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

Drosophila suzukii recently invaded North America and Europe. Populations
in Hawaii, California, New York and Nova Scotia are polymorphic for
Wolbachia, typically with <20% infection frequency. The Wolbachia
in D. suzukii, denoted wSuz, is closely related to wRi, the variant
prevalent in continental populations of D. simulans. wSuz 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. wSuz is imperfectly maternally transmitted, with wild
infected females producing on average 5-10% uninfected progeny. As
expected from its low frequency, wSuz produces no cytoplasmic
incompatibility (CI), i.e., no elevated embryo mortality when infected
males mate with uninfected females, and no appreciable sex-ratio
distortion. The persistence of wSuz 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 wSuz 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.