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Genome of Australian Fly Evolves to Coexist with Killer Bacteria


When a second Wolbachia strain is present, females flies produce female offspring—making it a "male-killer." Credit: Tank Monsternova
When a second Wolbachia strain is present, females flies produce female offspring—making it a "male-killer." Credit: Tank Monsternova

Researchers at the University of Melbourne have made a groundbreaking discovery about how the genome of the Drosophila pseudotakahashii fly, endemic to the east coast of Australia, has evolved to coexist with endosymbiont bacteria known as Wolbachia. In a paper published in the journal PLOS Biology, the team revealed that the genome of the fly has developed an ability to suppress the negative effects of Wolbachia, which can include killing the host's embryos and changing the sex ratio of offspring. This has led to an evolutionary "arms race" between the fly and its bacterial endosymbiont.


The Microbiome's Influence on Host Organisms


The composition of the microbiome, the community of fungi, bacteria and viruses that usually exist in the gut, can have a significant impact on the health of host organisms. Insects, in particular, are affected by microorganisms that exist not only in the gut of the host insect but also inside the host's cells. These intracellular microorganisms include bacteria that are passed on from mothers to their offspring.


Endosymbionts: Effects on Hosts and Arms Races


Endosymbionts can have both positive and negative effects on their hosts. The provision of nutrition to cells and protecting host cells against viruses are some of the positive effects. However, the negative effects can lead to killing the host's embryos and changing the sex ratio of offspring the host produces. When the endosymbionts are detrimental to the host, the host genome might evolve to block these effects. This can lead to an evolutionary "arms race" between the host and its bacterial endosymbiont.



The Case of Drosophila Pseudotakahashii


All members of the Drosophila pseudotakahashii fly carry an endosymbiont bacteria known as Wolbachia, which is passed from mother to offspring. However, when antibiotics are used to cure flies of this Wolbachia strain, the flies survive. In any mating that takes place between males infected by Wolbachia and females that no longer carry the infection, the offspring of these females die. This results in a significant advantage for females that carry the infection. The situation becomes more complicated when a second Wolbachia strain exists that can infect flies alongside the first strain. When the second strain is present, female flies only produce female offspring, making it effectively a "male-killer." This sets up another arms race where the host must evolve a way of suppressing the male-killer bacteria.


Suppression of the Male-Killer Bacteria


The research team observed this arms race in action in the Drosophila pseudotakahashii fly, producing very rapid changes in a specific region of the fly's genome. Over a few weeks, the fly populations with the male-killer bacteria were able to start producing males again, despite the presence of the second Wolbachia strain. The team was able to identify some genes that are likely to be involved in this "male-killer suppression."


Implications of the Research


This discovery has broader implications as we look towards manipulating the microbiome of organisms to encourage beneficial effects. For instance, Wolbachia strains living inside mosquitoes are being deliberately spread around the world because the bacteria can block the transmission of some mosquito-borne diseases like Dengue Fever. But what if the host's genome evolves to stop these effects? Or what if bacteria that kill males could be released to suppress pest populations? The research team warns that we are at the start of a journey when it comes to microorganisms and the more we learn, the more we need to expect the unexpected.


Journal Information: Kelly M. Richardson et al, A male-killing Wolbachia endosymbiont is concealed by another endosymbiont and a nuclear suppressor, PLOS Biology (2023). DOI: 10.1371/journal.pbio.3001879
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