Denis Poinsot

Identification of a Widespread Monomolecular Odor Differentially Attractive to Several Delia Radicum Ground-dwelling Predators in the Field

Dimethyl disulfide (DMDS) was identified as a major volatile constituent of Brassica napus roots heavily infested by Delia radicum, the cabbage root fly. Attractiveness of this widespread compound was tested in the field in a naturally complex odorous environment. By using an original setup especially designed for ground dwelling beetles, different concentrations of the pure molecule as well as attractiveness of the natural blend emitted by the rotten part of infested roots were tested simultaneously. The use of general linear model (GLM) statistics permitted us to finely discriminate the responses among the different treatments. The main predators of D. radicum (i.e., two staphylinids Aleochara bilineata and Aleochara bipustulata and carabid beetles of the genus Bembidion) were significantly attracted by DMDS, but responded in different ways to the natural blend and to the different concentrations tested. The dose–response curves were similar for the two staphylinids. However, whereas A. bilineata was more attracted by the natural volatile blend than by its preferred DMDS concentration, A. bipustulata was attracted as much by the natural blend as by its preferred DMDS concentration. Carabid beetles exhibited a different response. They were not attracted by the natural blend, but responded to a wider range of DMDS concentrations that included low concentrations that did not attract the staphylinid beetles. These results are discussed according to the potential resources searched by each taxon studied and their specificity for the resources. The possible use of DMDS for enhancing biological control of D. radicum is mentioned.

WOLBACHIA INFECTION IN DROSOPHILA SIMULANS: DOES THE FEMALE HOST BEAR A PHYSIOLOGICAL COST?

Fitness traits of three Drosophila simulans strains infected by endocellular bacteria belonging to the genus Wolbachia have been compared with those of replicate stocks previously cured from the infection by an antibiotic treatment. The traits measured were development time, egg‐to‐adult viability, egg hatch, productivity, fecundity, and the number of functional ovarioles. Individuals of the first strain were bi‐infected by two Wolbachia variants, wHa and wNo. The second strain was infected by wHa, the third one by wNo. The Wolbachia studied here cause cytoplasmic incompatibility (CI), a high embryonic mortality (70% to > 90%) when an infected male is crossed with an uninfected female. Three generations after antibiotic treatment, we observed in all strains a significant drop in productivity in the cured stocks. This drop was not due to antibiotic toxicity and was associated with the loss of the Wolbachia. However the effect had disappeared in two of the three strains five generations after treatment, and could not be found in the third strain in a third measurement carried out 14 generations after treatment. The temporary nature of the productivity difference indicates that Wolbachia do not enhance productivity in infected strains. On the other hand, in all traits measured, our results show the absence of any negative effects of the Wolbachia on their host. This could be explained when considering Wolbachia evolution, as maternally transmitted parasites bear a strong selective pressure not to harm their female host. However, CI would allow the bacteria to be maintained even when harming the female. The apparent absence of deleterious effects caused by these Wolbachia might result from a trade‐off, where a relatively low bacteria density would advantage the Wolbachia by suppressing any deleterious effects on the female host, at the cost of a weaker maternal transmission rate of the infection.

Kin discrimination and altruism in the larvae of a solitary insect

Kin selection theory predicts altruism between related individuals, which requires the ability to recognize kin from non-kin. In insects, kin discrimination associated with altruistic behaviour is well-known in clonal and social species but in very few solitary insects. Here, we report that the solitary larvae of a non-social insect Aleochara bilineata Gyll. (Coleoptera; Staphylinidae) show kin discrimination and sibling-directed altruistic behaviour. Larvae superparasitize more frequently the hosts parasitized by non-kin individuals than those hosts parasitized by siblings. Kin discrimination probably occurs by self-referent phenotype matching, where an individual compares its own phenotype with that of a non-familiar related individual, a mechanism rarely demonstrated in animals. The label used to recognize kin from non-kin corresponds to substances contained in the plug placed on the hosts by the resident larvae during the parasitization process. Kin competition induced by a limited larval dispersion may have favoured the evolution of kin recognition in this solitary species.

Wolbachia segregation rate in Drosophila simulans naturally bi-infected cytoplasmic lineages

Wolbachia are maternally transmitted endocellular bacteria infecting several arthropod species. In order to study Wolbachia segregation rate, Drosophila simulans females from an Indo-Pacific population (Seychelles) bi-infected by the two Wolbachia variants wHa and wNo were backcrossed to uninfected males in two conditions. In the first case, Seychelles males from a stock cured from its Wolbachia by tetracycline treatment were used. In the second case, the males came from a naturally uninfected Tunisian population. It was found that (i) the two Wolbachia variants can segregate, so that bi-infected females can produce a few offspring infected only by wHa or wNo. This occurs in both backcross conditions. (ii) Segregation leads more frequently to wHa than to wNo mono-infection. (iii) Wolbachia transmission is lower when the Seychelles genome is introgressed by the Tunisian genome, suggesting that host genomic factors might influence infection fate.

Effect of expected offspring survival probability on host selection in a solitary parasitoid

Optimal Foraging Theory predicts that parasitoid females should optimize their host selection to maximize their lifetime fitness gain and parasitize the most profitable hosts. In particular, in solitary parasitoids, females should avoid superparasitism, at least when sufficient unparasitized hosts are available. However, when unparasitized hosts are scarce, they should prefer, among already parasitized hosts, those that provide the best survival probability to their progeny, which depends on the age and the developmental stage of the first parasitoid.

Broccoli and turnip plants display contrasting responses to belowground induction by delia radicum infestation and phytohormone applications

Induced responses to insect herbivory are a common phenomenon in the plant kingdom. So far, induced responses have mostly investigated in aerial plant parts. Recently it was found that root herbivore may also elicit both local and systemic responses affecting aboveground herbivores and their natural enemies. Using broccoli (Brassica oleracea subsp. italica L.) and turnip (Brassica rapa subsp. rapa L.), two cultivated brassicaceaous plants differing in their chemistry and morphology, we analysed the local and systemic induced responses triggered by Delia radicum L. damage, JA and SA application. We also assessed whether the root induction treatments affected D. radicum larval performance. Both D. radicum damage and JA induced changes in glucosinolate and sugar content as well as affected D. radicum performance, while SA application did not. Despite the uniform chemical responses, the effect on larval performance on broccoli and turnip plants was very different. On broccoli, JA root treatment reduced herbivore performance, whereas in turnips the same treatment enhanced it. JA- and D. radicum-induced responses followed similar patterns, which suggests that the JA signalling pathway is involved in root-induced responses to larval feeding. Glucosinolate induction cannot fully explain the differences found in the performance of D. radicum on the different species. Changes in other resistance factors might significantly contribute to the induced resistance in these brassicaceaeous species as well.

Selection Strategies of Parasitized Hosts in a Generalist Parasitoid Depend on Patch Quality but Also on Host Size

Host rejection, superparasitism, and ovicide are three possible host selection strategies that parasitoid females can adopt when they encounter parasitized hosts. These differ in costs (in terms of time and energy required) and benefits (in terms of number and quality of offspring produced). Their relative payoff should vary with patch quality, (i.e., proportion of parasitized hosts present), and female choice between them should be adapted accordingly. We conducted behavioral observations to test the effect of the ratio of parasitized/unparasitized hosts present in a patch on the host selection strategies of Pachycrepoideus vindemmiae Rondani (Hymenoptera: Pteromalidae). This species being a generalist known to attack hosts of a great range of size, we also tested the impact of host size on female decisions with two host species differing greatly in size (Drosophila melanogaster and Delia radicum). We evaluated the adaptive value of each strategy in relation to host parasitization status and host size by measuring their duration and the potential number of offspring produced.

Intraspecific Variations in Host Discrimination Behavior in the Pupal Parasitoid Pachycrepoideus vindemmiae Rondani (Hymenoptera: Pteromalidae)

Abstract In solitary parasitoid species, superparasitism incurs a high cost because only one individual can emerge per host. While avoiding already-parasitized hosts seems advantageous, it requires an ability to discriminate between parasitized and unparasitized hosts. The ability to discriminate can be based on physical or chemical cues or signal associated either internally or externally with a given host. The type of stimuli used to recognize parasitized hosts generally depends on the features of these hosts but also on costs and benefits associated with reusing them. Some local factors such as mortality rate of females, host availability, and competition level can influence this trade-off. In species that occupy a large geographic range, local conditions may favor either external or internal mechanisms of host discrimination. We describe the behaviors associated with host discrimination in Pachycrepoideus vindemmiae Rondani (Hymenoptera: Pteromalidae), a solitary pupal parasitoid of cyclorraphous dipteran species. To detect potential intraspecific variability in their host discrimination behavior, we compared two P. vindemmiae populations originating from different geographical areas in France. Our results revealed different host discrimination strategies in both populations and indicated a potential trade-off between the speed and the accuracy of host discrimination. One population discriminated hosts externally (faster but less accurately), whereas the other discriminated internally (more slowly but more accurately). In our experimental conditions, these two strategies resulted in differences in the fitness gains of both populations. The ecological conditions that could have selected for such differences in the host discrimination and oviposition strategies of the two populations remain to be investigated.

Costs of secondary parasitism in the facultative hyperparasitoid Pachycrepoideus dubius: does host size matter?

Although hyperparasitism frequently occur in parasitic insects, many aspects of this strategy remain unknown. We investigated possible fitness costs of hyperparasitism as influenced by host size. Our study was conducted with the facultative hyperparasitoid Pachycrepoideus dubius Ashmead (Hymenoptera: Pteromalidae), which parasitizes host species differing greatly in size. We compared some fitness traits (level of successful parasitism, development time, sex ratio and offspring size) of P. dubius developing on large secondary/primary (Delia radicum L. (Diptera: Anthomyiidae)/Trybliographa rapae Westwood (Hymenoptera: Figitidae)) or small secondary/primary host species (Drosophila melanogaster L./Asobara tabida Nees (Hymenoptera: Braconidae)). In no‐choice and choice experiments, P. dubius was able to develop on different stages of T. rapae (L2 (endophagous), L4 (ectophagous), and pupae) but that it preferred to parasitize unparasitized D. radicum pupae over pupae parasitized by T. rapae. Furthermore, in P. dubius, hyperparasitism was associated with fitness costs (lower level of successful parasitism, smaller adult size) and these costs were greater on the smallest host complex. We hypothesize that the size of D. melanogaster pupae parasitized by A. tabida may be close to the suboptimal host size for P. dubius beneath which the costs of hyperparasitism make this strategy nonadaptive. Hyperparasitism in terms of trade‐offs between host quality and abundance of competitors is discussed.

INCIPIENT EVOLUTION OF WOLBACHIA COMPATIBILITY TYPES

Abstract Cytoplasmic incompatibility (CI) is induced in arthropods by the maternally inherited bacterium Wolbachia. When infected males mate with uninfected females or with females bearing a different Wolbachia variant, paternal chromosomes behave abnormally and embryos die. This pattern can be interpreted as resulting from two bacterial effects: One (usually termed mod, for modification) would affect sperm and induce embryo death, unless Wolbachia is also present in the egg, which implies the existence of a second effect, usually termed resc, for rescue. The fact that CI can occur in crosses between males and females infected by different Wolbachia shows that mod and resc interact in a specific manner. In other words, different compatibility types, or mod/resc pairs seem to have diverged from one (or a few) common ancestor(s). We are interested in the process allowing the evolution of mod/resc pairs. Here this question is addressed experimentally after cytoplasmic injection into a single host species (Drosophila simulans) by investigating compatibility relationships between closely related Wolbachia variants naturally evolving in different dipteran hosts: D. simulans, Drosophila melanogaster, and Rhagoletis cerasi. Our results suggest that closely related bacteria can be totally or partially incompatible. The compatibility relationships observed can be explained using a formal description of the mod and resc functions, implying both qualitative and quantitative variations.