Search

Some insects can develop immune resistance to koinobiont parasitoids. Reciprocally, adaptation to host immunology is critical for parasitoid success. Phylogenetic inertia and correlations between virulence against different hosts can act as constraints preventing these adaptations. Insights on these constraints may be obtained from the analysis of patterns of variations in the interactions at the species or genus level. Multivariate phylogenetic comparative methods were applied to virulence traits of 13 parasitoid strains of Leptopilina spp. (Hymenoptera: Figitidae) on five host strains of the Drosophila melanogaster species subgroup (Diptera Drosophilidae). Independent contrasts of virulence were calculated and principal component analysis (PCA) was performed on the independent contrasts to estimate the dimensionality of the interactions. Most of the variation of virulence was associated with the first component of the PCA (62.2%). But a significant proportion was explained by the second and third components, suggesting specific interactions. Strain-strain reciprocal specificity was observed in several pairs of host-parasitoid species. Significant phylogenetic inertia was observed on parasitoid virulence, but only at the genus level and only against hosts of intermediate resistance (phylogenetic R-2 between 0.62 and 0.85). Some parts of the interaction matrix exhibited specific interactions and others were fixed due to ancestral non-specific virulence (or avirulence). The results were interpreted viewing virulence as a threshold trait determined by underlying liability. When liability is far from the threshold, virulence is fixed. When liability is close to the threshold, virulence varies specifically and reciprocal adaptations can take place. These phylogenetic constraints may lead to a scenario of escape and radiation coevolution in the host-parasitoid system. Certains insectes développent des réactions immunitaires contre les parasitoïdes koinobiontes.

International audience

The cellular innate immune response of several species of Drosophila terminates with the encasement of large foreign objects within melanotic capsules comprised of several layers of adhering blood cells or hemocytes. This reaction is manifested by various Drosophila hosts in response to infection by endoparasitic wasps (i.e., parasitoids). Creditable assessments of the factor(s) causing, or contributing to, parasite mortality have long been considered as cytotoxic elements certain molecules associated with enzyme-mediated melanogenesis. However, observations that warrant additional or alternative considerations are those documenting parasitoid survival despite melanotic encapsulation, and those where parasitoids are destroyed with no evidence of this host response. Recent studies of the production of some reactive intermediates of oxygen and nitrogen during infection provide a basis for proposing that these molecules constitute important components of the immune arsenal of Drosophila. Studies of the virulence factors injected by female wasps during oviposition that suppress the host response will likely facilitate identification of the cytotoxic molecules as well as the cell-signaling pathways that regulate their synthesis.

International audience; Interactions between Drosophila hosts and parasitoid wasps are among the few examples in which occurrence of intraspecific variation of parasite success has been studied in natural populations. Such variations can originate from three categories of factors: environmental, host and parasitoid factors. Under controlled laboratory conditions, it is possible to focus on the two last categories, and, using specific reference lines, to analyze their respective importance. Parasitoid and host contributions to variations in parasite success have largely been studied in terms of evolutionary and mechanistic aspects in two Drosophila parasitoids, Asobara tabida and, in more details, in Leptopilina boulardi. This chapter focuses on the physiological and molecular aspects of L. boulardi interactions with two Drosophila host species, while most of the evolutionary hypotheses and models are presented in Chapter 11 of Dupas et al.

In this chapter, we describe the geographically widespread genetic fixation of traits involved in Drosophila-parasitoid immune interactions and the situations where such fixation is not observed. We then discuss how the three classes of coevolutionary dynamics that can occur at the local scale (coevolutionary escalation, coevolutionary alternation and coevolutionary polymorphism), the geographic mosaic of selection, and the phylogenetic constraints may explain such evolutionary patterns and drive diversification in the interactions. Most Drosophila parasitoid traits involved in virulence are host-species specific. Directional selection (coevolutionary escalation) on such traits can lead to their fixation or on the contrary maintain their polymorphism if these traits are associated with fitness costs. When hosts targeted by different host-specific virulence systems coexist, fluctuations in selective pressures on these systems, together with the ability of Drosophila parasitoids to select the most susceptible host for parasitization, can lead to coevolutionary alternation. Finally, we discuss the potential for parasitoid diversification in relation with the fact that most observed geographic situations, for different parasitoid clades, correspond to coevolutionary cold spots, due to fixation of virulence in parasitoid taxa.

Interactions between Drosophila hosts and parasitoid wasps are among the few examples in which occurrence of intraspecific variation of parasite success has been studied in natural populations. Such variations can originate from three categories of factors: environmental, host and parasitoid factors. Under controlled laboratory conditions, it is possible to focus on the two last categories, and, using specific reference lines, to analyze their respective importance. Parasitoid and host contributions to variations in parasite success have largely been studied in terms of evolutionary and mechanistic aspects in two Drosophila parasitoids, Asobara tabida and, in more details, in Leptopilina boulardi. This chapter focuses on the physiological and molecular aspects of L. boulardi interactions with two Drosophila host species, while most of the evolutionary hypotheses and models are presented in Chapter 11 of Dupas et al.

International audience

International audience

uploaded by Plazi from Taxodros