Wolbachia is an intracellular bacterium that infects many insects. In doing so, it actually alters how insects reproduce.
Wolbachia is inherited by female offspring, so these bacteria selfishly alter the sex ratios of insect populations, ensuring that more females than males are produced. Specifically, Wolbachia induces an embryonically lethal condition called cytoplasmic incompatibility in crosses between infected males and uninfected females (crosses that would not pass on Wolbachia to the insects’ offspring).
According to study author Seth Bordenstein, “we've known for decades that one of the secrets to Wolbachia's success is that it interferes with host reproduction in order to spread itself through females. But how the bacterium did it was a major mystery for the field.” In the most recent issue of Nature, researchers from Vanderbilt and Yale Universities report that the genes cifA and cifB control CI.
To identify the CI genes, the researchers compared the genomes of Wolbachia strains that do or do not cause CI. This approach identified two genes - cifA and cifB. When they inserted these genes into fruit flies, crosses between males expressing the CI genes and wild type females yielded hatch rates that were 68% lower than crosses between uninfected wild type flies.
This decreased hatch rate was rescued, however, if the males expressing the CI genes were crossed with infected females.
CI induced by Wolbachia is lethal to embryos for a number of reasons. Most often, cell division is arrested after two to three mitotic divisions, chromatin bridging occurs, or regional failures of cell division occur in the developing embryo.
When cifA and cifB were both expressed in male fruit flies, these types of abnormalities were observed in embryos that resulted from crosses with uninfected females. As expected, however, these abnormalities were prevented if the transgenic males were crossed with Wolbachia-infected females. Interestingly, both cifA and cifB must be expressed together for CI to occur - expressing just one of the genes does not produce a phenotype.
These findings have some big implications for controlling insect-borne diseases like Zika and malaria. Interestingly, Wolbachia-infected mosquitoes cannot spread viruses like dengue or Zika. Both Vanderbilt and Yale have applied for patents that would allow them to engineer insects or bacteria that halt the spread of vector-borne diseases.
Allowing Wolbachia to infect an entire population of mosquitoes would, in theory, curb the spread of these viruses.Alternately, introducing Wolbachia-infected males into the population would reduce the size of the mosquito population (assuming they mated with uninfected females).