Naturally-occurring variation in the relationship between a symbiont, Wolbachia, and its host, Drosophila simulans

Wolbachia, a maternally inherited endosymbiont of arthropods, is commonly known for its extreme reproductive manipulations of its hosts, and, more recently, for its potential use in reducing insect-borne diseases. Twenty years ago, a cytoplasmic incompatibility (CI)-inducing strain of Wolbachia, wRi, was shown to decrease the fecundity of infected Drosophila simulans females in California, making it an apparent parasite of this species, at least under laboratory conditions. However, extensive field surveys have documented the rapid spread of wRi-infected D. simulans through North America, Europe and Australia. To better understand this symbiont-host interaction, we assayed variation in levels of CI and fecundity effects in a natural orchard population. We found that multiple lines of wRi conferred significantly lower CI in their isofemale line than a control line. We also found an isofemale line, “Y36”, of D. simulans that showed severely decreased fecundity when uninfected, or when infected with wRi from other isofemale lines. To learn more about the cell biology of this aberrant line, we researched its cell localization patterns in D. simulans, compared to lines with no obvious fitness effects of infection. We found that Y36 showed higher concentration outside of cells in the brain than control lines, which primarily concentrate in the gonads. Our results suggest that, in wRi alone, there can be a wide variety of cell localization patterns, fitness effects, and more generally, host-symbiont relationships. In conclusion, we believe that this variation points to the untapped potential for naturally occurring variants of Wolbachia that could be useful in disease research.