Predator-specific inducible defenses in the rotifer Keratella tropica

1. A Patagonian strain of Keratella tropica has very different induced morphological responses to two predators – the carnivorous rotifer Asplanchna brightwelli and the interference competitor Daphnia pulex. Asplanchna induces the most exuberant morph. Compared to the basic morph, it has a fourfold longer right posterolateral spine (up to c. 115 μm), greatly elongated anterolateral and anterosubmedian spines, and no left posterolateral spine. Transitional morphs have an incompletely developed right posterolateral spine and a reduced left posterolateral spine. Daphnia induces moderate development of both posterior spines but no elongation of any anterior spines. Induction of these morphs by Asplanchna and Daphnia is mediated by kairomones. 2. The Asplanchna‐induced morph is much better defended against large (0.9 mm) Asplanchna than either the basic or Daphnia‐induced morph. The long right posterolateral spine usually prevents capture or ingestion. The Asplanchna‐ and Daphnia‐induced morphs are similarly susceptible to interference from large (3 mm) D. pulex. 3. Life‐table experiments with cohorts of the basic and Asplanchna‐induced morphs at 5 × 10³ and 2 × 10⁴ cells of Cryptomoas erosa per millilitre indicate little or no cost of the induced defense. Lifetime fecundity (13–15 offspring per female) did not differ significantly between morphs. The mean intrinsic rate of natural increase (r_m day⁻¹) of the induced morph was very slightly but significantly lower than that of the basic morph at the lower food concentration (0.46 versus 0.48) but no different from it at the higher food concentration (0.53 versus 0.54). However, spine development may involve undetermined allocation costs and environmental costs relating to interactions with other organisms. 4. It is not clear why K. tropica has separate induced responses to Asplanchna and Daphnia. Moderate spine development probably reduces damage or ingestion by small (<1.5 mm) daphniids, as in other species of Keratella, but further development may confer no protection against larger ones. Thus, the ratio of benefit to cost with daphniids (and other cladocerans) may be highest for intermediate spine development. In contrast, much greater spine development seems necessary for effective defense against Asplanchna. The more moderate response to Daphnia also may reflect less likely spatial and temporal overlap.     

The heritability of inducible defenses in tadpoles

The evolution of plastic traits requires phenotypic trade-offs and heritable traits, yet the latter requirement has received little attention, especially for predator-induced traits. Using a half-sib design, I examined the narrow-sense heritability of predator-induced behaviour, morphology, and life history in larval wood frogs (Rana sylvatica). Many of the traits had significant additive genetic variation in predator (caged Anax longipes) and no-predator environments. Whereas most traits had moderate to high heritability across environments, tail depth exhibited high heritability with predators but low heritability without predators. In addition, several traits had significant heritability for plasticity, suggesting a potential for selection to act on plasticity per se. Genetic correlations confirmed known phenotypic relationships across environments and identified novel relationships within each environment. This appears to be the first investigation of narrow-sense heritabilities for predator-induced traits and confirms that inducible traits previously shown to be under selection also have a genetic basis and should be capable of exhibiting evolutionary responses.     

Qualitative effects of inducible defenses in trophic chains

By means of qualitative techniques we analyze the consequences of inducible defenses of species embedded in trophic chains on the community stability and responses of population equilibrium densities to press perturbations. Our results show that the inclusion of inducible defenses in trophic chains leads to profound changes in system dynamics. Inducible defenses increase the likelihood of instability, especially when exhibited by species of lower trophic levels. We obtained biologically reasonable feedback conditions that must be satisfied to ensure stability. Species responses to press perturbation are modified by inducible defenses and their associated costs in multiple ways. Many of the direct effects in the community are reinforced, while indirect effects are either weakened, if they propagate in a top–down direction, or are unaffected if they propagate from basal species. The dominant view of inducible defenses as a stabilizing force seems to be valid only within a biologically constrained parameter space.     

Embryological aspects of inducible morphological defenses in Daphnia

Many cases of predator-induced morphological plasticity in daphnids are well studied examples of inducible defenses. However, little is known about the early development of these sometimes conspicuous traits. We compared for the first time in five different Daphnia species the embryonic development of predator-induced and noninduced animals using scanning electron microscopy (SEM). We observed significant morphological changes in the last embryonic stage in helmet formation in Daphnia cucullata and in neck-pedestal development in Daphnia pulex. In contrast, no morphological changes could be found during embryogenesis between induced and noninduced Daphnia lumholtzi, D. longicephala, and D. ambigua. Strategies for initiating the defensive traits differ among Daphnia species because of trade-offs between environmental requirements and developmental constraints. Some general features of Daphnia embryonic development are described using SEM. All Daphnia embryos have to shed at least three different membranes before leaving the brood pouch of the mother. After the embryos shed the third membrane, chemosensillae that are likely able to detect predator-released chemicals are exposed to the olfactory environment.     

The ecology and evolution of inducible defenses

Inducible defenses are responses activated through a previous encounter with a consumer or competitor that confer some degree of resistance to subsequent attacks. While the importance of inducible resistance has long been known in host-parasite interactions, it is only recently that its importance has emerged in other natural systems. Although the structural defenses produced by invertebrates to their competitors and predators are by no means the same as an immune response triggered by parasites, these responses all share the properties of (1) specificity, (2) amplification and (3) memory. This review discusses the following ecological consequences and evolutionary causes of inducible defenses: (1) Inducible defenses render historical factors important in biological interactions and can affect the probability of individual survival and growth, as well as affect population dynamics of consumers in some circumstances. (2) Although the benefits of inducible defenses are often balanced by fitness costs, including reduced growth, reproductive output and survivorship, the role of costs and benefits in the evolution of inducible defenses is by no means clear. A more integrated approach would involve a multivariate analysis of the role of natural selection on the inducible characters of interest, their norms of reaction and correlated fitness characters. (3) The disproportionate representation of inducible, morphological defenses among clonal organisms may be due to both a higher rate of origination and enhanced selection to maintain these defenses in clonal taxa. (4) Inducible defenses should be most common when reliable cues are available, attacks by biological agents are unpredictable, and the fitness gains of defenses are balanced by the costs. An integrated approach to studying inducible defenses would thus combine mechanistic estimates of costs, population-level estimates of defense effectiveness, and genetic estimates of correlations between fitness and inducible characters. This will allow us to estimate rates of evolution in these phenotypically plastic threshold characters.     

Parasitoid load affects plant fitness in a tritrophic system

Plants attacked by herbivorous insects emit volatile compounds that attract predators or parasitoids of the herbivores. Plant fitness increases when these herbivorous insects are parasitized by solitary parasitoids, but whether gregarious koinobiont parasitoids also confer a benefit to plant fitness has been disputed. We investigated the relationship between parasitoid load of the gregarious Cotesia glomerata (L.) (Hymenoptera: Braconidae), food consumption by larvae of their host Pieris brassicae L. (Lepidoptera: Pieridae), and seed production in a host plant, Brassica nigra L. (Brassicaceae), in a greenhouse experiment. Plants damaged by caterpillars containing single parasitoid broods produced a similar amount of seeds as undamaged control plants and produced significantly more seeds than plants with unparasitized caterpillars feeding on them. Increasing the parasitoid load to levels likely resulting from superparasitization, feeding by parasitized caterpillars was significantly negatively correlated with plant seed production. Higher parasitoid brood sizes were negatively correlated with pupal weight of Cotesia glomerata , revealing scramble competition leading to a fitness trade-off for the parasitoid. Our results suggest that in this tritrophic system plant fitness is higher when the gregarious parasitoid deposits a single brood into its herbivorous host. A prediction following from these results is that plants benefit from recruiting parasitoids when superparasitization is prevented. This is supported by our previous results on down-regulation of synomone production when Brassica oleracea was fed on by parasitized caterpillars of P. brassicae . We conclude that variable parasitoid loads in gregarious koinobiont parasitoids largely explain existing controversies about the putative benefit of recruiting these parasitoids for plant reproduction.     

Predator-specific inducible morphological defenses of a water flea against two freshwater predators

The expression of inducible morphological defenses in Daphnia in response to a single predator is a well-known phenomenon. However, predator-specific modifications of the same defensive traits as an adaption to different predator regimes is so far only described for Daphnia barbata. It is unknown if this accounts only for this species or if it is a more widespread, general adaptive response in the genus Daphnia. In the present study, we therefore investigated whether a clone of the pond-dwelling species Daphnia similis responds to different predatory invertebrates (Triops cancriformis; Notonecta maculata) with the expression of predator-specific modifications of the same defensive traits. We showed that Triops-exposed individuals express a significantly longer tail-spine, while body width decreased in comparison to control individuals. Additionally, they also expressed inconspicuous defenses, that is, significantly longer spinules on the dorsal ridge. The Notonecta-exposed D. similis showed a significantly longer tail-spine, longer spinules and a larger spinules bearing area on the dorsal ridge than control individuals as well. However, a geometric morphometric analysis of the head shape revealed significant, predator-specific changes. Triops-exposed individuals expressed a flattened head shape with a pronounced dorsal edge, while Notonecta-exposed individuals developed a high and strongly rounded head. Our study describes so far unrecognized inducible defenses of D. similis against two predators in temporary waters. Furthermore, the predator-dependent change in head shape is in concordance with the ‘concept of modality’, which highlights the qualitative aspect of natural selection caused by predators.     

Cry-wolf signals emerging from coevolutionary feedbacks in a tritrophic system

For a communication system to be stable, senders should convey honest information. Providing dishonest information, however, can be advantageous to senders, which imposes a constraint on the evolution of communication systems. Beyond single populations and bitrophic systems, one may ask whether stable communication systems can evolve in multitrophic systems. Consider cross-species signalling where herbivore-induced plant volatiles (HIPVs) attract predators to reduce the damage from arthropod herbivores. Such plant signals may be honest and help predators to identify profitable prey/plant types via HIPV composition and to assess prey density via the amount of HIPVs. There could be selection for dishonest signals that attract predators for protection from possible future herbivory. Recently, we described a case in which plants release a fixed, high amount of HIPVs independent of herbivore load, adopting what we labelled a ‘cry-wolf’ strategy. To understand when such signals evolve, we model coevolutionary interactions between plants, herbivores and predators, and show that both ‘honest’ and ‘cry-wolf’ types can emerge, depending on the assumed plant–herbivore encounter rates and herbivore population density. It is suggested that the ‘cry-wolf’ strategy may have evolved to reduce the risk of heavy damage in the future. Our model suggests that eco-evolutionary feedback loops involving a third species may have important consequences for the stability of this outcome.     

Modality matters for the expression of inducible defenses: introducing a concept of predator modality

Background

Inducible defenses are a common and widespread form of phenotypic plasticity. A fundamental factor driving their evolution is an unpredictable and heterogeneous predation pressure. This heterogeneity is often used synonymously to quantitative changes in predation risk, depending on the abundance and impact of predators. However, differences in ‘modality’, that is, the qualitative aspect of natural selection caused by predators, can also cause heterogeneity. For instance, predators of the small planktonic crustacean Daphnia have been divided into two functional groups of predators: vertebrates and invertebrates. Predators of both groups are known to cause different defenses, yet predators of the same group are considered to cause similar responses. In our study we question that thought and address the issue of how multiple predators affect the expression and evolution of inducible defenses.

Results

We exposed D. barbata to chemical cues released by Triops cancriformis and Notonecta glauca, respectively. We found for the first time that two invertebrate predators induce different shapes of the same morphological defensive traits in Daphnia, rather than showing gradual or opposing reaction norms. Additionally, we investigated the adaptive value of those defenses in direct predation trials, pairing each morphotype (non-induced, Triops-induced, Notonecta-induced) against the other two and exposed them to one of the two predators. Interestingly, against Triops, both induced morphotypes offered equal protection. To explain this paradox we introduce a ‘concept of modality’ in multipredator regimes. Our concept categorizes two-predator-prey systems into three major groups (functionally equivalent, functionally inverse and functionally diverse). Furthermore, the concept includes optimal responses and costs of maladaptions of prey phenotypes in environments where both predators co-occur or where they alternate.

Conclusion

With D. barbata, we introduce a new multipredator-prey system with a wide array of morphological inducible defenses. Based on a ‘concept of modality’, we give possible explanations how evolution can favor specialized defenses over a general defense. Additionally, our concept not only helps to classify different multipredator-systems, but also stresses the significance of costs of phenotype-environment mismatching in addition to classic ‘costs of plasticity’. With that, we suggest that ‘modality’ matters as an important factor in understanding and explaining the evolution of inducible defenses.

     

Nutrient dynamics in a tritrophic system of ants,aphids and beans

The interactions between ants (Lasius niger), aphids (Aphis fabae) and plants (Phaseolus vulgaris) were studied in a laboratory experiment with the following treatments: application of N‐fertilizer for plants, supply of mealworms or sugar solution or their combination as alternative food sources for ants. Three main questions were studied: (1) Do ants reduce tending to honeydew‐producing aphids when an alternative sugar or protein resource was available? (2) Is aphid predation/protein consumption by ants higher when additional carbon is offered to maintain the carbon/protein balance? (3) Does fertilizer treatment propagate in the food web? For the experimental analysis stable isotope techniques were applied. δ¹⁵N served as a marker for the pathway from plants to higher trophic levels. Low δ¹⁵N‐value of fertilizer spread from plant shoots to aphids and ants. To trace which sugar‐/protein source was consumed by ants, the different ¹³C/¹²C‐ratios of C₃‐ and C₄‐plants were used with aphids feeding on C₃‐plant material, while mealworm food and sugar solution originated from C₄‐plant material. Fertilizer application had no effect on biomasses of plants, consumers or microflora. Ant biomass was significantly higher when additional sugar solution was offered. Higher contents of ¹³C indicated a high incorporation rate of additional sugar. Additional protein had no effect on colony biomass and no increasing predation on aphids could be observed when carbon was in excess. However, due to the lack of queens and newly produced larvae, protein requirements of experimental colonies were lower than in natural systems. Ants positively affected aphid populations, but reduced tending, whilst having access to an alternative sugar resource. When sugar/protein was offered to ants, the host plant had an increased root/shoot ratio. This indicates that decreasing aboveground activity of ants could lead to reduced plant growth of aphid‐infested plants, presumably due to higher fungal attack on shoots.     

The effects of calcium in mediating the inducible morphological defenses of a freshwater snail,Physa acuta

Predation risk is known to induce numerous, often adaptive, plastic changes in the morphology, life history, and behavior of prey. However, little is known about how abiotic factors such as environmental nutrient availability affect these responses. In this experiment, a freshwater pulmonate snail (Physa acuta) was used to determine how predator-induced shell morphology was affected by the availability of an abiotic nutrient important for shell growth (calcium). Each snail was reared individually in a factorial experiment consisting of two predator treatments (chemical cues from crayfish present or absent) and three calcium concentrations (20, 45, or 70 mg/L). Calcium induced larger, heavier, and thicker shells, while predator cues induced heavier and thicker shells. Importantly, the magnitude of the predator effect on shell thickness (i.e., the inducible defense) increased with additional calcium availability. Our results demonstrate the importance of nutrient availability in mediating and limiting the amount of adaptive predator-induced plasticity.     

Understanding interactive inducible defenses of Daphnia and its phytoplankton prey

Cyanobacterial and zooplankton inducible defenses are important but understudied process that regulate the trophic interactions of freshwater ecosystem. Daphnia due to its large size is considered an important zooplankton with the high potential to control cyanobacterial blooms. It has been shown that Daphnia through maternal induction transfer tolerance to their next generation against Microcystis toxicity. Maternal induction has been investigated in different Daphnia species without considering phenotypic plasticity of prey. Laboratory experiments were performed to explore cyanobacteria-Daphnia inducible defenses in order to better understand their interactions. Two Daphnia species were fed either with Microcystis aeruginosa PCC7806 (Ma) or Microcystis flos-aquae (Mf) mixed with Chlorella vulgaris (Cv) (exposed Daphnia), and or pure Cv (unexposed Daphnia). Exposed prey cultures were produced by prior exposure to Daphnia infochemicals. Neonates produced by exposed and unexposed Daphnia were fed with mixed diet (Microcystis + Cv) of either exposed and or unexposed prey. Growth parameters and toxin production of exposed prey cultures were significantly different than that of control. Exposed Daphnia fecundity and survival was higher as compared to unexposed Daphnia. Growth and reproduction was reduced in exposed Daphnia when fed with exposed prey as compared to those fed with unexposed prey. This study provides information on the interactive inducible defenses between cyanobacteria and its grazer under laboratory conditions and may increase our understanding of cyanobacteria and Daphnia interactions in the freshwater ecosystem.     

Fungal entomopathogens in a tritrophic context

Variation in plant quality has an important impact on insect growth and development and there is considerable evidence that plants can also influence an insect’s natural enemies. Here we discuss the potential for plant-mediated effects on fungal entomopathogens. Fungi differ from other insect pathogens in that they infect an insect directly through its cuticle. This means that they are particularly vulnerable to changes in microclimate and properties of the insect cuticle. Potential direct and indirect mechanisms for plant-mediated effects on fungal entomopathogens are discussed. It is clear from these studies that fungal entomopathogens could be affected by plant volatiles and plant surface chemistry. Plant secondary chemicals can also inhibit fungal growth, potentially protecting the insect herbivore. However, the site of action and the mechanism behind these effects in plant-based studies is not always clear. The implications for biocontrol using fungal entomopathogens are discussed.     

Natal origin affects host preference and larval performance relationships in a tritrophic system

Many insects face the challenge to select oviposition sites in heterogeneous environments where biotic and abiotic factors can change over time. One way to deal with this complexity is to use sensory experiences made during developmental stages to locate similar habitats or hosts in which larval development can be maximized. While various studies have investigated oviposition preference and larval performance relationships in insects, they have largely overlooked that sensory experiences made during the larval stage can affect such relationships. We addressed this issue by determining the role of natal experience on oviposition preference and larval performance relationships in a tritrophic system consisting of Galerucella sagittariae, feeding on the two host plants Potentilla palustris and Lysimachia thyrsiflora, and its larval parasitoid Asecodes lucens. We firstly determined whether differences in host‐derived olfactory information could lead to divergent host selection, and secondly, whether host preference could result in higher larval performance based on the natal origin of the insects. Our results showed that the natal origin and the quality of the current host are both important aspects in oviposition preference and larval performance relationships. While we found a positive relationship between preference and performance for natal Lysimachia beetles, natal Potentilla larvae showed no such relationship and developed better on L. thyrsiflora. Additionally, the host selection by the parasitoid was mainly affected by the natal origin, while its performance was higher on Lysimachia larvae. With this study, we showed that the relationship between oviposition preference and larval performance depends on the interplay between the natal origin of the female and the quality of the current host. However, without incorporating the full tritrophic context of these interactions, their implication in insect fitness and potential adaptation cannot be fully understood.     

Priming of inducible defenses protects Norway spruce against tree-killing bark beetles

Plants can form an immunological memory known as defense priming, whereby exposure to a priming stimulus enables quicker or stronger response to subsequent attack by pests and pathogens. Such priming of inducible defenses provides increased protection and reduces allocation costs of defense. Defense priming has been widely studied for short-lived model plants such as Arabidopsis, but little is known about this phenomenon in long-lived plants like spruce. We compared the effects of pretreatment with sublethal fungal inoculations or application of the phytohormone methyl jasmonate (MeJA) on the resistance of 48-year-old Norway spruce (Picea abies) trees to mass attack by a tree-killing bark beetle beginning 35 days later. Bark beetles heavily infested and killed untreated trees but largely avoided fungus-inoculated trees and MeJA-treated trees. Quantification of defensive terpenes at the time of bark beetle attack showed fungal inoculation induced 91-fold higher terpene concentrations compared with untreated trees, whereas application of MeJA did not significantly increase terpenes. These results indicate that resistance in fungus-inoculated trees is a result of direct induction of defenses, whereas resistance in MeJA-treated trees is due to defense priming. This work extends our knowledge of defense priming from model plants to an ecologically important tree species.     

Poor phenotypic integration of blue mussel inducible defenses in environments with multiple predators

Aquatic prey encounter an array of threat cues from multiple predators and killed conspecifics, yet the vast majority of induced defenses are investigated using cues from single predator species. In most cases, it is unclear if odors from multiple predators will disrupt defenses observed in single-predator induction experiments. We experimentally compared the inducible defenses of the common marine mussel Mytilus edulis to waterborne odor from pairwise combinations of three predators representing two attack strategies. Predators included the sea star, Asterias vulgaris (= Asterias rubens ), and the crabs Carcinus maenas and Cancer irroratus . The mussels increased adductor muscle mass in response to cues from unfed Asterias (a predatory seastar that pulls mussel shells open) and increased shell thickness in response to unfed Carcinus, a predatory crab that crushes or peels shells. However, the mussels did not express either predator specific response when exposed to the combined cues of Asterias and Carcinus , and mussels did not increase shell thickness when exposed to cues from Cancer alone or any pairwise combination of the three predators. Shell closure or 'clamming up' did not occur in response to any predator combination. These results suggest that predator-specific responses to the Asterias and Carcinus are poorly integrated and cannot be expressed simultaneously. Simultaneous cues from multiple predators affect the integration of predator specific defenses and predator odors from functionally similar predators do not necessarily initiate similar defenses. Ultimately, the degree that prey can integrate potentially disparate defenses in a multiple predator environment may have ecological ramifications and represent a seldom explored facet of the evolution of inducible defenses.     

Indirect plant-mediated effects on insect immunity and disease resistance in a tritrophic system

Host plant quality can significantly influence the growth and condition of phytophagous insects, and consequently their susceptibility to pathogens. This study examined the relationship between host plant quality, insect condition, immune responsiveness and resistance to pathogens in the cabbage looper, Trichoplusia ni. Two baseline and induced immune parameters were estimated, haemocyte numbers and haemolymph phenoloxidase (PO) activity, for larvae on two host plants, broccoli and cucumber. Haemolymph protein concentration was assessed as an indication of insect condition, and the susceptibility of larvae to T. ni single nucleopolyhedrovirus (SNPV) was used as a measure of disease resistance. T. ni growth, survival and condition was much higher on broccoli than cucumber. Haemocyte numbers were significantly higher in broccoli-reared larvae, whereas PO activity was not. An immune challenge induced significantly elevated numbers of haemocytes for larvae reared on both host plants, but did not affect PO activity or protein concentrations. Susceptibility to T. ni SNPV was markedly higher in larvae reared on cucumber than on broccoli. These results clearly indicate that host plant quality can affect both immune response and disease resistance of T. ni larvae and that bottom-up effects could be important in interactions between insects and entomopathogens.     

Parasitism on seed predators overcomes the detrimental effects of defoliation on plant fitness in a tritrophic system

It is widely recognised that the interaction between plants and herbivores cannot be completely understood if the natural enemies of the latter are not included. Most studies looking at the effects of herbivores and their enemies on plant fitness only consider one herbivore species or guild; however, plants in nature usually face the attack of more than one herbivore guild simultaneously and these herbivores may have a non-additive effect on the attraction with bodyguards and plant fitness. In this study, we asked whether folivory affects the activity of parasitoids on seed predators and whether this effect cascades down to plant fitness. We assessed these questions in a tritrophic system: the plant Ruellia nudiflora, its pre-dispersal seed predators and the parasitoids of the latter. Plants were submitted to either 50 % artificial defoliation or no defoliation (control). The number of seeds, fruit production and parasitoid incidence was assessed periodically in both sets of plants. Parasitoids indirectly and positively affected seed number, while defoliation had a direct negative effect on the number of seeds and an indirect negative effect on parasitoid incidence. However, the combined effect of defoliation and seed predation increased the indirect positive effect of the parasitoids on seed production, which overcame the negative effects of defoliation.     

Population dynamics of prey exhibiting inducible defenses: the role of associated costs and density-dependence

The effect of antipredator behavior on the dynamics of a resource-consumer model was analyzed in relation to the magnitude of associated costs, and the strength of density-dependence. For this purpose, I present a deterministic continuous resource-consumer model that exhibits biomass conversion, structural homogeneity, and competition for renewable and fixed resources as separate processes. Antipredator behavior is incorporated as an inducible response to consumer density, and has metabolic and feeding costs. By means of numerical methods, I show: (1) that antipredator behavior is stabilizing for certain parameter ranges, where other stabilizing forces do not dominate the dynamics; (2) intraspecific competition for both fixed and renewable resources have a stabilizing role; (3) metabolic cost is always stabilizing, and feeding cost can be stabilizing or destabilizing, depending on the relative strength of the two competition forces.