼虫在以高度防御的植物为食时对其天敌具有更高的耐受性（Laurentz 等人， 2012 ；Nieminen 等人， 2003 ；Reudler 等人， 2011 ）。评估寄主植物中高 IG 水平对幼虫性能影响的研究各不相同，从无影响到体重和发育速度小幅增加（Laurentz 等， 2012 ；Reudler 等， 2011 ；Saastamoinen 等， 2007 ）。除了IG之外， P. 披针草还产生高水平的酚类化合物，苯乙醇苷毛蕊花苷（也称为毛蕊花苷）（Tamura & Nishibe， 2002 ），其抗菌和抗草食特性仍然存在争议（Fazly Bazzaz 等， 2018 ；Holeski 等， 2013） ；Pardo 等人， 1993 ；Reichardt 等人， 1988 ）。
在这项研究中，我们的第一个目标是调查M. cinxia的微生物群是否影响幼虫的性能。我们选择滞育前幼虫，因为它们作为群居家庭生活在单一寄主植物上（与滞育后幼虫相反），并且它们的状况部分解释了它们在滞育期间的生存概率（Duplouy 等人， 2018 ；Kahilainen 等人， 2018 ；Kuussaari 和 Singer， 2017 ；塔克等人， 2015 ）。我们的第二个目标是评估性能差异是否是由微生物群对宿主幼虫应对宿主植物防御能力的影响所介导的。我们的第三个目标是评估幼虫肠道微生物组的组成及其对幼虫性能的影响是否存在差异，这些幼虫的父母起源于具有不同寄主植物使用历史的种群。
为了研究幼虫微生物群对幼虫性能和幼虫应对IG的能力的影响，我们解决了以下三个问题：（1）微生物群是否为其宿主幼虫带来了优势？为此，我们使用叶子抗生素处理来操纵幼虫微生物组，并测试以未处理叶子为食的幼虫是否比以抗生素处理叶子为食的幼虫具有更高的性能（存活率、发育率、生物量产生）。 (2) 微生物群赋予的任何优势是否与减轻摄入的宿主 IG 的影响有关？为此，我们使用了P. lanceolata的选择系，其中根据高或低的叶 IG 组成水平（高 IG 和低 IG 系）选择植物（Marak 等， 2000 ）。我们评估了饲喂抗生素处理和未处理的高 IG 和低 IG 植物叶子的幼虫的代谢组（包括 IG 和赖氨酸水平），以测试 (i) 饲喂高 IG 品系的幼虫是否具有较高水平的 IG 和降低的 IG 水平。赖氨酸水平，(ii) 微生物群是否减轻了赖氨酸水平的降低，以及 (iii) 以未经处理的高 IG 植物为食的幼虫是否比经抗生素处理的高 IG 植物的幼虫具有更高的性能。 (3) 幼虫的亲本来源是否影响幼虫的微生物组组成及其对幼虫生产性能和应对IG的能力的影响？为此，我们使用了M. cinxia幼虫，其父母起源于芬兰奥兰群岛内两个不同地区（Eckerö 和 Sund）的种群，这两个地区的宿主使用进化历史不同。 由于存在两种寄主植物物种： Veronica spicata和Plantago lanceolata ，这两种植物通常分别含有低水平和高水平的 IG，因此 Eckerö 的种群历史上遭遇过较低的防御水平。相比之下，源自松德的幼虫历史上只遇到防御能力更强的宿主植物披针叶松。我们研究了暴露于对比 IG 浓度后的微生物群组成和性能。因此，我们专门测试了（i）幼虫组装的肠道细菌和真菌微生物组在亲本群体起源之间是否存在差异，以及（ii）来自具有更强的高IG植物（Sund）进化历史的地区的幼虫是否从其相关微生物组中受益更多当以高 IG 植物为食时，比来自高 IG 植物进化历史较弱的地区的幼虫 (Eckerö) 更有效。
京公网安备 11010802027423号