Response of life-history traits to artificial and natural selection for virulence and nonvirulence in a Drosophila parastitoid, Asobara tabida.

Co-evolution of host-parasitoid interactions is determined by the costs of host resistance, which received empirical evidence, and the costs of parasitoid virulence, which have been mostly hypothesized. Asobara tabida is a parasitoid, which mainly parasitizes Drosophila melanogaster and D. subobscura, the first species being able to resist to the parasitoid development while the second species is not. To parasitize resistant hosts, including D. melanogaster, A. tabida develops sticky eggs, which prevent encapsulation, but this virulence mechanism may be costly. Interindividual and interpopulation variation in the proportion of sticky eggs respectively allowed us to (i) artificially select and compare life-history traits of a virulent and a nonvirulent laboratory strain, and (ii) compare a virulent and a nonvirulent field strain, to investigate the hypothetical costs of virulence. We observed strong differences between the 2 laboratory strains. The nonvirulent strain invested fewer resources in reproduction and walked less than the virulent one but lived longer. Concerning the field strains, we observed that the nonvirulent strain had larger wings while the virulent one walked more and faster. All together, our results suggest that virulence may not always be costly, but rather that different life histories associated with different levels of virulence may coexist at both intra- and interpopulation levels.

Biochemical characterization and comparison of aspartylglucosaminidases secreted in venom of the parasitoid wasps Asobara tabida and Leptopilina heterotoma.

Aspartylglucosaminidase (AGA) is a low-abundance intracellular enzyme that plays a key role in the last stage of glycoproteins degradation, and whose deficiency leads to human aspartylglucosaminuria, a lysosomal storage disease. Surprisingly, high amounts of AGA-like proteins are secreted in the venom of two phylogenetically distant hymenopteran parasitoid wasp species, Asobara tabida (Braconidae) and Leptopilina heterotoma (Cynipidae). These venom AGAs have a similar domain organization as mammalian AGAs. They share with them key residues for autocatalysis and activity, and the mature α- and ß-subunits also form an (αß)2 structure in solution. Interestingly, only one of these AGAs subunits (α for AtAGA and ß for LhAGA) is glycosylated instead of the two subunits for lysosomal human AGA (hAGA), and these glycosylations are partially resistant to PGNase F treatment. The two venom AGAs are secreted as fully activated enzymes, they have a similar aspartylglucosaminidase activity and are both also efficient asparaginases. Once AGAs are injected into the larvae of the Drosophila melanogaster host, the asparaginase activity may play a role in modulating their physiology. Altogether, our data provide new elements for a better understanding of the secretion and the role of venom AGAs as virulence factors in the parasitoid wasps' success.

The Wide Potential Trophic Niche of the Asiatic Fruit Fly Drosophila suzukii: The Key of Its Invasion Success in Temperate Europe?

The Asiatic fruit fly Drosophila suzukii has recently invaded Europe and North and South America, causing severe damage to fruit production systems. Although agronomic host plants of that fly are now well documented, little is known about the suitability of wild and ornamental hosts in its exotic area. In order to study the potential trophic niche of D. suzukii with relation to fruit characteristics, fleshy fruits from 67 plant species were sampled in natural and anthropic ecosystems (forests, hedgerows, grasslands, coastal areas, gardens and urban areas) of the north of France and submitted to experimental infestations. A set of fruit traits (structure, colour, shape, skin texture, diameter and weight, phenology) potentially interacting with oviposition choices and development success of D. suzukii was measured. Almost half of the tested plant species belonging to 17 plant families allowed the full development of D. suzukii. This suggests that the extreme polyphagy of the fly and the very large reservoir of hosts producing fruits all year round ensure temporal continuity in resource availability and contribute to the persistence and the exceptional invasion success of D. suzukii in natural habitats and neighbouring cultivated systems. Nevertheless, this very plastic trophic niche is not systematically beneficial to the fly. Some of the tested plants attractive to D. suzukii gravid females stimulate oviposition but do not allow full larval development. Planted near sensitive crops, these "trap plants" may attract and lure D. suzukii, therefore contributing to the control of the invasive fly.

Trans-generational effects of mild heat stress on the life history traits of an aphid parasitoid.

Temperature changes are common in nature and insects are particularly exposed and sensitive to such variations which can be potential stresses, ultimately affecting life history traits and overall fitness. Braconids have been widely used to study the effects of temperature on host-parasitoid interactions and the present work focused on the solitary endoparasitoid Aphidius ervi Haliday (Hymenoptera: Braconidae Aphidiidae), an efficient biological control agent commercially used against aphids such as the potato aphid Macrosiphum euphorbiae Thomas (Sternorrhyncha: Aphididae). Contrary to previous studies using heat shocks at extreme temperatures, we evaluated the effects of mild heat stresses by transferring young parasitoid adults from the constant temperature of 20°C to either a warm (25°C) or hot (28°C) temperature, for either 1 h or 48 h. Such treatments are consistent with situations commonly experienced by parasitoids when moved from their rearing conditions to greenhouses or field conditions. The effects were evaluated both on the heat stressed A. ervi adults (G0) (immediate effects) and on their first generation (G1) progeny (trans-generational effects). G0 wasps' mortality was significantly affected by the temperature in interaction with the duration of the stress. Longevity of G0 wasps surviving the heat stress was negatively affected by the temperature and females lived longer than males. Heat stress applied to A. ervi parents also had consequences on their G1 progeny whose developmental time, rates of mummification and percentage of parasitoid completing total development were negatively affected. Surprisingly, the egg load at emergence of the G1 female progeny was increased when their mothers had been submitted to a mild heat stress of 25°C or 28°C. These results clearly demonstrate trans-generational phenotypic plasticity, showing that adaptation to thermal stresses may be achieved via maternal effects. This study also sheds light on the complexity of insect responses and underlying mechanisms to fluctuating conditions in their natural environment.

Structural and functional characterization of pseudopodocyte, a shaggy immune cell produced by two Drosophila species of the obscura group.

We recently reported that most of the Drosophila species of the obscura group were unable to mount cellular capsules and no lamellocyte was ever found in the hemolymph of any of the tested species. Only three species were able to encapsulate, despite lacking lamellocytes. Their encapsulation ability was always associated with the presence of an unpreviously described kind of capsule-forming immunocytes designated as "atypical hemocytes". Here, we describe the ultrastructural and functional characteristics of this type of hemocyte. We show that these cells share many ultrastructural and morphological features with Drosophila melanogaster plasmatocytes, although they are involved in the formation of the external layers of the cellular capsule, a functional property exhibited by lamellocytes in D. melanogaster. Due to the high number of pseudopodes in these cells, we suggest to name them "pseudopodocytes". After structural and functional characterization of these atypical hemocytes, their ambiguous status between plasmatocytes and lamellocytes is discussed.

Immune resistance of Drosophila hosts against Asobara parasitoids: cellular aspects.

The immunity of Drosophila relies on a variety of defenses cooperating to fight parasites and pathogens. The encapsulation reaction is the main hemocytic response neutralizing large parasites like endophagous parasitoids. The diversity of the mechanisms of immunoevasion evolved by Asobara parasitoids, together with the wide spectrum of Drosophila host species they can parasitize, make them ideal models to study and unravel the physiological and cellular aspects of host immunity. This chapter summarizes what could be learnt on the cellular features of the encapsulation process in various Drosophila spp., and also on the major role played by Drosophila hosts hemocytes subpopulations, both in a quantitative and qualitative manner, regarding the issue of the immune Asobara-Drosophila interactions.

Components of Asobara venoms and their effects on hosts.

Hymenoptera of the Asobara genus are endophagous parasitoids of Drosophila larvae. In these apocrita insects whose venom gland is associated with the female reproductive tract, the wasp venom is injected into the host along with the parasitoid egg during oviposition. We conducted a comparative study of the venom apparatuses from three Asobara spp.: the European Asobara tabida, the Asiatic A. japonica and the African A. citri. Light and electron microscopy of venom glands, together with the biochemical analysis of their contents, revealed important differences between Asobara spp. In addition, the physiological effects of female wasp's venom injected into Drosophila larvae differed greatly between the tested Asobara spp.

Strategies of avoidance of host immune defenses in Asobara species.

Eggs and larvae of endophagous parasitoids face the host's immunity reaction once they penetrate the insect host's hemocele. In order to overcome the host's immune barrier, endoparasitoids have developed various strategies. Conformer parasitoids hide and/or get protected from the attack by the host's immunity cells without interfering with the host's immune system. Differently, regulator parasitoids directly attack the host's hemocytes, therefore totally inhibiting the immunity reaction of encapsulation in the parasitized host. Female wasps may also discriminate immunoreactive hosts from nonreactive, permissive ones before laying an egg. These different strategies coexist within the same genus of the braconids Asobara, endoparasitoids of Drosophila larvae. The physiological mechanisms underlying the conformer and regulator strategies in Asobara are exposed. The factors which may contribute to the diversity of the means developed by Asobara parasitoids to overcome the hosts' immunity defenses are discussed.

Conversely to its sibling Drosophila melanogaster, D. simulans overcomes the immunosuppressive effects of the parasitoid Asobara citri.

The endoparasitoid Asobara citri avoids Drosophila melanogaster immune defenses, thanks to immune suppressive effects. We investigated whether this parasitoid could also circumvent the immune reaction of D. simulans, a sibling species of D. melanogaster. The rates of infestation, successful parasitism, total encapsulation and mortality were measured after complete development of both D. melanogaster and D. simulans larvae parasitized by A. citri. Results showed that the parasitoids were almost never encapsulated in D. melanogaster larvae, while 45% were encapsulated in D. simulans. A. citri induced a targeted disruption of the hematopoietic organs and a decrease of the hemocytes load in host larvae of both species. Despite such disruptive immune effects most D. simulans larvae succeeded in encapsulating A. citri eggs, probably thanks to their ability to immediately mount a capsule after infestation. This work brings some insight into the diversity of the immune potentials evolved by Drosophila species towards parasitoids.