The effects of, and interactions between, Cardinium and Wolbachia in the doubly infected spider mite Bryobia sarothamni.

Many arthropods are infected with vertically transmitted, intracellular bacteria manipulating their host's reproduction. Cytoplasmic incompatibility (CI) is commonly observed and is expressed as a reduction in the number of offspring in crosses between infected males and uninfected females (or females infected with a different bacterial strain). CI is often related to the presence of Wolbachia, but recent findings indicate that a second reproductive parasite, Cardinium, is also capable of inducing CI. Although both Wolbachia and Cardinium occur in arthropods and may infect the same host species, little is known about their interactions. We observed Wolbachia and Cardinium in the sexual spider mite Bryobia sarothamni (Acari: Tetranychidae) and investigated the effects of both bacteria on reproduction. We performed all possible crossing combinations using naturally infected strains, and show that Cardinium induces strong CI, expressed as an almost complete female mortality. B. sarothamni is the third host species in which Cardinium-induced CI is observed, and this study reveals the strongest CI effect found so far. Wolbachia, however, did not induce CI. Even so, CI was not induced by doubly infected males, and neither singly Wolbachia-infected nor doubly infected females could rescue CI induced by Cardinium-infected males. Possibly, this is related to the differences between Cardinium strains infecting singly and doubly infected individuals. We found a cost of infection in single infected individuals, but not in doubly infected individuals. We show that infection frequencies in field populations ranged from completely uninfected to a polymorphic state. In none of the populations infections were fixed.