The evolution of constitutive and induced defences to infectious disease

Evidence for a constitutive cost of host resistance on body fat growth in ewe lambs from lines selected for resistance or susceptibility to experimental infections with Haemonchus contortus

Although benefits of selection for host resistance to gastro-intestinal nematodes have long been recognized, its costs on production traits remain unclear. A main difficulty when studying those costs is to disentangle genetic effects due to selection from plastic responses induced by infection. Putative costs of host resistance have been extensively investigated in growing sheep. However, while most of those studies have relied on live weight to assess body growth, more comprehensive assessments accounting for body composition are advocated to detect trade-offs. In this study we used 90 female lambs from lines divergently selected on resistance to Haemonchus contortus that we experimentally infected (n = 60) or not (n = 30) under controlled conditions. As those conditions were defined to enable uninfected lambs to fully express their growth potential, we sought to precisely identify the effects of selection for host resistance on health traits and on growth traits. We assessed muscular and fat growth based on repeated measurements with dorsal ultrasonography for all lambs on farm, and with whole-body computed tomography (CT) scans for a subgroup of 18 infected lambs. Lambs achieved a high growth rate, including infected ones despite their high worm burden (confirmed at necropsy in the subgroup). As expected, lambs from the resistant (R) line were less infected than those from the susceptible (S) line. However, the clear pathogenic effects observed on muscular growth and voluntary feed intake were similar between lines. In contrast, a line difference in body fat was supported both by dorsal and volumetric CT measurements. Specifically, lower fat in the R line compared with the S line was observed equally in infected and uninfected groups, thus providing evidence for a constitutive cost of host resistance. Although this cost is not necessarily disadvantageous in nutrient-rich environments exposing animals to excess fat deposition, its consequences in nutrient-scarce environments may be important to promote sustainable breeding strategies for host resistance.

Mapping the functional form of the trade-off between infection resistance and reproductive fitness under dysregulated immune signaling

Immune responses benefit organismal fitness by clearing parasites but also exact costs associated with immunopathology and energetic investment. Hosts manage these costs by tightly regulating the induction of immune signaling to curtail excessive responses and restore homeostasis. Despite the theoretical importance of turning off the immune response to mitigate these costs, experimentally connecting variation in the negative regulation of immune responses to organismal fitness remains a frontier in evolutionary immunology. In this study, we used a dose-response approach to manipulate the RNAi-mediated knockdown efficiency of cactus (IκBα), a central regulator of Toll pathway signal transduction in flour beetles (Tribolium castaneum). By titrating cactus activity across four distinct levels, we derived the shape of the relationship between immune response investment and traits associated with host fitness, including infection susceptibility, lifespan, fecundity, body mass, and gut homeostasis. Cactus knock-down increased the overall magnitude of inducible immune responses and delayed their resolution in a dsRNA dose-dependent manner, promoting survival and resistance following bacterial infection. However, these benefits were counterbalanced by dsRNA dose-dependent costs to lifespan, fecundity, body mass, and gut integrity. Our results allowed us to move beyond the qualitative identification of a trade-off between immune investment and fitness to actually derive its functional form. This approach paves the way to quantitatively compare the evolution and impact of distinct regulatory elements on life-history trade-offs and fitness, filling a crucial gap in our conceptual and theoretical models of immune signaling network evolution and the maintenance of natural variation in immune systems.

The evolution of constitutively active humoral immune defenses in Drosophila populations under high parasite pressure

Both constitutive and inducible immune mechanisms are employed by hosts for defense against infection. Constitutive immunity allows for a faster response, but it comes with an associated cost that is always present. This trade-off between speed and fitness costs leads to the theoretical prediction that constitutive immunity will be favored where parasite exposure is frequent. We selected populations of Drosophila melanogaster under high parasite pressure from the parasitoid wasp Leptopilina boulardi. With RNA sequencing, we found the evolution of resistance in these populations was associated with them developing constitutively active humoral immunity, mediated by the larval fat body. Furthermore, these evolved populations were also able to induce gene expression in response to infection to a greater level, which indicates an overall more activated humoral immune response to parasitization. The anti-parasitoid immune response also relies on the JAK/STAT signaling pathway being activated in muscles following infection, and this induced response was only seen in populations that had evolved under high parasite pressure. We found that the cytokine Upd3, which induces this JAK/STAT response, is being expressed by immature lamellocytes. Furthermore, these immune cells became constitutively present when populations evolved resistance, potentially explaining why they gained the ability to activate JAK/STAT signaling. Thus, under intense parasitism, populations evolved resistance by increasing both constitutive and induced immune defenses, and there is likely an interplay between these two forms of immunity.

Activation of immune defences against parasitoid wasps does not underlie the cost of infection

Parasites reduce the fitness of their hosts, and different causes of this damage have fundamentally different consequences for the evolution of immune defences. Damage to the host may result from the parasite directly harming its host, often due to the production of virulence factors that manipulate host physiology. Alternatively, the host may be harmed by the activation of its own immune defences, as these can be energetically demanding or cause self-harm. A well-studied model of the cost of infection is Drosophila melanogaster and its common natural enemy, parasitoid wasps. Infected Drosophila larvae rely on humoral and cellular immune mechanisms to form a capsule around the parasitoid egg and kill it. Infection results in a developmental delay and reduced adult body size. To disentangle the effects of virulence factors and immune defences on these costs, we artificially activated anti-parasitoid immune defences in the absence of virulence factors. Despite immune activation triggering extensive differentiation and proliferation of immune cells together with hyperglycaemia, it did not result in a developmental delay or reduced body size. We conclude that the costs of infection do not result from these aspects of the immune response and may instead result from the parasite directly damaging the host.

Mapping the functional form of the trade-off between infection resistance and reproductive fitness under dysregulated immune signaling

Immune responses benefit organismal fitness by clearing parasites but also exact costs associated with immunopathology and energetic investment. Hosts manage these costs by tightly regulating the induction of immune signaling to curtail excessive responses and restore homeostasis. Despite the theoretical importance of turning off the immune response to mitigate these costs, experimentally connecting variation in the negative regulation of immune responses to organismal fitness remains a frontier in evolutionary immunology. In this study, we used a dose-response approach to manipulate the RNAi-mediated knockdown efficiency of cactus (IκBα), a central regulator of Toll pathway signal transduction in flour beetles (Tribolium castaneum). By titrating cactus activity along a continuous gradient, we derived the shape of the relationship between immune response investment and traits associated with host fitness, including infection susceptibility, lifespan, fecundity, body mass, and gut homeostasis. Cactus knock-down increased the overall magintude of inducible immune responses and delayed their resolution in a dsRNA dose-dependent manner, promoting survival and resistance following bacterial infection. However, these benefits were counterbalanced by dsRNA dose-dependent costs to lifespan, fecundity, body mass, and gut integrity. Our results allowed us to move beyond the qualitative identification of a trade-off between immune investment and fitness to actually derive its functional form. This approach paves the way to quantitatively compare the evolution and impact of distinct regulatory elements on life-history trade-offs and fitness, filling a crucial gap in our conceptual and theoretical models of immune signaling network evolution and the maintenance of natural variation in immune systems.

Exploring the molecular basis of resistance to Botrytis cinerea in chickpea genotypes through biochemical and morphological markers

Chickpea (Cicer arietinum L.) is an important pulse crop around the globe and a valuable source of protein in the human diet. However, it is highly susceptible to various plant pathogens such as fungi, bacteria, and viruses, which can cause significant damage from the seedling phase until harvest, leading to reduced yields and affecting its production. Botrytis cinerea can cause significant damage to chickpea crops, especially under high humidity and moisture conditions. This fungus can cause grey mould disease, which can lead to wilting, stem and pod rot, and reduced yields. Chickpea plants have developed specific barriers to counteract the harmful effects of this fungus. These barriers include biochemical and structural defences. In this study, the defence responses against B. cinerea were measured by the quantification of biochemical metabolites such as antioxidant enzymes, malondialdehyde (MDA), proline, glutathione (GSH), H₂O₂, ascorbic acid (AA) and total phenol in the leaf samples of chickpea genotypes (one accession of wild Cicer species, viz. Cicer pinnatifidum188 identified with high level of resistance to Botrytis grey mould (BGM) and a cultivar, Cicer arietinumPBG5 susceptible to BGM grown in the greenhouse). Seedlings of both the genotypes were inoculated with (1 × 10⁴ spore mL^-1) inoculum of isolate 24, race 510 of B. cinerea and samples were collected after 1, 3, 5, and 7 days post-inoculation (dpi). The enhanced enzymatic activity was observed in the pathogen-inoculated leaf samples as compared to uninoculated (healthy control). Among inoculated genotypes, the resistant one exhibited a significant change in enzymatic activity, total phenolic content, MDA, proline, GSH, H₂O₂, and AA, compared to the susceptible genotype. The study also examined the isozyme pattern of antioxidant enzymes at various stages of B. cinerea inoculation. Results from scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy revealed that BGM had a more significant impact on susceptible genotypes compared to resistant ones when compared to the control (un-inoculated). In addition, SEM and FTIR spectroscopy analyses confirmed the greater severity of BGM on susceptible genotypes compared to their resistant counterparts. Our results suggest the role of antioxidant enzymes and other metabolites as defence tools and biochemical markers to understand compatible and non-compatible plant-pathogen interactions better. The present investigation will assist future plant breeding programs aimed at developing resistant varieties.

Homo medicus: The transition to meat eating increased pathogen pressure and the use of pharmacological plants in Homo

The human lineage transitioned to a more carnivorous niche 2.6 mya and evolved a large body size and slower life history, which likely increased zoonotic pathogen pressure. Evidence for this increase includes increased zoonotic infections in modern hunter-gatherers and bushmeat hunters, exceptionally low stomach pH compared to other primates, and divergence in immune-related genes. These all point to change, and probably intensification, in the infectious disease environment of Homo compared to earlier hominins and other apes. At the same time, the brain, an organ in which immune responses are constrained, began to triple in size. We propose that the combination of increased zoonotic pathogen pressure and the challenges of defending a large brain and body from pathogens in a long-lived mammal, selected for intensification of the plant-based self-medication strategies already in place in apes and other primates. In support, there is evidence of medicinal plant use by hominins in the middle Paleolithic, and all cultures today have sophisticated, plant-based medical systems, add spices to food, and regularly consume psychoactive plant substances that are harmful to helminths and other pathogens. We propose that the computational challenges of discovering effective plant-based treatments, the consequent ability to consume more energy-rich animal foods, and the reduced reliance on energetically-costly immune responses helped select for increased cognitive abilities and unique exchange relationships in Homo. In the story of human evolution, which has long emphasized hunting skills, medical skills had an equal role to play.

Pleiotropy promotes the evolution of inducible immune responses in a model of host-pathogen coevolution

Components of immune systems face significant selective pressure to efficiently use organismal resources, mitigate infection, and resist parasitic manipulation. A theoretically optimal immune defense balances investment in constitutive and inducible immune components depending on the kinds of parasites encountered, but genetic and dynamic constraints can force deviation away from theoretical optima. One such potential constraint is pleiotropy, the phenomenon where a single gene affects multiple phenotypes. Although pleiotropy can prevent or dramatically slow adaptive evolution, it is prevalent in the signaling networks that compose metazoan immune systems. We hypothesized that pleiotropy is maintained in immune signaling networks despite slowed adaptive evolution because it provides some other advantage, such as forcing network evolution to compensate in ways that increase host fitness during infection. To study the effects of pleiotropy on the evolution of immune signaling networks, we used an agent-based modeling approach to evolve a population of host immune systems infected by simultaneously co-evolving parasites. Four kinds of pleiotropic restrictions on evolvability were incorporated into the networks, and their evolutionary outcomes were compared to, and competed against, non-pleiotropic networks. As the networks evolved, we tracked several metrics of immune network complexity, relative investment in inducible and constitutive defenses, and features associated with the winners and losers of competitive simulations. Our results suggest non-pleiotropic networks evolve to deploy highly constitutive immune responses regardless of parasite prevalence, but some implementations of pleiotropy favor the evolution of highly inducible immunity. These inducible pleiotropic networks are no less fit than non-pleiotropic networks and can out-compete non-pleiotropic networks in competitive simulations. These provide a theoretical explanation for the prevalence of pleiotropic genes in immune systems and highlight a mechanism that could facilitate the evolution of inducible immune responses.

Constitutively-expressed and induced immune effectors in the house fly (Musca domestica) and the transcription factors that may regulate them

Insects possess both infection-induced and constitutively expressed innate immune defences. Some effectors, such as lysozymes and antimicrobial peptides (AMPs), are constitutively expressed in flies, but expression patterns vary across tissues and species. The house fly (Musca domestica L.) has an impressive immune repertoire, with more effector genes than any other flies. We used RNA-seq to explore both constitutive and induced expression of immune effectors in flies. House flies were fed either Pseudomonas aeruginosa or Escherichia coli, or sterile control broth, and gene expression in the gut and carcass was analysed 4 h post-feeding. Flies fed either bacterium did not induce AMP expression, but some lysozyme and AMP genes were constitutively expressed. Prior transcriptome data from flies injected with bacteria also were analysed, and these constitutively expressed genes differed from those induced by bacterial injection. Binding sites for the transcription factor Myc were enriched upstream of constitutively expressed AMP genes, while upstream regions of induced AMPs were enriched for NF-κB binding sites resembling those of the Imd-responsive transcription factor Relish. Therefore, we identified at least two expression repertoires for AMPs in the house fly: constitutively expressed genes that may be regulated by Myc, and induced AMPs likely regulated by Relish.

Predation shifts coevolution toward higher host contact rate and parasite virulence

Hosts can avoid parasites (and pathogens) by reducing social contact, but such isolation may carry costs, e.g. increased vulnerability to predators. Thus, many predator-host-parasite systems confront hosts with a trade-off between predation and parasitism. Parasites, meanwhile, evolve higher virulence in response to increased host sociality and consequently, increased multiple infections. How does predation shift coevolution of host behaviour and parasite virulence? What if predators are selective, i.e. predators disproportionately capture the sickest hosts? We answer these questions with an eco-coevolutionary model parametrized for a Trinidadian guppy-Gyrodactylus spp. system. Here, increased predation drives host coevolution of higher grouping, which selects for higher virulence. Additionally, higher predator selectivity drives the contact rate higher and virulence lower. Finally, we show how predation and selectivity can have very different impacts on host density and prevalence depending on whether hosts or parasites evolve, or both. For example, higher predator selectivity led to lower prevalence with no evolution or only parasite evolution but higher prevalence with host evolution or coevolution. These findings inform our understanding of diverse systems in which host behavioural responses to predation may lead to increased prevalence and virulence of parasites.

Metabolism of carbohydrates and activity of the antioxidant system in mosses on a post-technogenic salinized territory

Adaptive physiological and biochemical reactions of mosses Didymodon rigidulus Hedw., Barbula unguiculata Hedw. and Brachythecium campestre (Müll. Hal.) Schimp. to salt stress have been investigated from the territory of the tailings storage of the Stebnyk Mining and Chemical Enterprise “Polymineral” (Lviv region, Ukraine). The peculiarities of carbohydrate metabolism in mosses under salinity conditions have been studied. The content of soluble carbohydrates and proline, the antioxidant activity, the content of ascorbate and reduced glutathione as well as the activity of enzymes of their metabolism – ascorbate peroxidase and glutathione reductase at the initial stages of the stress (salt shock) and prolonged stress exposure (salt stress) have been evaluated. It has been found that the increase of α-amylase activity, enhancement of the hydrolysis of starch and the increase of the concentration of soluble carbohydrates under salt stress were the reactions of the studied species of mosses. It has been established that there was an increase in the concentration of soluble carbohydrates by 1.2–1.5 times in moss shoots under salinity conditions, compared with plants from the background area (vicinity of Stebnyk). Experimental studies have shown that under salinity conditions sucrose dominates in the pool of soluble carbohydrates (59.0–79.5% of the total sugars content). The sucrose content was 1.5–2.0 times higher in the plants B. unguiculata and D. rigidulus from the highly saline area of the tailings storage. It has been indicated that under stress conditions constitutive adaptive mechanisms are more expressed in resistant moss species, and plants with a lower level of resistance adapt to the stressor, mainly due to induced protective systems. Experimental studies have shown that plants B. unguiculata and D. rigidulus, which are resistant to abiotic stressors, have a high constitutive pool of soluble carbohydrates both at the beginning of the experiment and under prolonged exposure of the salt stress. In the shoots of the sensitive moss B. campestre the stress-induced character of the sugars accumulation has been revealed. The accumulation of proline in mosses cells under salt stress depended on their species characteristics. The stress-induced accumulation of proline can be considered as a part of the bryophytes’ protective system, but this osmolyte does not play a key role in the formation of the mosses’ resistance to salt stress. Obviously, soluble carbohydrates are the main osmolytes in the moss cells. It has been found that resistant moss species have a high constitutive antioxidant status, while in the sensitive moss B. campestre the increase in the antioxidant activity occurred during prolonged salt stress, which may indicate its induced nature. It has been shown that the resistant mosses B. unguiculata and D. rigidulus have 3–4 times higher levels of glutathione and ascorbate content and 1.6–2.5 times higher activity of enzymes of their metabolism – glutathione reductase and ascorbate peroxidase, compared to plants of the less tolerant moss species B. campestre, which provided reduction of the lipid peroxidation process in plasma membranes and decreased the content of TBA-active products under stress.

Simultaneous evolution of host resistance and tolerance to parasitism

Tolerance and resistance are two modes of defence mechanisms used by hosts when faced with parasites. Here, we assume tolerance reduces infection-induced mortality rate and resistance reduces the susceptibility of getting infected. Importantly, a negative association between these two strategies has often been found experimentally. We study the simultaneous evolution of resistance and tolerance in a host population where they are related by such a trade-off. Using evolutionary invasion theory, we examine the patterns of optimal investment in each defence strategy, under different ecological scenarios. Our focus is on predicting which of the two strategies is favoured under various epidemiological and ecological conditions. Our key findings surround the impact of recovery and sterility of infected hosts. As the rate at which infected hosts recover from the infection, that is the recovery rate increases, the investment in tolerance increases (resistance decreases) when infected hosts are sterile, but this pattern reverses when infected hosts can reproduce. We further found that a change in the parameter determining the intraspecies competition for resources leading to a reduction in birth rate, that is the crowding factor affects investments in tolerance and resistance only when infected hosts can reproduce. These results emphasize the role of fecundity in driving the evolutionary dynamics of a host. We also find that disease prevalence can increase or decrease depending on whether or not the host evolves: prevalence is highest at low recovery rates when the host does not evolve, but the feedback of a change in tolerance and resistance reverses this pattern, leading to lower prevalence at low recovery rates as host evolves.

Risk-Induced Trait Responses and Non-consumptive Effects in Plants and Animals in Response to Their Invertebrate Herbivore and Parasite Natural Enemies

Predators kill and consume prey, but also scare living prey. Fitness of prey can be reduced by direct killing and consumption, but also by non-consumptive effects (NCEs) if prey show costly risk-induced trait responses (RITRs) to predators, which are meant to reduce predation risk. Recently, similarities between predators and parasites as natural enemies have been recognized, including their potential to cause victim RITRs and NCEs. However, plant-herbivore and animal host-parasite associations might be more comparable as victim-enemy systems in this context than either is to prey-predator systems. This is because plant herbivores and animal parasites are often invertebrate species that are typically smaller than their victims, generally cause lower lethality, and allow for further defensive responses by victims after consumption begins. Invertebrate herbivores can cause diverse RITRs in plants through various means, and animals also exhibit assorted RITRs to increased parasitism risk. This synthesis aims to broadly compare these two enemy-victim systems by highlighting the ways in which plants and animals perceive threat and respond with a range of induced victim trait responses that can provide pre-emptive defense against invertebrate enemies. We also review evidence that RITRs are costly in terms of reducing victim fitness or abundance, demonstrating how work with one victim-enemy system can inform the other with respect to the frequency and magnitude of RITRs and possible NCEs. We particularly highlight gaps in our knowledge about plant and animal host responses to their invertebrate enemies that may guide directions for future research. Comparing how potential plant and animal victims respond pre-emptively to the threat of consumption via RITRs will help to advance our understanding of natural enemy ecology and may have utility for pest and disease control.

The Defense Response Involved in Sweetpotato Resistance to Root-Knot Nematode Meloidogyne incognita: Comparison of Root Transcriptomes of Resistant and Susceptible Sweetpotato Cultivars With Respect to Induced and Constitutive Defense Responses

Sweetpotato (Ipomoea batatas [L.] Lam) is an economically important, nutrient- and pigment-rich root vegetable used as both food and feed. Root-knot nematode (RKN), Meloidogyne incognita, causes major yield losses in sweetpotato and other crops worldwide. The identification of genes and mechanisms responsible for resistance to RKN will facilitate the development of RKN resistant cultivars not only in sweetpotato but also in other crops. In this study, we performed RNA-seq analysis of RKN resistant cultivars (RCs; Danjami, Pungwonmi and Juhwangmi) and susceptible cultivars (SCs; Dahomi, Shinhwangmi and Yulmi) of sweetpotato infected with M. incognita to examine the induced and constitutive defense response-related transcriptional changes. During induced defense, genes related to defense and secondary metabolites were induced in SCs, whereas those related to receptor protein kinase signaling and protein phosphorylation were induced in RCs. In the uninfected control, genes involved in proteolysis and biotic stimuli showed differential expression levels between RCs and SCs during constitutive defense. Additionally, genes related to redox regulation, lipid and cell wall metabolism, protease inhibitor and proteases were putatively identified as RKN defense-related genes. The root transcriptome of SCs was also analyzed under uninfected conditions, and several potential candidate genes were identified. Overall, our data provide key insights into the transcriptional changes in sweetpotato genes that occur during induced and constitutive defense responses against RKN infection.

Constitutive activation of cellular immunity underlies the evolution of resistance to infection in Drosophila

Organisms rely on inducible and constitutive immune defences to combat infection. Constitutive immunity enables a rapid response to infection but may carry a cost for uninfected individuals, leading to the prediction that it will be favoured when infection rates are high. When we exposed populations of Drosophila melanogaster to intense parasitism by the parasitoid wasp Leptopilina boulardi, they evolved resistance by developing a more reactive cellular immune response. Using single-cell RNA sequencing, we found that immune-inducible genes had become constitutively upregulated. This was the result of resistant larvae differentiating precursors of specialized immune cells called lamellocytes that were previously only produced after infection. Therefore, populations evolved resistance by genetically hard-wiring the first steps of an induced immune response to become constitutive.

Tomato Domestication Attenuated Responsiveness to a Beneficial Soil Microbe for Plant Growth Promotion and Induction of Systemic Resistance to Foliar Pathogens

Crop domestication events followed by targeted breeding practices have been pivotal for improvement of desirable traits and to adapt cultivars to local environments. Domestication also resulted in a strong reduction in genetic diversity among modern cultivars compared to their wild relatives, though the effect this could have on tripartite relationships between plants, belowground beneficial microbes and aboveground pathogens remains undetermined. We quantified plant growth performance, basal resistance and induced systemic resistance (ISR) by Trichoderma harzianum, a beneficial soil microbe against Botrytis cinerea, a necrotrophic fungus and Phytophthora infestans, a hemi-biotrophic oomycete, in 25 diverse tomato genotypes. Wild tomato related species, tomato landraces and modern commercial cultivars that were conventionally or organically bred, together, representing a domestication gradient were evaluated. Relationships between basal and ISR, plant physiological status and phenolic compounds were quantified to identify potential mechanisms. Trichoderma enhanced shoot and root biomass and ISR to both pathogens in a genotype specific manner. Moreover, improvements in plant performance in response to Trichoderma gradually decreased along the domestication gradient. Wild relatives and landraces were more responsive to Trichoderma, resulting in greater suppression of foliar pathogens than modern cultivars. Photosynthetic rate and stomatal conductance of some tomato genotypes were improved by Trichoderma treatment whereas leaf nitrogen status of the majority of tomato genotypes were not altered. There was a negative relationship between basal resistance and induced resistance for both diseases, and a positive correlation between Trichoderma-ISR to B. cinerea and enhanced total flavonoid contents. These findings suggest that domestication and breeding practices have altered plant responsiveness to beneficial soil microbes. Further studies are needed to decipher the molecular mechanisms underlying the differential promotion of plant growth and resistance among genotypes, and identify molecular markers to integrate selection for responsiveness into future breeding programs.

The impact of Spodoptera exigua herbivory on Meloidogyne incognita-induced root responses depends on the nematodes' life cycle stages

Induced responses to above-ground and below-ground herbivores may interact via systemic signalling in plants. We investigated whether the impact of above-ground herbivory on root-knot nematode-induced responses depends on the nematode's life cycle stages. Tomato plants were infected with the nematode (Meloidogyne incognita) for 5, 15 or 30 days before receiving Spodoptera exigua caterpillars above-ground. We collected root materials after 24 h of caterpillar feeding. We investigated phytohormones and α-tomatine levels, and the expression of defence and glycoalkaloid metabolism (GAME) marker genes in tomato roots. Nematode infection alone increased the endogenous root levels of jasmonic acid (JA), salicylic acid (SA), abscisic acid (ABA), α-tomatine and the expression of the GLYCOALKALOID METABOLISM 1 (GAME1) gene mostly at 30 days post-nematode inoculation. Caterpillar feeding alone upregulated Lipoxygenase D and downregulated Basic-β-1-glucanase and GAME1 expression in roots. On nematode-infected plants, caterpillar feeding decreased JA levels, but it increased the expression of Leucine aminopeptidase A. The induction patterns of ABA and SA suggest that caterpillars cause cross-talk between the JA-signalling pathway and the SA and ABA pathways. Our results show that caterpillar feeding attenuated the induction of the JA pathway triggered by nematodes, mostly in the nematodes' reproduction stage. These results generate a better understanding of the molecular and chemical mechanisms underlying frequent nematode-plant-caterpillar interactions in natural and agricultural ecosystems.

Young but not defenceless: antifungal activity during embryonic development of a social insect

Termites live in environments heavily colonized by diverse microorganisms, including pathogens. Eggs laid within the nest are likely to experience similar pathogenic pressures as those experienced by older nest-mates. Consequently, eggs may be under selective pressures to be immune-competent. Through in vitro experiments using developing embryos of the dampwood termite, Zootermopsis angusticollis, we tested the ontogeny, location and strength of their antifungal activity against the fungus, Metarhizium brunneum. Exterior washes of the chorion (extra-chorionic) and components within the chorion (intra-chorionic) were incubated with fungal conidia, which were then scored for viability. The fungistatic activity was location and developmental stage dependent. Extra-chorionic washes had relatively weak antifungal activity. Intra-chorionic homogenates were highly antifungal, exhibiting increased potency through development. The positive correlation between intra-chorionic fungistasis and developmental stage is probably due to the expression of endogenous proteins during embryogenesis. Boiling of both the extra-chorionic washes and the intra-chorionic contents rescued conidia viability, indicating the antifungal agent(s) is (are) heat-sensitive and probably proteinaceous. This study is the first to address embryonic antifungal activity in a hemimetabolous, eusocial taxon. Our results support the hypothesis that microbes have been significant agents of selection in termites, fostering the evolution of antifungal properties even in the most immature stage of development.

Stripe rust induced defence mechanisms in the leaves of contrasting barley genotypes (Hordeum vulgare L.) at the seedling stage

Puccinia striiformis f. sp. hordei, the causal organism of stripe rust in barley poses serious threats to its production. The present study examined the seedling response and changes in antioxidant defence system along with NADPH oxidase, hydrogen peroxide, and lipid peroxidation marker-malondialdehyde (MDA) in the four barley genotypes namely Jyoti, RD2900, RD2901, and RD2552 in response to M and G-races of stripe rust pathogen. Disease reaction showed Jyoti as susceptible genotype, RD2901 and RD2552 as resistant, whereas RD2900 behaved differentially to both the races. M-race which is predominant was found to be more virulent than G-race of barley stripe rust pathogen. RD2901 showed an increase in activities of NADPH oxidase, catalase, peroxidase, and enzymes of ascorbate-glutathione pathway along with ascorbate and glutathione pool on inoculation with M-race, which was accompanied by the decrease in hydrogen peroxide and MDA contents. Jyoti, on the other hand, showed an increase in peroxidase and glutathione-S-transferase activities only which were unable to maintain redox homeostasis. The scrutiny of data indicated an increase in ASA/DHA ratio on infection in all the genotypes irrespective of their behaviour towards the races. However, GSH/GSSG ratio significantly declined in Jyoti and increased or remained unaffected in the resistant genotypes which suggested that GSH/GSSG might be playing a vital role in imparting tolerance against stripe rust. Further, correlation studies also revealed that leaf damage was positively correlated with H₂O₂ and MDA contents.

Strategies of host resistance to pathogens in spatially structured populations: An agent-based evaluation

There is growing theoretical evidence that spatial structure can affect the ecological and evolutionary outcomes of host-parasite interactions. Locally restricted interactions have been shown in particular to affect host resistance and tolerance. In this study we investigate the evolution of several types of host disease resistance strategies, alone or in combination, in spatially structured populations. We construct a spatially explicit, individual-based stochastic model where hosts and parasites interact with each other in a spatial lattice, and interactions are restricted to a given neighbourhood of varying size. We investigate several host resistance strategies, including constitutive (expressed in all resistant hosts), induced (expressed only upon infection), and combinations thereof. We show that a costly constitutive resistance cannot reach fixation, whereas an inducible resistance strategy may become fixed in the population if the cost remains low, particularly if it impacts host recovery. We also demonstrate that mixed strategies can be maintained in the host population, and that a higher investment in a recovery-boosting inducible resistance allows for a higher investment in a constitutive response. Our simulations reveal that the spatial structure of the population impacts the selection for resistance in a complex fashion. While single strategies of resistance are generally favoured in less structured populations, mixed strategies can sometimes prevail only in highly structured environments, e.g. when combining constitutive and transmission-blocking induced responses Overall these results shed new light on the dynamics of disease resistance in a spatially-structured host-pathogen system, and advance our theoretical understanding of the evolutionary dynamics of disease resistance, a necessary step to elaborate more efficient and sustainable strategies for disease management.