Wolbachia in Rhagoletis cerasi

Research project P 14024 Wolbachia in Rhagoletis cerasi Christian STAUFFER 24.01.2000 Characterization of the cytoplasmic incompatibility types of Wolbachia in single and double infected Rhagoletis cerasi (Diptera, Tephritidae) Crossing experiments between populations of the European cherry fruit fly Rhagoletis cer4si showed patterns of unidirectional incompatibility. Females from northern populations crossed with males from southern populations did not yield viable eggs whereas the reciprocal cross resulted in normal numbers of offspring. Central European R. cerasi populations were screened by PCR using Wolbachia specific primers. All individuals were found to be infected by Wolbachia, a protcobacteria which can cause cytoplasmic incompatibility in insects. RFLP revealed the presence of two different Wolbachia strains in southwestern, populations and single infections with one of the two strains in northeastern -populations. This project will focus on the characterization of the cytoplasmic incompatibility types of the Wolbachia infections in R. cerasi. The present geographic distribution of both Wolbachia strains will bestudied by screening European populations and more detailed by the screening of individuals collected in transects through the incompatibility zone. Crossing experiments of field collected individuals will reveal the level of cytoplasmic incompatibility, transmission rate and segregation of the Wolbachia strains in the field. Transinfection experiments will allow the comparison of both Wolbachia strains from R. cerasi with a closely related strain of DrosophiIa simulans, from Coffs Harbour.

Wolbachia (a-proteobacteria) is an intracellular, maternally inherited bacterium which has been detected in many arthropods and nematodes. Wolbachia infections disperse in host populations by mechanisms such as cytoplasmic incompatibility (CI). CI leads to embryonic mortality which occurs when infected males mate with uninfected females or females infected with a different Wolbachia strain. Populations of the European cherry fruit fly Rhagoletis cerasi (Diptera, Tephritidae) were found to be infected by two different strains, wCer1 and wCer2. Infections with wCer1 were fixed in all European populations, and superinfections with both strains, wCer1&2 occurred in southern and central Europe. Strong unidirectional incompatibility between the southern - central and the surrounding European populations of R. cerasi had already been reported in the 1970s. From the conformity in the geographic distribution of the Wolbachia infections and CI it was deduced that wCer2 caused CI in R. cerasi. A comparison between the geographic distributions showed that wCer1&2 spread into wCer1 infected populations in some areas, which was the first report on the dispersal of a superinfection in already infected field populations. In some regions wCer1&2 spread up to 350 km in 25 years. In other regions, a spread of wCer1&2 was prevented by dispersal barriers. There, a sharp transition from singly to doubly infected populations was detected and suggested regional isolation between fixed wCer1 and wCer1&2 infected populations. By the process of CI, the mitochondrial haplotype infected with wCer1&2 was observed to hitchhike through wCer1 infected populations, replacing thereby the original haplotype associated with wCer1 and reducing mitochondrial polymorphism in its host. Phylogenetic analysis of the haplotypes associated with the Wolbachia strains in R. cerasi revealed that wCer1 is the "older" infection which occurred more than 330,000 years ago. wCer1 was associated with four different haplotypes, whereas wCer2 had only one. wCer2 invaded R. cerasi after the last glacial period, which was less than 20,000 years ago. A phylogeographic study indicated that the wCer2 infection occurred first in Italy. Wolbachia is mainly transmitted vertically, from mothers to the offspring. Data of the infection and haplotype screening of R. cerasi indicated that intraspecific horizontal transmission, from wCer1&2 to wCer1 infected individuals, happened occasionally. In an experimental approach, wCer1&2 were transferred from R. cerasi to Drosophila simulans (Diptera, Drosophilidae). Both Wolbachia strains segregated in the new host. wCer1 was eventually lost, but wCer2 was maintained in laboratory lines. wCer2 induced CI in the new host, however at a lower level than in R. cerasi. wCer2 would not invade natural populations of D. simulans because of low CI, inefficient maternal transmission and negative fitness costs to the host. D. simulans is naturally infected by different Wolbachia strains. Some of them are inducing high levels of CI, others do not, like wAu, phylogenetically close to wCer2. It was unknown whether the absence of CI expression in D. simulans by wAu infections is due to a bacterial loss of function or due to host control. The results of the transfer experiment of wCer2 into D. simulans suggested that the expression of CI might be controlled by the new host. The varying levels of CI induced by wCer2 in D. simulans lines suggested that the wCer2 infection was polymorphic in its original host. A specialised parasitoid of R. cerasi, Opius rhagoleticola (Hymenoptera, Braconidae), was found infected with wCer2, which indicated that a horizontal transfer of wCer2 between both host species must have occurred in nature.