Variation In Defensive Traits Research Articles

Variation in defensive traits against herbivores of native and invasive populations of Carpobrotus edulis

Variation in defensive traits against herbivores of native and invasive populations of Carpobrotus edulis

Ecological determinants of variation in phenotypic selection on quantitative immune defence traits

Immune defence is an important determinant of organismal fitness. While theoretical models based on tradeoffs in resource allocation predict quantitative immune traits to be subject to stabilizing selection due to associated energetic costs and self‐harm, empirical studies report mainly positive directional selection. This discrepancy may arise from multiple ecological factors that vary in nature and could influence selection. We examined if selection on immune activity varies depending on immune challenge/infection risk, between immune traits and among populations in the freshwater snail Lymnaea stagnalis. We assessed selection on the phenoloxidase‐like and antibacterial activity of snail haemolymph while manipulating the level of immune challenge imposed by environmental microbes. We did this using snails from multiple populations and also quantified within‐population family‐level variation (i.e. evolutionary potential) in the snails' immune activity. We found that the strength of immune challenge and the examined immune trait determined selection on the snails' immune function. Thus, variation in infection risk can be an important factor in maintaining genetic variation in defence traits. Additionally, immune traits showed low among‐population differentiation but high within‐population genetic variation. This pattern could arise if natural snail populations are exposed to higher temporal than spatial variation in infection risk.

Geographic variation and temporal changes in plant–herbivore interaction: Case studies with Solidago altissima in native and introduced ranges

AbstractPlants are subjected to environmental gradients and may encounter various herbivores, leading to geographic variation in defensive traits. The present review highlights that biological invasions are remarkable natural experiments for studying geographic variation in plant–herbivore interaction and tracking temporal dynamics in plant defense in response to environmental changes. Studies from this viewpoint can challenge various general topics in plant ecology, including the evolution of plant defense and indirect interactions among plants. First, I provide a brief overview on how the introduction of exotic herbivores drives rapid evolution after the establishment of exotic plants and its impacts on native plants. Second, I present a series of case studies investigating the patterns and mechanisms of geographic variation in the interaction between Solidago altissima and Corythucha marmorata (lace bug) in the native range in the United States and the introduced range in Japan. By combining biogeographical and experimental approaches, my collaborators and I unraveled the temporal dynamics of S. altissima's resistance to lace bugs and explored the drivers of differentiation in resistance between native and introduced ranges. These studies provide new insight into the geographic variation in exotic plant–herbivore interaction by unraveling the mechanisms and the temporal scale that cause the variation. These findings are vital not only for predicting invasiveness of exotic plants but also for understanding the evolution of plant–herbivore interaction in community contexts and under climate change.

Observational evidence of herbivore-specific associational effects between neighboring conspecifics in natural, dimorphic populations of Datura wrightii.

Associational effects—in which the vulnerability of a plant to herbivores is influenced by its neighbors—have been widely implicated in mediating plant–herbivore interactions. Studies of associational effects typically focus on interspecific interactions or pest–crop dynamics. However, associational effects may also be important for species with intraspecific variation in defensive traits. In this study, we observed hundreds of Datura wrightii—which exhibits dimorphism in its trichome phenotype—from over 30 dimorphic populations across California. Our aim was to determine whether a relationship existed between the trichome phenotype of neighboring conspecifics and the likelihood of being damaged by four species of herbivorous insects. We visited plants at three timepoints to assess how these effects vary both within and between growing seasons. We hypothesized that the pattern of associational effects would provide rare morphs (i.e., focal plants that are a different morph than their neighbors) with an advantage in the form of reduced herbivory, thereby contributing to the negative frequency‐dependent selection previously documented in this system. We found the best predictor of herbivory/herbivore presence on focal plants was the phenotype of the focal plant. However, we also found some important neighborhood effects. The total number of plants near a focal individual predicted the likelihood and/or magnitude of herbivory by Tupiochoris notatus, Lema daturaphila, and Manduca sexta. We also found that velvety focal plants with primarily sticky neighbors are more susceptible to infestation by Tupiochoris notatus and Lema daturaphila. This does not align with the hypothesis that associational effects at the near‐neighbor scale contribute to a rare‐morph advantage in this system. Overall, the results of our study show that the number and trichome‐morph composition of neighboring conspecifics impact interactions between D. wrightii and insect herbivores.

Negative associations between parasite avoidance, resistance and tolerance predict host health in salmonid fish populations.

Genetic variation in defence against parasite infections is fundamental for host-parasite evolution. The overall level of defence of a host individual or population includes mechanisms that reduce parasite exposure (avoidance), establishment (resistance) or pathogenicity (tolerance). However, how these traits operate and evolve in concert is not well understood. Here, we investigated genetic variation in and associations between avoidance, resistance and tolerance in a natural host-parasite system. Replicated populations of Atlantic salmon (Salmo salar) and sea trout (an anadromous form of brown trout, Salmo trutta) were raised under common garden conditions and infected with the eye fluke Diplostomum pseudospathaceum. We demonstrate significant genetic variation in the defence traits across host populations and negative associations between the traits, with the most resistant populations showing the weakest avoidance and the lowest infection tolerance. These results are suggestive of trade-offs between different components of defence and possibly underlie the genetic variation in defence traits observed in the wild. Because the three defence mechanisms affect host-parasite evolution in profoundly different ways, we emphasize the importance of studying these traits in concert.

Are populations of mayflies living in adjacent fish and fishless streams genetically differentiated?

Summary1. Conspecific populations living in habitats with different risks of predation often show phenotypic variation in defensive traits. Traits of two species of mayflies (Baetidae: Baetis bicaudatus and Baetis sp. nov.) differ between populations living in fish and fishless streams in a high altitude drainage basin in western Colorado, U.S.A. We tested for genetic differentiation between mayfly populations in these two habitat types, assuming that lack of genetic differentiation would be consistent with the hypothesis that those traits are phenotypically plastic.2. Previous work has shown that larvae of both species behave differently and undergo different developmental pathways in adjacent fish and fishless streams. These phenotypic differences in behaviour and development have been induced experimentally, suggesting that populations from fishless streams have the genetic capability to respond to fish.3. During summer 2001 we collected Baetis larvae from several fish and fishless streams, and from fish and fishless sections of the same streams. We used allozymes and a fragment of the cytochrome oxidase subunit 1 mitochondrial gene to examine genetic variation of Baetis individuals within and among streams.4. Results showed that genetic variation exists among populations of the same species of Baetis from different streams, but none of that variation was associated with the presence or absence of fish. These data confirm that populations of Baetis living in fish and fishless streams are not genetically distinct, and are consistent with the hypothesis that traits associated with environments of different risk are phenotypically plastic.

POPULATION VARIATION IN THE COST AND BENEFIT OF TOLERANCE AND RESISTANCE AGAINST HERBIVORY IN DATURA STRAMONIUM

In this study we examine the hypothesis that divergent natural selection produces genetic differentiation among populations in plant defensive strategies (tolerance and resistance) generating adaptive variation in defensive traits against herbivory. Controlled genetic material (paternal half-sib families) from two populations of the annual Datura stramonium genetically differentiated in tolerance and resistance to herbivory were used. This set of paternal half-sib families was planted at both sites of origin and the pattern of genotypic selection acting on tolerance and resistance was determined, as well as the presence and variation in the magnitude of allocational costs of tolerance. Selection analyses support the adaptive differentiation hypothesis. Tolerance was favored at the site with higher average level of tolerance, and resistance was favored at the site with higher average level of resistance. The presence of significant environmentally dependent costs of tolerance was in agreement with site variation in the adaptive value of tolerance. Our results support the expectation that environmentally dependent costs of plant defensive strategies can generate differences among populations in the evolutionary trajectory of defensive traits and promote the existence of a selection mosaic. The pattern of contrasting selection on tolerance suggests that, in some populations of D. stramonium, tolerance may alter the strength of reciprocal coevolution between plant resistance and natural enemies.

POPULATION VARIATION IN THE COST AND BENEFIT OF TOLERANCE AND RESISTANCE AGAINST HERBIVORY IN DATURA STRAMONIUM

In this study we examine the hypothesis that divergent natural selection produces genetic differentiation among populations in plant defensive strategies (tolerance and resistance) generating adaptive variation in defensive traits against herbivory. Controlled genetic material (paternal half-sib families) from two populations of the annual Datura stramonium genetically differentiated in tolerance and resistance to herbivory were used. This set of paternal half-sib families was planted at both sites of origin and the pattern of genotypic selection acting on tolerance and resistance was determined, as well as the presence and variation in the magnitude of allocational costs of tolerance. Selection analyses support the adaptive differentiation hypothesis. Tolerance was favored at the site with higher average level of tolerance, and resistance was favored at the site with higher average level of resistance. The presence of significant environmentally dependent costs of tolerance was in agreement with site variation in the adaptive value of tolerance. Our results support the expectation that environmentally dependent costs of plant defensive strategies can generate differences among populations in the evolutionary trajectory of defensive traits and promote the existence of a selection mosaic. The pattern of contrasting selection on tolerance suggests that, in some populations of D. stramonium, tolerance may alter the strength of reciprocal coevolution between plant resistance and natural enemies.

The Ecological Concept of Costs of Induced Systemic Resistance (ISR)

Plant defence is thought to provide benefits for the defended plants. Theoretical concepts must, therefore, explain why there is variation in defensive traits, which naively might be assumed to be present constitutively in fixed high amounts. Explanations are mainly based on the assumption of fitness costs. Investment in defence is thought to reduce the fitness of plants in enemy-free environments. Fitness costs often result from allocation costs, i.e. allocation of limited resources to defence, which then cannot be used for growth or other fitness-relevant processes. This theoretical concept can provide a useful tool for the interpretation of induced plant responses against pathogens, named induced systemic (or systemic acquired) resistance (ISR or SAR). Phenotypic plasticity, leading to induced responses, might have evolved mainly to reduce costs, since investment in defence is restricted to situations actually requiring defence. ISR can incur allocation costs and other, indirect costs, which ultimately may lead to fitness costs. Evolution of any defensive trait depends on both what a plant ideally 'should do’ and what it actually 'is able to do’. Costs of defence constrain its expression. This might have important influences on the evolution of plant defensive traits, as well as on the exploitation of natural defences in agricultural crop protection.