Hidden Wolbachia diversity in field populations of the European cherry fruit fly, Rhagoletis cerasi (Diptera, Tephritidae)

The European cherry fruit fly Rhagoletis cerasi has been a field model for cytoplasmic incompatibility since the mid 1970s. Two Wolbachia strains were detected in this tephritid species and wCer2 was described as the CI inducing agent dividing European populations into two unidirectional incompatible groups, i.e. southern females produce viable offspring with northern males, whereas the reciprocal cross results in incompatibility. We detected three new Wolbachia strains by sequencing a multitude of plasmids derived from Wolbachia surface protein gene (wsp) polymerase chain reaction (PCR) products. Strain-specific primers were developed allowing individual diagnosis without need for cloning. Hybridization of specific PCR products with a wsp oligonucleotide enhanced the detection limit significantly and revealed the presence of low-titre infections in some strains, in different ontogenetic stages and in adults of different age. We then performed a survey of strain prevalence and infection frequency in eight European regions. wCer1 was fixed in all populations, whereas wCer2 was detected only in the South. wCer3 frequency was the lowest without a clear distribution pattern. The abundance of wCer4 was homogenous across Europe. Like wCer2, wCer5 showed significant differences in spatial distribution. Our new findings of previously undetected and recombinant Wolbachia strains in R. cerasi reveal a major caveat to the research community not to overlook hidden Wolbachia diversity in field populations. Low-titres and geographical variability in Wolbachia diversity are expected to influence the outcome of Wolbachia population dynamics and Wolbachia-based insect population control and may create invasion barriers for expanding and artificially introduced Wolbachia strains.