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Host-parasite interaction; Apis mellifera; Nosema ceranae; Tolerance; Proteome; Coevolution; Host manipulation is a common strategy by parasites to reduce host defense responses, enhance development, host exploitation, reproduction and, ultimately, transmission success. As these parasitic modifications can reduce host fitness, increased selection pressure may result in reciprocal adaptations of the host. Whereas the majority of studies on host manipulation have explored resistance against parasites (i.e. ability to prevent or limit an infection), data describing tolerance mechanisms (i.e. ability to limit harm of an infection) are scarce. By comparing differential protein abundance, we provide evidence of host-parasite interactions in the midgut proteomes of N. ceranae-infected and uninfected honey bees from both Nosema-tolerant and Nosema-sensitive lineages. We identified 16 proteins out of 661 protein spots that were differentially abundant between experimental groups. In general, infections of Nosema resulted in an up-regulation of the bee's energy metabolism. Additionally, we identified 8 proteins that were differentially abundant between tolerant and sensitive honey bees regardless of the Nosema infection. Those proteins were linked to metabolism, response to oxidative stress and apoptosis. In addition to bee proteins, we also identified 3 Nosema ceranae proteins. Interestingly, abundance of two of these Nosema proteins were significantly higher in infected Nosema-sensitive honeybees relative to the infected Nosema-tolerant lineage. This may provide a novel candidate for studying the molecular interplay between N. ceranae and its honey bee host in more detail.

International audience; Natural enemies such as predators and parasites are known to shape intraspecific variability of behaviour and personality in natural populations, yet several key questions remain: (i) What is the relative importance of predation vs. parasitism in shaping intraspecific variation of behaviour across generations? (ii) What are the contributions of genetic and plastic effects to this behavioural divergence? (iii) And to what extent are responses to predation and parasitism repeatable across independent evolutionary lineages? We addressed these questions using Trinidadian guppies (Poecilia reticulata) (i) varying in their exposure to dangerous fish predators and Gyrodactylus ectoparasites for (ii) both wild-caught F0 and laboratory-reared F2 individuals and coming from (iii) multiple independent evolutionary lineages (i.e. independent drainages). Several key findings emerged. First, a population’s history of predation and parasitism influenced behavioural profiles, but to different extent depending on the behaviour considered (activity, shoaling or boldness). Second, we had evidence for some genetic effects of predation regime on behaviour, with differences in activity of F2 laboratory-reared individuals, but not for parasitism, which had only plastic effects on the boldness of wild-caught F0 individuals. Third, the two lineages showed a mixture of parallel and nonparallel responses to predation/parasitism, with parallel responses being stronger for predation than for parasitism and for activity and boldness than for shoaling. These findings suggest that different sets of behaviours provide different pay-offs in alternative predation/parasitism environments and that parasitism has more transient effects in shaping intraspecific variation of behaviour than does predation

Host–parasite systems provide convincing examples of Red Queen co-evolutionary dynamics.Yet, a key process underscored in Van Valen's theory – that arms race dynamics can result in extinction – has never been documented.One reason for this may be that most sampling designs lack the breadth needed to illuminate the rapid pace of adaptation by pathogen populations. In this study, we used a 25-year temporal sampling to decipher the demographic history of a plant pathogen: the poplar rust fungus, Melampsora larici-populina. A major adaptive event occurred in 1994 with the breakdown of R7 resistance carried by several poplar cultivars widely planted in Western Europe since 1982.The corresponding virulence rapidly spread in M. larici-populina populations and nearly reached fixation in northern France, even on susceptible hosts. Using both temporal records of virulence profiles and temporal population genetic data, our analyses revealed that (i) R7 resistance breakdown resulted in the emergence of a unique and homogeneous genetic group, the so-called cultivated population, which predominated in northern France for about 20 years, (ii) selection for Vir7 individuals brought with it multiple other virulence types via hitchhiking, resulting in an overall increase in the population-wide number of virulence types and (iii) – above all – the emergence of the cultivated population superseded the initial population which predominated at the same place before R7 resistance breakdown.Our temporal analysis illustrates how antagonistic co-evolution can lead to population extinction and replacement, hence providing direct evidence for the escalation process which is at the core of Red Queen dynamics.

Host–parasite systems provide convincing examples of Red Queen co-evolutionary dynamics. Yet, a key process underscored in Van Valen's theory – that arms race dynamics can result in extinction – has never been documented. One reason for this may be that most sampling designs lack the breadth needed to illuminate the rapid pace of adaptation by pathogen populations. In this study, we used a 25-year temporal sampling to decipher the demographic history of a plant pathogen: the poplar rust fungus, Melampsora larici-populina. A major adaptive event occurred in 1994 with the breakdown of R7 resistance carried by several poplar cultivars widely planted in Western Europe since 1982. The corresponding virulence rapidly spread in M. larici-populina populations and nearly reached fixation in northern France, even on susceptible hosts. Using both temporal records of virulence profiles and temporal population genetic data, our analyses revealed that (i) R7 resistance breakdown resulted in the emergence of a unique and homogeneous genetic group, the so-called cultivated population, which predominated in northern France for about 20 years, (ii) selection for Vir7 individuals brought with it multiple other virulence types via hitchhiking, resulting in an overall increase in the population-wide number of virulence types and (iii) – above all – the emergence of the cultivated population superseded the initial population which predominated at the same place before R7 resistance breakdown. Our temporal analysis illustrates how antagonistic co-evolution can lead to population extinction and replacement, hence providing direct evidence for the escalation process which is at the core of Red Queen dynamics.

Key message For an optimal deployment of poplar resistance to the gall-inducing aphid Phloeomyzus passerinii, a laboratory susceptibility assay has been developed. The nature of aphid–tree interactions during compatible and incompatible situations has been studied in detail. This should help at identifying specific resistance markers and at testing effects of site conditions on tree–pest interactions. Context P. passerinii is a major pest of poplar plantations in Europe, and the plantation of resistant poplar genotypes is regarded as the best long-term management strategy for this pest. This requires a sound knowledge of the interactions among the pest, its host and their environment. Aims Here, we review the recent advances aiming at developing an optimal deployment of host resistance versus P. passerinii. Results Investigations on aphid-host interactions demonstrated that P. passerinii induces pseudogalls within the bark of susceptible hosts. This results in a reduction of starch bark content during aphid outbreaks, which could be involved in tree death. The constitutive level of starch in the bark could be related to the tolerance level of trees. A susceptibility test has been designed for poplar genotypes, discriminating three categories of susceptibility depending on tree’s ability to totally or partially inhibit pseudogall induction. The test still has several limitations however. It neither takes into account the large level of individual genetic diversity of the aphid in France, nor the environmental modulation of tree resistance and tolerance, while water deficit and fertilization could potentially affect these parameters. Conclusion The hypotheses drawn regarding the processes leading to tree death or resistance should help at identifying resistance markers, and at testing effects of site conditions on tree–pest interactions.

The network approach is increasingly used by food-web ecologists and ecological parasitologists and has shed light on how parasite–host assemblages are organized, as well as on the role of parasites on the structure and stability of food webs. With accelerating rates of nonnative parasites being introduced around the world, there is an increasing need to predict their ecological impacts and the network approach can be helpful in this regard. There is inherent complexity in parasite invasions as parasites are highly diverse in terms of taxa and life strategies. Furthermore, they may depend on their cointroduced host to successfully overcome some crucial steps in the invasion process. Free-living introduced species often experience enemy release during invasion, which reduces the number of introduced parasites. However, introduced parasites that successfully establish may alter the structure of the recipient network through various mechanisms including parasite spill-over and spill-back, and manipulative and nonmanipulative phenotypic alterations. Despite limited literature on biological invasions in infectious food webs, some outstanding methodological issues and the considerable knowledge gaps that remain, the network approach provides valuable insights on some challenging questions, such as the link between structure and invasibility by parasites. Additional empirical data and theoretical investigations are needed to go further and the predictive power of the network approach will be improved by incorporating weighted methods that are based on trophic data collected using quantitative methods, such as stable isotope analyses.

The Trypanosomatidae family includes the genera Trypanosoma and Leishmania, protozoan parasites displaying complex digenetic life cycles requiring a vertebrate host and an insect vector. Trypanosoma brucei gambiense, Trypanosorna cruzi, and Leishmania spp. are important human pathogens causing human African trypanosomiasis (HAT or sleeping sickness), Chagas' disease, and various clinical forms of Leishmaniasis, respectively. They are transmitted to humans by tsetse flies, triatomine bugs, or sandflies, and affect millions of people worldwide. In humans, extracellular African trypanosomes (T. brucei) evade the hosts' immune defenses, allowing their transmission to the next host, via the tsetse vector. By contrast, T. cruzi and Leishmania sp. have developed a complex intracellular lifestyle, also preventing several mechanisms to circumvent the host's immune response. This review seeks to set out the immune evasion strategies developed by the different trypanosomatids resulting from parasite-host interactions and will focus on: clinical and epidemiological importance of diseases; life cycles: parasites-hosts-vectors; innate immunity: key steps for trypanosomatids in invading hosts; deregulation of antigen-presenting cells; disruption of efficient specific immunity; and the immune responses used for parasite proliferation.

Les invasions biologiques se sont intensifiées au cours des dernières décennies en raison d’une accélération des échanges commerciaux. Ces invasions représentent une menace pour les écosystèmes et de nombreuses activités anthropiques, il est donc crucial de comprendre les mécanismes qui les sous-tendent afin de mieux prévoir et limiter leurs impacts. Dans cette thèse, j’aborde la question du potentiel dispersif d’espèces natives et non natives impliquées dans une association phorétique nouvellement formée. En particulier, je me focalise sur le cas de l’association entre un nématode invasif ravageur des pinèdes, le nématode du pin (Bursaphelenchus xylophilus) et son insecte vecteur endémique en Europe (Monochamus galloprovincialis). J’ai tout d’abord étudié les flux de gènes de l’insecte vecteur seul afin d’identifier les barrières à sa dispersion. J’ai ensuite simulé l’expansion spatiale du couple nématode-vecteur à l’aide d’un modèle de dispersion, en intégrant l’effet synergique de cette nouvelle association. Les résultats obtenus au cours de cette thèse montrent qu’il existe un important potentiel de dispersion du nématode invasif en Europe par le biais de cette association phorétique. En revanche, certains paramètres de l’environnement tels que les reliefs et les températures basses qui leur sont associées, ainsi que les fortes densités en pins constituent des barrières à la dispersion du vecteur et donc des obstacles potentiels à l’expansion du nématode invasif. Au-delà des apports relatifs au modèle d’étude, cette thèse a conduit au développement de plusieurs méthodes pouvant être adaptées à d’autres cas d’associations phorétiques nouvelles et, par extension, contribuer à la compréhension de la dispersion des espèces au sein de ces systèmes complexes et peu étudiés., Biological invasions dramatically increased over the last decades due to the intensification of international trade. These invasions constitute a threat for ecosystems and many anthropic activities, therefore it is crucial to understand underlying processes in order to better predict and manage their impacts. In this PhD thesis, I explore the potential of dispersion of native and non-native species involved in a novel phoretic association. I focus on the case of the association between a pest for pine forests, the pinewood nematode (Bursaphelenchus xylophilus) introduced in Europe, and its endemic insect vector (Monochamus galloprovincialis). I first estimated gene flows of the insect vector alone in order to identify the barriers and corridors to dispersal of this species. Then I have simulated the spatial spread of the nematode-vector couple using a spread model, and accounting for the synergistic effect of this novel association. The results obtained during this PhD showed that the invasive nematode has an important potential to spread through this phoretic association. However, some environmental features such as elevation, areas with low temperatures, and the high pine densities constitute barriers to dispersal of the vector and so, potential obstacles to the spread of the invasive nematode. Beyond these results focused on the model of study, this thesis has led to the development of several methods that may be adapted to other cases of novel phoretic association and, by extension, may contribute to a better understanding of dispersal of species involved in those complex and poorly known systems.

Publication Inra prise en compte dans l'analyse bibliométrique des publications scientifiques mondiales sur les Fruits, les Légumes et la Pomme de terre. Période 2000-2012. http://prodinra.inra.fr/record/256699; Nonpathogenic Fusarium oxysporum endophytes from healthy banana roots were evaluated for their ability to reduce Fusarium wilt of banana (Panama disease). Isolates were identified morphologically and by using species-specific primers. Pathogenicity was confirmed by inoculating banana plantlets in the greenhouse. Nonpathogenic F. oxysporum isolates were grouped into 14 haplotype groups by polymerase chain reaction restriction fragment length polymorphism analysis of the intergenic spacer region, and representative isolates evaluated for biocontrol of F. oxysporum f. sp. cubense. In the greenhouse, 10 nonpathogenic F. oxysporum isolates were able to significantly reduce Fusarium wilt of banana. The isolate that protected banana plantlets best in the greenhouse, a nonpathogenic F. oxysporum from the root rhizosphere, and Pseudomonas fluorescens WCS 417 were then field tested. When the putative biological control organisms were tested in the field, neither the nonpathogenic F. oxysporum, P. fluorescens, nor combinations thereof reduced Fusarium wilt development significantly. A number of factors could contribute to the lack of field protection, including soil microbial and chemical composition and reduced survival of biocontrol organisms in banana roots. A lack of knowledge regarding the etiology of Fusarium wilt of ‘Cavendish’ banana in the subtropics and the effect of F. oxysporum f. sp. cubense race and banana cultivar in protection of banana by biocontrol organisms should be further investigated.

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Host plant interactions are likely key drivers of evolutionary processes involved in the diversification of phytophagous insects. Granivory has received substantial attention for its crucial role in shaping the interaction between plants and their seed parasites, but fine-scale mechanisms explaining the role of host plant reproductive biology on specialization of seed parasites remain poorly described. In a comparative approach using plant histological techniques, we tested the hypotheses that different seed parasite species synchronize their life cycles to specific stages in seed development, and that the stage they target depends on major differences in seed development programs. In a pinaceous system, seed storage products are initiated before ovule fertilization and the wasps target the ovule’s nucellus during megagametogenesis, a stage at which larvae may benefit from the by-products derived from both secreting cells and dying nucellar cells. In a cupressaceous system, oviposition activity peaks later, during embryogenesis, and the wasps target the ovule’s megagametophyte where larvae may benefit from cell disintegration during embryogenesis. Our cytohistological approach shows for the first time how, despite divergent oviposition targets, different parasite species share a common strategy that consists of first competing for nutrients with developing plant structures, and then consuming these developed structures to complete their development. Our results support the prediction that seed developmental program is an axis for specialization in seed parasites, and that it could be an important parameter in models of their ecological and taxonomic divergence. This study provides the basis for further investigating the possibility of the link between plant ontogeny and pre-dispersal seed parasitism

International audience; Eosinophils are one of the major mammalian effector cells encountered by helminths during infection. In the present study, we investigated the effects of eosinophil granule exposure on the sheep parasitic nematode Haemonchus contortus as a model. H. contortus eggs exposed to eosinophil granule products showed increased rhodamine 123 efflux and this effect was not due to loss of egg integrity. Rh123 is known to be a specific P-glycoprotein (Pgp) substrate and led to the hypothesis that in addition to their critical role in xenobiotic resistance, helminth ABC transporters such as Pgp may also be involved in the detoxification of host cytotoxic products. We showed by quantitative RT-PCR that, among nine different H. contortus Pgp genes, Hco-pgp-3, Hco-pgp-9.2, Hco-pgp-11 and, Hco-pgp-16 were specifically up-regulated in parasitic life stages suggesting a potential involvement of these Pgps in the detoxification of eosinophil granule products. Using exsheathed L3 larvae that mimic the first life stage in contact with the host, we demonstrated that eosinophil granules induced a dose dependent overexpression of Hco-pgp-3 and the closely related Hco-pgp-16. Taken together, our results provide the first evidence that a subset of helminth Pgps interact with, and could be involved in the detoxification of, host products. This opens the way for further studies aiming to explore the role of helminth Pgps in the host-parasite interaction, including evasion of the host immune response.

Le parasitisme est un des principaux problèmes dans les élevages des ruminants. Les nématodes parasites du tractus digestif des ovins et caprins sont responsables d’importantes baisses de rendement. La maîtrise de ces parasitoses a été longtemps basée sur l’utilisation de molécules anthelminthiques. Cependant, l’efficacité des traitements est fréquemment remise en cause par l’émergence d’isolats résistants à une ou plusieurs de ces molécules. Dans ce contexte, une meilleure connaissance des mécanismes impliqués dans l’installation et la survie des parasites dans leur hôte est essentielle pour le développement de méthodes de lutte efficaces. Les P-glycoprotéines sont des pompes membranaires de la superfamille des transporteurs ABC. Ces pompes transportent des molécules très variées qui ont en commun leur caractère hydrophobe. Nous avons émis l’hypothèse de l’implication de ces transporteurs dans l’interaction hôte-parasite. Dans le contexte de ce travail nous avons identifié des séquences partielles ou complètes d’ADNc de 9 Pgps du nématode parasite Haemonchus contortus. Une forte activation des Pgps des nématodes en présence des produits de dégranulation des éosinophiles de l’hôte a été observée, démontrant ainsi l’interaction entre les Pgps des nématodes et les produits issus de l’hôte. De plus, l’exposition in vitro des nématodes parasites aux produits de l’hôte montrent après analyse par PCR quantitative une induction significative de l’expression de deux Pgps (Hco-pgp-3, et Hco-pgp-16). Chez le nématode murin Heligmosomoides polygyrus bakeri 5 Pgps ont été identifiés. D’autre part, l’analyse du niveau d’expression des Pgps d’H. bakeri a permis de montrer que le gène Hba-pgp-2 est exprimé uniquement chez les stades en contact avec les produits de l’hôte (oeufs, L4 et adultes). De plus, une induction spécifique d’Hba-pgp-2 par le cholestérol a été observée suggérant ainsi l’implication d’Hba-pgp-2 dans la capture et/ou la distribution des stérols des cellules de l’hôte indispensable aux nématodes. Ce travail constitue la première mise en évidence de l’interaction entre les Pgps des nématodes parasites et des produits issus de leur hôte. Ces résultats constituent une base solide pour le développement d’une méthode efficace permettant de bloquer ces transporteurs et d’éliminer les nématodes parasites.

National audience

3 pages; International audience; To combat parasitism hosts often rely on their immune system, which is the last line of defense. However, the immune system may not always be effective, and other non-immunological defenses might be favored to reduce the cost of parasite infection. Here we report that larvae of the moth Lobesia botrana can rapidly accelerate their development and reach maturity earlier in response to cues perceived at a distance from parasitoids. Such a phenotypically plastic life history shift, induced by the perception of deadly enemies in the environment, is likely to be an adaptive defensive strategy to prevent parasitoid attack, and has important implications in host-parasite dynamics.

© 2012 Lockyer et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. The outcome of infection in the host snail Biomphalaria glabrata with the digenean parasite Schistosoma mansoni is determined by the initial molecular interplay occurring between them. The mechanisms by which schistosomes evade snail immune recognition to ensure survival are not fully understood, but one possibility is that the snail internal defence system is manipulated by the schistosome enabling the parasite to establish infection. This study provides novel insights into the nature of schistosome resistance and susceptibility in B. glabrata at the transcriptomic level by simultaneously comparing gene expression in haemocytes from parasite-exposed and control groups of both schistosome-resistant and schistosome-susceptible strains, 2 h post exposure to S. mansoni miracidia, using an novel 5 K cDNA microarray. Differences in gene expression, including those for immune/stress response, signal transduction and matrix/adhesion genes were identified between the two snail strains and tests for asymmetric distributions of gene function also identified immune-related gene expression in resistant snails, but not in susceptible. Gene set enrichment analysis revealed that genes involved in mitochondrial electron transport, ubiquinone biosynthesis and electron carrier activity were consistently up-regulated in resistant snails but down-regulated in susceptible. This supports the hypothesis that schistosome-resistant snails recognize schistosomes and mount an appropriate defence response, while in schistosome-susceptible snails the parasite suppresses this defence response, early in infection. This work was carried out with funding from the Wellcome Trust (068589/Z/02/Z). CMA and BH acknowledge support from the United States National Institutes of Health (NIH), USA R01AI052363 (CMA) and COBRE 1P20RR018754 IdEA, NCRR.

Varroa : Treatment and control ; Acarapis parasitic mites ; Tropilaelaps parasitic mites ; Questions and answers.

The impact of five phenolic acids (ferulic, coumaric, caffeic, syringic, and p-hydroxybenzoic acids) on fungal growth and type B trichothecene production by four strains of Fusarium graminearum was investigated. All five phenolic acids inhibited growth but the degree of inhibition varied between strains. Our results suggested that the more lipophilic phenolic acids are, the higher is the effect they have on growth. Toxin accumulation in phenolic acid-supplemented liquid glucose, yeast extract, and peptone cultures was enhanced in the presence of ferulic and coumaric acids but was reduced in the presence of p-hydroxybenzoic acid. This modulation was shown to correlate with a regulation of TRI5 transcription. In this study, addition of phenolic acids with greater antioxidant properties resulted in a higher toxin accumulation, indicating that the modulation of toxin accumulation may be linked to the antioxidant properties of the phenolic acids. These data suggest that, in planta, different compositions in phenolic acids of kernels from various cultivars may reflect different degrees of sensitivity to “mycotoxinogenesis.”

7 arbres adultes d’une même parcelle ont été répartis en 4 groupes en fonction de la gravité du chancre causé par N. ditissima dans le houppier. La sensibilité de leurs descendances (issues de graines) a été appréciée à partir de la taille du chancre développé entre novembre et juin à la suite d’une inoculation artificielle de différentes souches de N. ditissima. A une seule exception près les descendances les plus sensibles correspondent aux arbres mères les plus gravement atteints, et à l’inverse, les descendances les moins sensibles, aux arbres mères peu chancreux ou indemnes de chancre. La stabilité du comportement des descendances d’un essai à l’autre (vérifiée pour 4 descendances) et la qualité de la liaison mère/enfant révèle la dépendance génétique du caractère étudié. De plus, l’interaction différentielle entre souches de N. ditissima et les descendances étudiées implique l’existence d’une résistance de type vertical., 17 mature trees of the same stand, have been classified into four groups according to canker severity in their branches. Susceptibility of their progenies (from seeds) has been estimated from the canker size, developped between November and June, induced by wound inoculation of various N. ditissima strains. Except for one, the more susceptible progenies correspond to the more cankered mother trees, and on the contrary, the less susceptibles ones, to slighlty cankered or healthy mother trees. The steady response from one inoculation to the other for 4 progenies and the high parent-offspring relationship suggest a high genetic dependance of the characteristic. The interaction between N. ditissima strains and beech progenies implicates a vertical resistance.

Etude de l'influence de quelques facteurs mésologiques sur le développement larvaire et eonymphal de Diprion pini (Hympénoptère ; Sympte )

Calcium nutrition (0.1-15 meq |-1 Ca; hydroponic culture under controlled conditions) had a significant influence on the susceptibility of N. tabacum to Phytophthora parasitica. In inoculated leaf experiments and in inoculated stem experiments; the tobacco plants grown in 2 meq . |-1 Ca (total calcium content = 2.5% of the dry matter) were the most resistant to P. parasitica var. nicofianx, i.e., in the compatible interaction. In the incompatible interaction, the tobacco plants grown above 0.5 meq - |-1 Ca (total calcium content > 1.5% of the dry matter) were the most resistant to P. parasitica in inoculated leaf experiments. The pathogenesis-related protein (PR) accumulation was determined in the different interactions and compared with that obtained after induction by an abiotic inductor (a by-product of benzoic acid). Firstly, in the inoculated leaf experiments, the rapidity of PR appearance depended upon the inductor type or on its aggressivity and appeared to be correlated with the horizontal resistance in the two types of interactions. Then regardless of the inductor type and the tobacco cultivar (Xanthi nc or Escambray the weaker the calcium nutrition level, the faster the PR appearance. These results suggested that there was no correlation between the rapidity of PR appearance and tobacco resistance level against the compatible and incompatible strains in interaction with calcium nutrition., La nutrition calcique (0,1 à 15 meq . |-1 Ca; culture hydroponique en conditions contrôlées) a un effet statistiquement significatif. Pour les tests sur disques de feuille comme pour les tests sur tiges de plants décapités, ce sont les tabacs de la dose 2 meq .|-1 de Ca (teneur en calcium total = 2,5% de la masse de matière sèche) qui semblent les plus résistants au Phytophthora parasitica var. nicotianx (interaction compatible). Pour l’interaction incompatible et pour les tests sur disques de feuilles, ce sont les tabacs des doses supérieures à 0,5 meq . |-1 Ca (teneur en calcium total > 1,5% de la masse de matière sèche) qui semblent les plus résistants au Phytophthora parasitica. La cinétique d’accumulation des protéines reliées à la pathogénèse (PRs) a été également observée pour les différents niveaux d’interaction et comparée à celle obtenue après induction par un inducteur abiotique (dérivé de l’acide benzoïque) en fonction de la nutrition calcique. D’une part, pour les tests feuilles, la rapidité d apparition des PRs dépend du type d’inducteur ou de son agressivité et est corrélée au niveau de résistance horizontale pour les deux types d’interaction. D’autre part, quels que soient le type d’inducteur et la variété de tabac (Xanthi nc ou Escambray) , l’apparition des PRs est d’autant plus rapide que le niveau de nutrition calcique est faible, ceci indique qu'il n’apparait pas de corrélation nette entre rapidité de synthèse des PRs et niveau de résistance à la souche compatible ou incompatible en interaction avec la nutrition calcique.

La toxicité de 20 pesticides est étudiée sur le couple hôte-parasite Trialeurodes vaporariorum - Encarsia formosa. La méthode de laboratoire utilisée fait appel à des essais de longue durée, les produits étant appliqués sur tous les stades de l’hôte et du parasite. La plupart des fongicides testés sont utilisables en lutte intégrée, certains avec réserve (imazalil, triforine) ; seul le pyrazophos détruisant plus de 80 p. 100 des populations d’E. formosa est à exclure formellement. Le cyhexatin, plus toxique pour l’hôte que pour le parasite, semble être un bon acaricide dans l’optique de la lutte intégrée. Tous les insecticides étudiés présentent une certaine toxicité pour E. formosa : la bioresméthrine et le dichlorvos, peu persistants, sont utilisables selon certaines modalités, de même que le pyrimicarbe, aphicide considéré comme spécifique qui est toxique pour les adultes du parasite. Les insecticides très persistants tels la deltaméthrine, le parathion éthyl et le pyrimiphos-méthyl sont absolument à proscrire (populations d’E. formosa réduites de plus de 95 p. 100). L’étude des facteurs de toxicité de ces pesticides montre le danger de trop simplifier les méthodes d’appréciation de l’effet toxique des produits vis-à-vis des entomophages, des méthodes simples étant le plus souvent peu fiables. L’accent est également mis sur la nécessité de raisonner en termes d’équilibre de population et non de toxicité absolue lorsqu’on a affaire comme ici à un entomophage lié spécifiquement à un hôte en milieu clos., The toxicity of 20 pesticides has been studied on the host-parasite couple Trialeurodes vaporariorum - Encarsia formosa. Long-term trials were used under laboratory conditions. Pesticides were applied to the plant at all stages of host and parasite development. Most of the tested fungicides could be used in integrated control, some of them with caution (imazalyl, triforine) ; only pyrazophos which kills more than 80 % of E. formosa populations, must be excluded. Cyhexatin, which was more toxic for the host than for the parasite, seems to be a suitable miticide for integrated control. All insecticides studied showed some toxicity to E. formosa : poorly persistent products such as bioresmethrin and dichlorvos, a’s well as pirimicarb, a specific aphicide, toxic to adult parasites, could be used with care. The highly persistent insecticides, like deltamethrin, ethyl-parathion and methylpirimiphos, must be excluded (E. formosa populations reduced more than 95 %). When studying the toxicity factors of these pesticides, we observed that it is dangerous to simplify the methods for determining the toxic effects of these products to entomophagous insects. Simple methods are often insufficiently reliable. We also emphasize the need to think in terms of balance of populations instead of absolute toxicity when the entomophagous insect under study is specifically host-dependent in glasshouse conditions.

Une étude au microscope électronique des altérations apparues dans les cellules des gaines foliaires de diverses Triticinées lors de l’infection par Cercosporella herpotrichoides Fron, agent du piétin-verse, a été effectuée après culture en chambre de vernalisation ainsi qu’au champ. Cette étude a été réalisée au stade plantule sur 4 lignées de Triticinées choisies selon leur niveau de résistance à la maladie : 2 variétés de blé tendre (Triticum aestivum L.), l’une « Moisson », très sensible, et l’autre « Cappelle », qui présente une moindre sensibilité à la maladie, une lignée résistante « V.P.M. » obtenue à partir du croisement (Aegilops ventricosa x Triticum persicum) x Marne3 et une lignée d’Aegilops ventricosa Tausch. n° «Vent 11 », particulièrement résistante. Pour l’ensemble des lignées étudiées, le développement du parasite est essentiellement intracellulaire. La pénétration des hyphes à travers les parois pectocellulosiques se fait par digestion enzymatique de celles-ci. L’attaque du champignon provoque, chez l’hôte, une forte réactivité pariétale et la dégénérescence du contenu des cellules qui apparaissent hypertrophiées. Chez les lignées résistantes, une réaction cytoplasmique très intense est observée lors de la lyse cellulaire. Elle s’accompagne de l’accumulation d’une grande quantité de précipités osmioréducteurs dans les cellules infectées. Des différences importantes sont mises en évidence entre génotypes sensibles et résistants dans l’ultrastructure des hyphes développés tant au contact de l’épiderme des gaines qu’au sein des tissus. Chez les lignées sensibles, la maladie atteint rapidement l’ensemble des tissus des 3 premières gaines foliaires et le contenu des hyphes est dense et riche en organites différenciés. Le mycélium se présente à la fois sous la forme de filaments et d’éléments uni- ou pluricellulaires qui paraissent provenir du cloisonnement de la partie terminale des hyphes. Ces éléments, à paroi très épaisse, sont riches en matériel de réserve et ont un système vacuolaire très réduit. Assimilables à des chlamydospores, ils secrètent un exsudat d’apparence mucilagineuse qui semble favoriser leur adhérence sur les parois de l’hôte. Le rôle de ces éléments dans l’extension de la maladie est discuté. Chez les lignées résistantes, les hyphes, qui demeurent localisés généralement dans les couches cellulaires les plus externes de la 1er gaine, apparaissent le plus souvent dilatés et dégénérescents. D’importantes altérations de la paroi fongique sont observées. Elles pourraient modifier les propriétés de perméabilité de cette paroi et, permettant l’entrée massive d’eau dans les hyphes, entraîner leur dilatation et leur vésiculisation dans les cellules-hôtes. Lors de l’attaque par le champignon, les tissus résistants semblent donc le siège d’une forte réaction fongitoxique. Cette réaction, de type hypersensible, pourrait être déclenchée au niveau de la membrane plasmique. La synthèse d’enzymes capables de lyser la paroi fongique paraît en particulier être exaltée dans ces tissus. L'hypothèse d’une spécificité variétale dans les systèmes hôte-pathogène dans le cas de l’infection par le piétin-verse est avancée., The cell alterations induced in leaf-sheaths of various resistant or susceptible Triticinae by the pathogen Cercosporella herpotrichoides Fron, the eyespot disease agent, were studied with an electron microscope. Two lines of susceptible soft wheat (Triticum aestivum L.), « Moisson » and « Cappelle », one resistant line « V.P.M. » issue from crossing between (Aegilops ventricosa x Triticum persicum) x Marne3 and one line of Aegilops ventricosa Tausch, « Vent 11 », were investigated. The plants were grown in a greenhouse or in the field. The study was carried out 8 weeks after inoculation for the plants grown in the greenhouse, and 4 months after the infection for the plants grown in the field. In all these lines an intracellular development of the mycelium was mainly observed. The primary penetration of the hyphae occurred by enzymatic degradation of host cell wall constituents. Under attack by the fungus, the staining density of cell walls increased, all the intracellular organelles quickly disrupted and the parasitized cells swelled. In susceptible lines, a strong cytoplasmic reaction paralleled the lysis and the accumulation of very osmiophilic material in infected cells. Differences occurred in the ultrastructure of mycelium whether in contact with the epiderm cells or located on the leaf-sheath cells. In susceptible plants, in which the eyespot disease reached successively the three leaf-sheaths, the hyphae content was dense and rich in differentiated organelles. A stroma was constituted of filaments and of uni- and pluricellular elements. The elements proceeded from the partition of the hyphae apex. As in the chlamydospores, they were surrounded by a thick wall, contained a rich storage material and were poorly vacuolized. They could be a dissemination agent of the eyespot disease. In resistant plants, the hyphae remained in the outer layer cells of the first leaf-sheath and appeared empty, dilated and degenerescent. In addition, important alterations occurred in the fungus walls which could very well change the wall permeability and consequently cause the dilatation and vesiculisation of the hyphae by osmotic water stress. In resistant tissues, a violent toxic reaction of hypersensibility type seemed to take place in the host cell plasmalemma in response to the fungus infection. Then it is suggested that enzymes could be synthetized inducing the lysis of the fungus walls. The hypothesis of a varietal specificity in host-pathogen systems for the eyespot disease was advanced.

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