Superinfection of cytoplasmic incompatibility-inducing Wolbachia is not additive in Orius strigicollis (Hemiptera: Anthocoridae)

Cytoplasmic incompatibility (CI) allows the intracellular, maternally inherited bacterial symbiont Wolbachia to invade arthropod host populations by inducing infertility in crosses between infected males and uninfected females. The general pattern is consistent with a model of sperm modification, rescued only by egg cytoplasm infected with the same strain of symbiont. The predacious flower bug Orius strigicollis is superinfected with two strains of Wolbachia, wOus1 and wOus2. Typically, superinfections of CI Wolbachia are additive in their effects; superinfected males are incompatible with uninfected and singly infected females. In this study, we created an uninfected line, and lines singly infected with wOus1 and wOus2 by antibiotic treatment. Then, all possible crosses were conducted among the four lines. The results indicated that while wOus2 induces high levels of CI, wOus1 induces very weak or no CI, but can rescue CI caused by wOus2 to a limited extent. Levels of incompatibility in crosses with superinfected males did not show the expected pattern. In particular, superinfected males caused extremely weak CI when mated with either singly infected or uninfected females. An analysis of symbiont densities showed that wOus1 densities were significantly higher than wOus2 densities in superinfected males, and wOus2 densities were lower, but not significantly, in superinfected relative to singly infected males. These data lend qualified support for the hypothesis that wOus1 interferes with the ability of wOus2 to cause CI by suppressing wOus2 densities. To our knowledge, this is the first clear case of non-additive CI in a natural superinfection.