Wolbachia do not live by reproductive manipulation alone: infection polymorphism in Drosophila suzukii and D. subpulchrella

Abstract

Drosophila suzukii recently invaded North America and Europe. Populations in Hawaii, California, New York and Nova Scotia are polymorphic for Wolbachia , typically with Wolbachia in D. suzukii , denoted w S uz, is closely related to w R i, the variant prevalent in continental populations of D. simulans . w S uz is also nearly identical to Wolbachia found in D. subpulchrella , plausibly D. suzukii 's sister species. This suggests vertical Wolbachia transmission through cladogenesis (‘cladogenic transmission’). The widespread occurrence of 7–20% infection frequencies indicates a stable polymorphism. w S uz is imperfectly maternally transmitted, with wild infected females producing on average 5–10% uninfected progeny. As expected from its low frequency, w S uz produces no cytoplasmic incompatibility (CI), that is, no increased embryo mortality when infected males mate with uninfected females, and no appreciable sex‐ratio distortion. The persistence of w S uz despite imperfect maternal transmission suggests positive fitness effects. Assuming a balance between selection and imperfect transmission, we expect a fitness advantage on the order of 20%. Unexpectedly, Wolbachia ‐infected females produce fewer progeny than do uninfected females. We do not yet understand the maintenance of w S uz in D. suzukii . The absence of detectable CI in D. suzukii and D. subpulchrella makes it unlikely that CI‐based mechanisms could be used to control this species without transinfection using novel Wolbachia . Contrary to their reputation as horizontally transmitted reproductive parasites, many Wolbachia infections are acquired through introgression or cladogenesis and many cause no appreciable reproductive manipulation. Such infections, likely to be mutualistic, may be central to understanding the pervasiveness of Wolbachia among arthropods.

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