Facultative endosymbionts of aphid populations from coastal dunes of the North Sea

Aphids establish symbiotic associations with a diverse assemblage of mutualistic bacteria. Some of them are not required for the host's survival but still have a crucial impact on the biology and ecology of their host. Facultative symbionts may modify important host-life-history traits and affect the interactions of aphids with other members of the community. So far several species of aphid have been reported to occur in coastal dunes. Given the extreme environmental conditions of this type of habitat and the wide distribution along the European coast of some aphid species, these aphids would be expected to show variation in their facultative endosymbionts. However, there is currently no information available for these species. To address this question, we collected specimens from different populations of aphids (i.e. Schizaphis rufula, Laingia psanunae and Rhopalosiphum padi) associated with the dune grass Ammophila arenaria in several locations of the North and the Irish Sea. By means of specific diagnostic PCR's we checked for the presence of facultative bacterial endosymbionts in these populations. Results of this explorative assessment showed variation in the endosymbiont community according to species and location. All populations sampled along the North Sea coast were associated with the facultative endosymbiont Serratia symbiotica. Hamiltonella defensa was only detected in some specimens coming from the population in Het Zwin, Belgium. Regiella insecticola and the gamma-protobacteria X-type were only found associated with the population of Schizaphis rufula in De Panne, Belgium. Although further experiments are necessary to characterize the nature of these symbiotic relationships, our correlation analyses showed a significant co-occurrence of S. symbiotica with H. defensa and R. insecticola with X-type protobacteria suggesting reciprocal regulatory functions. No significant correlation was detected between the number of mummies (i.e. carcasses of aphids parasitized by wasps) and the occurrence of bacterial symbionts. The potential role of these symbionts in coastal dune ecosystems is discussed.