Laboratoř evoluční ekologie

Laboratory of Evolutionary Ecology

Our laboratory has a broad conceptual focus on trying to understand how extant diversity has been driven through ecological interactions over evolutionary time. We mostly work on model systems comprising tropical plants and their associated insects (herbivores, pollinators and parasitoids), as the majority of described of terrestrial species fall into this group. Our work often includes the use of molecular tools to provide an evolutionary baseline onto which we add layers of complexity in the form of detailed ecological measurements (e.g. plant chemistry data relevant to herbivores or pollinators). We rely on the field collection of samples and work collaboratively with others to achieve our goals, performing a large component of the molecular work in the Department of Ecology. We are currently working on four main themes: 1) evolution of anti-herbivore defences in plants 2) roles of biotic and abiotic factors in the genesis of host-plant chemical diversity 3) elucidating the role of evolutionary relationships and defensive traits in structuring the insect herbivore communities associated with Salix and Ficus, 4) multi-trophic interactions promoting the diversity of arboreal arthropods.

Řešené výzkumné projekty

Why is there such high diversity of chemical defences? The role of insect herbivory in promoting chemical diversity in willows

Background and rationale of proposed research
Over 75% of all macroscopic terrestrial life is a plant or an insect (Price 2002). It has been proposed that reciprocal interactions between the two are one of the main sources of their diversity (Ehrlich and Raven 1964, Janz 2011). The interactions between plants and insects are largely mediated by plant metabolites, which reach astonishing diversity themselves (Richards et al. 2015, Salazar et al. 2016, Sedio et al. 2017, Volf et al. 2018). The bewildering diversity of plant secondary metabolites has been a source of constant scientific attention ever since the isolation of morphine by Friedrich Wilhelm Sertürner over 200 years ago. Here we propose an integrative, phylogenetically controlled study of plant defences to address the question: Why is there such high diversity of chemical defences? Our aim is to unravel the ecological and evolutionary mechanisms by which chemical diversity of plant defences arises from the specificity of insect-plant interactions and the pressure exerted by insect herbivores on coexisting hosts.

Table 1. Used terms and definitions.

 

 

 

 

 

 

Herbivory pressure contributes to the diversity of chemical defences in plants (Becerra 2015). In their seminal paper, Ehrlich & Raven (Ehrlich and Raven 1964) proposed that escalation leads to diversification of plant chemical defences (Table 1). According to this hypothesis, acquiring escalated or diversified chemical defences should allow plants to escape herbivory. However, the costs of herbivory are not universal, and neither are insect responses to defences. The herbivore community composition and interaction specificity matter (Volf et al. 2015a, Segar et al. 2017). Whereas generalists are deterred by host specific and toxic defences, specialists may have higher fitness on hosts rich in specific defensive compounds (Volf et al. 2015a, Volf et al. 2015b, Endara et al. 2017, Volf et al. 2018). Such variable selective pressures result in mixed defence strategies including divergent defences emerging alongside escalating or conserved ones (Becerra 2007, Kursar et al. 2009, Sedio et al. 2017, Volf et al. 2018) (Fig. 1). Divergent traits promote chemical variation between closely related hosts, making them chemically distinct and less likely to share specialized herbivores adapted to a certain combination of host traits (Becerra 2007). There can be an especially strong selection for divergence as a source of chemical variation and diversity in sympatric congeneric plants that would otherwise share their specialized herbivores (Becerra 2007). Relating plant defences to herbivore community composition is a critical step in revealing how divergence and other evolutionary trends contribute to chemical diversity in plants in detail.
Importantly, the evolution of chemical diversity needs to be approached from the perspective of overall defensive strategies and chemical profiles rather than individual traits. For example, selection towards divergence should lead to formation of clusters of species with certain combinations of traits rather than to a total trait overdispersion (Rocha et al. 2018). Empirical evidence shows that plant traits often form defensive syndromes (Agrawal and Fishbein 2006) in order to maintain protection against diverse insect communities. Some specialists can cope with variation in the production of individual, structurally related metabolites (Volf et al. 2015a). However, divergence in defensive syndromes has a potential to change the host’s chemical profile and diversity as a whole, efficiently limiting the overlap in specialist herbivore communities. We thus suggest that divergence in plant defensive syndromes can be one of the major factors affecting insect communities on the one hand and promoting plant chemical diversity on the other.


Fig. 1. Similarity of defences along the phylogeny of Ficus (A), Piper (B), Salix (C), and Macaranga (D) species based on the previous studies and our preliminary results (Richards et al. 2015, Volf et al. 2015a, Volf et al. 2015b, Volf et al. 2018). Left-oriented columns suggest trait dissimilarity (divergence), right-oriented columns suggest trait similarity. Traits with significant deviation in local Moran’s index are in red. We expect a similar divergence in higher categories of plant defensive strategies to further promote disparity between willows.


Furthermore, our previous work suggested that defensive syndromes may consist of traits following various evolutionary trajectories, making them even harder to adapt to by herbivores (Volf et al. 2018).
Apart from syndromes based on constitutive defences, alternative strategies can involve tolerance to herbivory, fast growth or induced defences. There seems to be a trade-off between investment into constitutive and induced defences (Moreira et al. 2014, Pellissier et al. 2016). While this has been studied along environmental gradients (Moreira et al. 2014, Pellissier et al. 2016), we suggest that such trade-offs can exist among sympatric hosts. Some species may simply rely heavily on induced defences, while others may employ mainly constitutive defence. After herbivore attack, plants can upregulate their direct defences targeted at the herbivore, affecting its preference and performance (Agrawal 1999). In addition, plants can employ indirect defences, such as herbivore induced plant volatiles (HIPVs), to attract predators and parasitoids (Amo et al. 2013, Pellissier et al. 2016). Induced defences can be specific to individual herbivores and differ based on their life history (Erb et al. 2012, Danner et al. 2017). Plants facing different herbivores may thus emit different chemical signals. We thus propose that induced defences are an ideal community composition and specificity of insect-plant interactions contribute to chemical diversity in plants. We argue that induced defences represent an additional axis of variation and increase chemical diversity among closely related sympatric hosts, helping them to coexist. We suggest that much defensive chemical diversity among closely related hosts stems from divergence in defensive syndromes and corresponding specificity of insect-plant responses. The production of specific HIPVs is a good example. In order to reveal how plant chemical diversity is generated, studies on the diversification of plant defences based on field observations need to be combined with studies on associated insect communities and manipulative experiments in controlled conditions. Given the interdisciplinary nature of this field, studies from individual plant systems have so far excelled in individual aspects on metabolomics, insect community ecology, variety of explored forms of defence, or evolutionary ecology (Kursar et al. 2009, Richards et al. 2015, Salazar et al. 2016, Endara et al. 2017, Sedio et al. 2017, Volf et al. 2018). Now, novel methods allow for a synthesis using multiple complementary approaches. Only such a combination of complementary approaches can identify how the diversity of defensive secondary metabolites evolves through interactions with insect herbivores.

Objectives and outline of research
Our main objective is to elucidate the evolutionary and ecological mechanisms driving chemical diversification in Salix as an especially diverse and ecologically dominant woody-plant genus. Previous research has focused on tropical plant genera (e.g. Gentry 1982, Whitfeld et al. 2012, Sedio et al. 2017). Temperate plant genera such as Salix (hereafter referred to as willows) may represent even better model systems. For a genus of temperate woody-plants, willows reach astonishing diversity (Skvortsov 1999). They are ecologically dominant in several temperate habitats and often form diverse assemblages of closely related species. They thus represent a good parallel to hyper diverse tropical genera, allowing to test for large-scale evolutionary patterns. They can be easily grown in green-house conditions, allowing for controlled manipulative experiments. Furthermore, they harbour diverse communities of herbivores form several guilds, such as leaf-chewers, galler, miners, or sap-suckers(Volf et al. 2015a, Volf et al. 2015b, Volf et al. 2017). We can build on our previous studies and databases on willows (Volf et al. 2015a, Volf et al. 2015b, Volf et al. 2017). This allows us to select suitable host species and diverse sets of herbivores for manipulative experiments. Using willows as a study system, we aim at bridging the field studies from diverse but poorly known herbivore-plant systems, on the one hand, and experimental studies from model but species poor systems on the other. Our approach will initially involve analysing the broader evolutionary trends (formation of defensive syndromes (Agrawal and Fishbein 2006)) and large scale patterns (comparison of divergent traits between sympatric and allopatric species (Kursar et al. 2009)) before analysing how they arise from bipartite interactions (divergence in the response to individual herbivores) or specificity in plant responses (effect of herbivore traits on the induced defences). By combining advanced metabolomics and in-depth ecological knowledge we can investigate the mechanistic and evolutionary responses of plants to insect herbivores to an unprecedented detail. The core of the project will be a detailed dissection of both constitutive and induced plant defences as elicited by herbivores of different guilds in a phylogenetically controlled context. By studying both constitutive and induced plant defences in the field, we can accurately quantify ‘real world’ responses to insect herbivore attack on a community level. The field work will inform greenhouse experiments aimed at analysing the specificity of plant responses to individual herbivores. This integrative approach will allow us to address several fundamental hypotheses falling into four research modules:
 

Module A: Are closely related sympatric willow species divergent in their defensive syndromes and how does this affect herbivores? Here we will analyse how individual defensive traits, their higher categories (direct chemical defences, direct physical defences, indirect induced defences) and leaf resource acquisition traits (as a proxy for fast growth as a defensive strategy (Agrawal and Fishbein 2006)) contribute to formation of willow defensive syndromes. Our main aim is to demonstrate that the divergence in defensive syndromes, not individual traits, among closely related sympatric hosts is the key driver of their herbivore communities. Furthermore, we will show how exactly the defensive syndromes are composed of individual traits and whether these traits tend to follow similar evolutionary trajectories. To achieve this, we will study a set of 18 sympatric willow species and their herbivores occurring in Central Europe. We expect: A1: Willow defences will form syndromes that will be divergent among closely related willows. A2: Divergence
in defensive syndromes will be a key driver of the community structure of specialized herbivores. A3: Defensive syndromes will consist of conserved, divergent, and escalating traits, making them especially hard for herbivores to adapt to.
 

Module B: Is the composition of willow chemical defences related to the shared pools of herbivores? Here we aim at showing how exposure to the same pool of herbivores promotes divergence and chemical diversity among closely related hosts. In addition to species studied in Module A, we will sample willow species from Minnesota and study their defences and herbivores. First, we will test if the divergence in defences is greater among sympatric species sharing herbivores than among allopatric species. Second, we will test if allopatric species harbouring functionally similar herbivores are similar in their defences, suggesting some universality in the effect of willow defences against herbivores. Finally, we will ask whether the divergence in defences is a major driver of willow chemical diversity. To achieve this, we will “visualize” the metabolome using network analysis and identify its most divergent and diverse parts by quantifying structural diversity of metabolites (Sedio et al. 2018). We expect: B1: Divergence in defences will be higher among sympatric than among allopatric species. B2: Allopatric species with similar composition of herbivore communities will have convergent defences. B3: The divergent parts of the metabolome will show the highest diversity of compounds. This will show how exactly the divergence in defences contributes to overall diversity in plant defences.
 

Module C: Induced defences as a model system: how the divergence arises from bipartite interactions? In the previous modules, we aim at analysing the evolution and divergence in willow defences on large spatial or community scales. Here, we will show how the divergence and defence diversity arises from bipartite interactions. We will focus on induced defences as a suitable study system because they can be quantified as a direct response to the herbivores and show a high degree of specificity. We will select six closely related species of willows from Module A. We will expose them to three species of herbivores with different life-histories, feeding modes, and host preferences and observe their response. We expect: C1: Willow species will show divergent responses to the individual herbivores tested. C2: When comparisons are made between willow species, the similarity in the induced responses will depend more strongly on the defensive syndrome similarity than on their phylogenetic distance. C3: When compared within species, individual willow species will be most induced by the herbivores showing the highest preference for them.
 

Module D: What herbivore traits govern the specificity in plant responses? This question is crucial for understanding how the diversity of plant defences is formed under the diffuse co-evolution with diverse sets of herbivores. We will use S. fragilis, a common species with diverse chemistry (Volf et al. 2015b), and expose it to a broad set of herbivores. We will decompose the variation in the HIPV signals and direct induced defences explained by herbivore feeding guild, specialization, and relatedness. In addition, we will explore if the recovered trends relate to willow defences and insect communities found in Module A. Mainly, we will examine if the willow species naturally harbouring high densities of the herbivores studied in module D produce the expected volatiles under natural conditions. Furthermore, we will compare the β-diversity in HIPVs induced by different herbivores in Module D with intraspecific and interspecific variability from Module A. This will help to establish if the specificity in HIPV signals can significantly contribute to overall chemical diversity in plants. We expect: D1: Herbivore feeding guild will explain the largest share of variation in willow responses. D2: Specialists will induce relatively stronger response in indirect defences than in direct defences which they are able to cope with. D3: Generalists will induce stronger response in direct defences than specialists as direct defences can be relatively more efficient against generalists than against specialists.

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Biologické centrum AV ČR, v.v.i.
Entomologický ústav
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370 05 České Budějovice

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