Geographic variation and trade‐offs in parasitoid virulence

Abstract

Summary1. Host-parasitoid systems are characterized by a continuous development of new defence strategies in hosts and counter-defence mechanisms in parasitoids. This co-evolutionary arms race makes host-parasitoid systems excellent for understanding trade-offs in host use caused by evolutionary changes in host immune responses and parasitoid virulence. However, knowledge obtained from natural host-parasitoid systems on such trade-offs is still limited. 2. In this study, the aim was to examine trade-offs in parasitoid virulence in Asecodes parviclava (Hymenoptera: Eulophidae) when attacking three closely related beetles: Galerucella pusilla, Galerucella calmariensis and Galerucella tenella (Coleoptera: Chrysomelidae). A second aim was to examine whether geographic variation in parasitoid infectivity or host immune response could explain differences in parasitism rate between northern and southern sites. 3. More specifically, we wanted to examine whether the capacity to infect host larvae differed depending on the previous host species of the parasitoids and if such differences were connected to differences in the induction of host immune systems. This was achieved by combining controlled parasitism experiments with cytological studies of infected larvae. 4. Our results reveal that parasitism success in A. parviclava differs both depending on previous and current host species, with a higher virulence when attacking larvae of the same species as the previous host. Virulence was in general high for parasitoids from G. pusilla and low for parasitoids from G. calmariensis. At the same time, G. pusilla larvae had the strongest immune response and G. calmariensis the weakest. These observations were linked to changes in the larval hemocyte composition, showing changes in cell types important for the encapsulation process in individuals infected by more or less virulent parasitoids. 5. These findings suggest ongoing evolution in parasitoid virulence and host immune response, making the system a strong candidate for further studies on host race formation and speciation.

Comparative genetics of invasive populations of walnut aphid, Chromaphis juglandicola, and its introduced parasitoid, Trioxys pallidus, in California

“…However, empirical evidence for reciprocal selection and coadaptation remains limited and best represented among species with antagonistic interactions (Carmona, Fitzpatrick, & Johnson, ). In this context, the conflict between insect hosts and their parasitoids is of particular interest as the interaction involves the death of one of the two participants, and thus, strong reciprocal selection might be expected to result in dynamic coevolution (Abrams, ; Dupas, Carton, & Poirie, ; Fors, Markus, Theopold, Ericson, & Hambäck, ; Kraaijeveld, Van Alphen, & Godfray, ). Both variations in host resistance to parasitism and in parasitoid counter resistance have been documented for Drosophila melanogaster Meigen and its parasitoids (Kraaijeveld & Godfray, , ; Lynch, Schlenke, & de Roode, ), and the influence of defensive symbionts on the antagonistic coevolution of aphids and their parasitoids has been particularly well studied (e.g., Dion, Zele, Simon, & Outreman, ; Nyabuga, Loxdale, Heckel, & Weisser, ; Oliver, Russell, Moran, & Hunter, ; Rouchet & Vorburger, ; Schmid, Sieber, Zimmerman, & Vorburger, ; Vorburger, , ).…”

“…Both variations in host resistance to parasitism and in parasitoid counter resistance have been documented for Drosophila melanogaster Meigen and its parasitoids (Kraaijeveld & Godfray, , ; Lynch, Schlenke, & de Roode, ), and the influence of defensive symbionts on the antagonistic coevolution of aphids and their parasitoids has been particularly well studied (e.g., Dion, Zele, Simon, & Outreman, ; Nyabuga, Loxdale, Heckel, & Weisser, ; Oliver, Russell, Moran, & Hunter, ; Rouchet & Vorburger, ; Schmid, Sieber, Zimmerman, & Vorburger, ; Vorburger, , ). However, we have surprisingly little knowledge of how genetic variation influences host–parasitoid coevolution in the field (but see Henter, ; Henter & Via, ; and Fors et al., ).…”

Selection by parasitoid females among closely related hosts based on volatiles: Identifying relevant chemical cues

“…In this system, the host species vary in resistance to parasitoid attack, and it is notable that G. calmariensis (that was selected by parasitoids originating from G. calmariensis in the trials) is the species showing the lowest resistance to parasitoid attack (Fors et al., , ). In fact, Fors et al.…”

“…However, as the parasitoid females in this system do not have zero fitness when attacking the resistant host, the ability to attack this host could be retained in the population, assuming that it allows survival during times when the susceptible host is not available. Indeed, females of A. parviclava lay eggs in G. pusilla when no other option is at hand (Fors et al., , ). On the other hand, what we are observing may be an evolutionary transition phase, where the ability to differentiate between the two hosts will change from an acquired to a genetically fixed behavior where volatile proportions determine host selection (Davis & Stamps, ; De Moraes et al., ).…”

“…Studies on the population variation in the interactions between A. parviclava and its host species also suggest that there is population variation in parasitoid virulence. Parasitoids originating from sites with a relatively high proportion of G. pusilla (the resistant host) have a higher virulence (Fors et al., ). It is possible that there is also genetic differentiation in the ability of females to select among the host species, but we currently have no data to support such a pattern.…”