Vincent Foray

A handbook for uncovering the complete energetic budget in insects: the van Handel's method (1985) revisited

Insects comprise relevant biological models for investigating nutrient acquisition and allocation processes in the context of life‐history ecology and evolution. However, empirical investigations are still partly limited by the lack of availability of simple methods for simultaneously estimating the four major energetic components (i.e. lipids, free sugars, glycogen and proteins) in the same individual. In the present work, we validate a fast, reproducible and cheap method for overcoming this problem that uses different solvents successively. First, proteins are solubilized in a phosphate‐lysis buffer and then quantified according to the classical Bradford assay procedure. In a second step, a chloroform–methanol mixture is added to the aqueous phase, which allows assay of the total lipid fraction, as well as the free sugars and glycogen in the same insect homogenate. In addition, a micro‐separation procedure is adapted to partition the total lipids into neutral (mainly stored lipids) and polar (mainly structural lipids) components. Although these assays are conducted sequentially in the same individual, the sensitivity of our method remains high: the estimated amount of each energetic compartment does not differ from that obtained with former, partial methods. Our method should thus largely improve our knowledge about nutrient acquisition and allocation among insects not only in laboratory‐reared individuals, but also in animals caught in the wild. Descriptions and recommendations are given at each step of the protocol to adapt the procedure to various insect species. Finally, to prevent misinterpretation of data generated in accordance with this protocol, the limits of our method are discussed in the light of life‐history studies.

Does cold tolerance plasticity correlate with the thermal environment and metabolic profiles of a parasitoid wasp

Tolerance of ectotherm species to cold stress is highly plastic according to thermal conditions experienced prior to cold stress. In this study, we investigated how cold tolerance varies with developmental temperature (at 17, 25 and 30°C) and whether developmental temperature induces different metabolic profiles. Experiments were conducted on the two populations of the parasitoid wasp, Venturia canescens, undergoing contrasting thermal regimes in their respective preferential habitat (thermally variable vs. buffered). We predicted the following: i) development at low temperatures improves the cold tolerance of parasitoid wasps, ii) the shape of the cold tolerance reaction norm differs between the two populations, and iii) these phenotypic variations are correlated with their metabolic profiles. Our results showed that habitat origin and developmental acclimation interact to determine cold tolerance and metabolic profiles of the parasitoid wasps. Cold tolerance was promoted when developmental temperatures declined and population originating from variable habitat presented a higher cold tolerance. Cold tolerance increases through the accumulation of metabolites with an assumed cryoprotective function and the depression of metabolites involved in energy metabolism. Our data provide an original example of how intraspecific cold acclimation variations correlate with metabolic response to developmental temperature.

Fuelling flight in a parasitic wasp: Which energetic substrate to use?

1. Flight is an energy‐demanding behaviour in insects. In parasitic wasps, strategies of nutrient acquisition and allocation, resulting life‐history trade‐offs and relationships with foraging strategies and resource availability have received much attention. However, despite the ecological importance of dispersal between host and food patches, and the great impact energy diverted to flight should have on lifetime reproductive success, the eco‐physiology of flight in parasitoids is poorly understood.

The impact of thermal fluctuations on reaction norms in specialist and generalist parasitic wasps

Reaction norms depict the environmental effects on phenotypic traits and are used to predict the global change consequences on species distributions. However, studies performed at constant temperatures have limited ecological significance because expressed phenotypes depend on the range and frequency of environmental states. Using Jensens inequality (i.e. a mathematical property of nonlinear functions), we predicted that the effect of thermal fluctuations on the phenotype depends on the shape of the reaction norm. Thermal fluctuations around the optimal temperature are expected to reduce the phenotypic trait values, especially for specialists because of their narrower reaction norms. This study measured the effects of diel fluctuations in developmental temperature on phenotypic expression of traits related to fitness and energetic resources in two strains of the parasitoid wasp Venturia canescens from different habitats: a thermal generalist strain and a specialist one. In a first experiment, we compared the effect of constant thermal regimes versus fluctuating ones having the same means (20, 25 and 30 °C) on reaction norms of life-history traits and of energetic reserves. In a second experiment, we examined the effects of a natural thermoperiod in the field on these traits. Our results show that the shape of the reaction norm defines the phenotypic changes induced by the development under fluctuating thermal conditions. These results match the predictions of the Jensens inequality. Moreover, our results emphasize the significance of taking into account several phenotypic life-history traits to study the adaptive value of phenotypic plasticity. We also show that the level of energetic resources depends on the mean developmental temperature and not on the thermal regime. Finally, the field experiment confirms that the phenotype of these parasitoids depends on the temperature variation. Our study highlights the relevance of the Jensens inequality to predict the effect of thermal fluctuations on fitness of parasitoids with contrasted thermal sensitivities.

Accessing the Hidden Microbial Diversity of Aphids: an Illustration of How Culture-Dependent Methods Can Be Used to Decipher the Insect Microbiota

Microorganism communities that live inside insects can play critical roles in host development, nutrition, immunity, physiology, and behavior. Over the past decade, high-throughput sequencing reveals the extraordinary microbial diversity associated with various insect species and provides information independent of our ability to culture these microbes. However, their cultivation in the laboratory remains crucial for a deep understanding of their physiology and the roles they play in host insects. Aphids are insects that received specific attention because of their ability to form symbiotic associations with a wide range of endosymbionts that are considered as the core microbiome of these sap-feeding insects. But, if the functional diversity of obligate and facultative endosymbionts has been extensively studied in aphids, the diversity of gut symbionts and other associated microorganisms received limited consideration. Herein, we present a culture-dependent method that allowed us to successfully isolate microorganisms from several aphid species. The isolated microorganisms were assigned to 24 bacterial genera from the Actinobacteria, Firmicutes, and Proteobacteria phyla and three fungal genera from the Ascomycota and Basidiomycota phyla. In our study, we succeeded in isolating already described bacteria found associated to aphids (e.g., the facultative symbiont Serratia symbiotica), as well as microorganisms that have never been described in aphids before. By unraveling a microbial community that so far has been ignored, our study expands our current knowledge on the microbial diversity associated with aphids and illustrates how fast and simple culture-dependent approaches can be applied to insects in order to capture their diverse microbiota members.

Toward a better understanding of the mechanisms of symbiosis: a comprehensive proteome map of a nascent insect symbiont

Symbiotic bacteria are common in insects and can affect various aspects of their hosts’ biology. Although the effects of insect symbionts have been clarified for various insect symbiosis models, due to the difficulty of cultivating them in vitro, there is still limited knowledge available on the molecular features that drive symbiosis. Serratia symbiotica is one of the most common symbionts found in aphids. The recent findings of free-living strains that are considered as nascent partners of aphids provide the opportunity to examine the molecular mechanisms that a symbiont can deploy at the early stages of the symbiosis (i.e., symbiotic factors). In this work, a proteomic approach was used to establish a comprehensive proteome map of the free-living S. symbiotica strain CWBI-2.3T. Most of the 720 proteins identified are related to housekeeping or primary metabolism. Of these, 76 were identified as candidate proteins possibly promoting host colonization. Our results provide strong evidence that S. symbiotica CWBI-2.3T is well-armed for invading insect host tissues, and suggest that certain molecular features usually harbored by pathogenic bacteria are no longer present. This comprehensive proteome map provides a series of candidate genes for further studies to understand the molecular cross-talk between insects and symbiotic bacteria.

Impacts of differences in nutritional quality of wingless and winged aphids on parasitoid fitness

ABSTRACT Winged aphids are described as hosts of lesser quality for parasitoids because a part of their resources is used to produce wings and associated muscles during their development. Host lipid content is particularly important for parasitoid larvae as they lack lipogenesis and therefore rely entirely on the host for this resource. The goal of this study was to determine to what extent winged and wingless aphids differ from a nutritional point of view and whether these differences impact parasitoid fitness, notably the lipid content. We analysed the energetic budget (proteins, lipids and carbohydrates) of aphids of different ages (third instars, fourth instars and adults) according to the morph (winged or wingless). We also compared fitness indicators for parasitoids emerging from winged and wingless aphids (third and fourth instars). We found that in third instars, parasitoids are able to inhibit wing development whereas this is not the case in fourth instars. Both winged instars allow the production of heavier and fattier parasitoids. The presence of wings in aphids seems to have little effect on the fitness of emerging parasitoids and did not modify female choice for oviposition. Finally, we demonstrate that Aphidius colemani, used as a biological control agent, is able to parasitize wingless as well as winged Myzus persicae, at least in the juvenile stages. If the parasitism occurs in third instars, the parasitoid will prevent the aphid from flying, which could in turn reduce virus transmission. Summary: Developing winged aphids should not be considered as low-quality hosts as they bring a higher fitness to parasitoids.

Evidence for Gut-Associated Serratia symbiotica in Wild Aphids and Ants Provides New Perspectives on the Evolution of Bacterial Mutualism in Insects

Many insects engage in symbiotic associations with diverse assemblages of bacterial symbionts that can deeply impact on their ecology and evolution. The intraspecific variation of symbionts remains poorly assessed while phenotypic effects and transmission behaviors, which are key processes for the persistence and evolution of symbioses, may differ widely depending on the symbiont strains. Serratia symbiotica is one of the most frequent symbiont species in aphids and a valuable model to assess this intraspecific variation since it includes both facultative and obligate symbiotic strains. Despite evidence that some facultative S. symbiotica strains exhibit a free-living capacity, the presence of these strains in wild aphid populations, as well as in insects with which they maintain regular contact, has never been demonstrated. Here, we examined the prevalence, diversity, and tissue tropism of S. symbiotica in wild aphids and associated ants. We found a high occurrence of S. symbiotica infection in ant populations, especially when having tended infected aphid colonies. We also found that the S. symbiotica diversity includes strains found located within the gut of aphids and ants. In the latter, this tissue tropism was found restricted to the proventriculus. Altogether, these findings highlight the extraordinary diversity and versatility of an insect symbiont and suggest the existence of novel routes for symbiont acquisition in insects.

The French touch in entomological biology: synthesis of the “16th Colloque Biologie de l’Insecte”

E-mail: nathalie.mondy@univ-lyon1.fr, fl oriane.chardonnet@orange.fr, emilie.delava@univ-lyon1.fr; vincent.foray@univ-lyon1.fr, ceciclia.multeau@supagro.inra.fr, aurelien.vigneron@insa-lyon.fr Accepté le 16 juin 2011 T present paper represents a synthesis of communications collected in the “16e Colloque Biologie de l’Insecte” which gathered about 160 researchers from France and other European countries at Lyon in October 2010 (on-line reports of the symposium http://www.insavalor.fr/CBI2010). Because insects are some of the most familiar organisms of global ecosystems and because of their diversity, they are suitable models to investigate a broad spectrum of questions, in genetics and ecology as well as on populations and physiological responses to environmental changes. Molecular ecology approaches, spurred on by the recent advances in genomics, were omnipresent during this congress and revealed their relevance for the study of specifi c proximal mechanisms. As a consequence, most of presentations were focused on functional and mechanistic biology rather than conceptual approaches, following several of the recent trends outlined below.