Project description:The bacterial endosymbiont Wolbachia has been used to control insect pests owing to its ability to manipulate their life history and suppress infectious diseases. Therefore, knowledge on Wolbachia dynamics in natural populations is fundamental. The European cherry fruit fly, Rhagoletis cerasi, is infected with the Wolbachia strain wCer2, mainly present in southern and central European populations, and is currently spreading into wCer2-uninfected populations driven by high unidirectional cytoplasmic incompatibility. Here, we describe the distribution of wCer2 along two transition zones where the infection is spreading into wCer2-uninfected R. cerasi populations. Fine-scale sampling of 19 populations in the Czech Republic showed a smooth decrease of wCer2 frequency from south to north within a distance of less than 20 km. Sampling of 12 Hungarian populations, however, showed a sharp decline of wCer2 infection frequency within a few kilometres. We fitted a standard wave equation to our empirical data and estimated a Wolbachia wave speed of 1.9 km yr-1 in the Czech Republic and 1.0 km yr-1 in Hungary. Considering the univoltine life cycle and limited dispersal ability of R. cerasi, our study highlights a rapid Wolbachia spread in natural host populations.
Project description:The endosymbiont Wolbachia can manipulate arthropod host reproduction by inducing cytoplasmic incompatibility (CI), which results in embryonic mortality when infected males mate with uninfected females. A CI-driven invasion of Wolbachia can result in a selective sweep of associated mitochondrial haplotype. The co-inheritance of Wolbachia and host mitochondrial DNA can therefore provide significant information on the dynamics of an ongoing Wolbachia invasion. Therefore, transition zones (i.e., regions where a Wolbachia strain is currently spreading from infected to uninfected populations) represent an ideal area to investigate the relationship between Wolbachia and host mitochondrial haplotype. Here, we studied Wolbachia-mitochondrial haplotype associations in the European cherry fruit fly, Rhagoletis cerasi, in two transition zones in the Czech Republic and Hungary, where the CI-inducing strain wCer2 is currently spreading. The wCer2-infection status of 881 individuals was compared with the two known R. cerasi mitochondrial haplotypes, HT1 and HT2. In accordance with previous studies, wCer2-uninfected individuals were associated with HT1, and wCer2-infected individuals were mainly associated with HT2. We found misassociations only within the transition zones, where HT2 flies were wCer2-uninfected, suggesting the occurrence of imperfect maternal transmission. We did not find any HT1 flies that were wCer2-infected, suggesting that Wolbachia was not acquired horizontally. Our study provides new insights into the dynamics of the early phase of a Wolbachia invasion.
Project description:Elucidating the mechanisms and conditions facilitating the formation of biodiversity are central topics in evolutionary biology. A growing number of studies imply that divergent ecological selection may often play a critical role in speciation by counteracting the homogenising effects of gene flow. Several examples involve phytophagous insects, where divergent selection pressures associated with host plant shifts may generate reproductive isolation, promoting speciation. Here, we use ddRADseq to assess the population structure and to test for host-related genomic differentiation in the European cherry fruit fly, Rhagoletis cerasi (L., 1758) (Diptera: Tephritidae). This tephritid is distributed throughout Europe and western Asia, and has adapted to two different genera of host plants, Prunus spp. (cherries) and Lonicera spp. (honeysuckle). Our data imply that geographic distance and geomorphic barriers serve as the primary factors shaping genetic population structure across the species range. Locally, however, flies genetically cluster according to host plant, with consistent allele frequency differences displayed by a subset of loci between Prunus and Lonicera flies across four sites surveyed in Germany and Norway. These 17 loci display significantly higher FST values between host plants than others. They also showed high levels of linkage disequilibrium within and between Prunus and Lonicera flies, supporting host-related selection and reduced gene flow. Our findings support the existence of sympatric host races in R. cerasi embedded within broader patterns of geographic variation in the fly, similar to the related apple maggot, Rhagoletis pomonella, in North America.