The Wolbachia pandemic | Symbionts spread rapidly across highly diverged flies
Words: Perran Ross
Cover photo: Andrew Weeks
Wolbachia are perhaps the most prevalent bacterial symbionts on earth. Of the millions of insect species, Wolbachia are estimated to infect up to half of them. These bacteria are renowned for the effects they exert on their hosts, which can often be quite dramatic. Some Wolbachia strains are highly pathogenic; for instance, the wMelPop, or “popcorn” strain causes the cells of their fly hosts to rupture, leading to an early death. In mosquitoes, the infection can render their proboscis bendy and completely useless for blood-sucking. However as Wolbachia can only live within the cells of host insects, it’s in their best interests to keep their hosts alive.
Wolbachia are transmitted from mother to offspring, and so they often employ strategies that increase the number of females that are infected with the bacteria. For instance, they can turn male offspring into females or even induce parthenogenesis, allowing females to produce more females without the need for males at all. Some strains can also provide their hosts with direct benefits, such as increasing the number of eggs they lay, or protecting them against pathogens. Please see our recent blog post for a general overview of Wolbachia.
These features enable Wolbachia infections to spread throughout populations of insects. In some cases, these Wolbachia “pandemics” can occur extremely rapidly. In the 1990s, Australian Drosophila simulans flies were mostly Wolbachia-free, with a low proportion being infected with a strain called wAu. By 2012, almost all flies across the eastern coast of Australia were now infected with the Wolbachia strain wRi, with the wAu strain nowhere to be seen. The wRi strain induces cytoplasmic incompatibility, which reduces the fertility of uninfected females that mate with males infected with Wolbachia. But infected females can produce viable offspring regardless of who they mate with, giving them the upper hand in reproduction.
Wolbachia strains have also been introduced deliberately into mosquito populations hijacking this same mechanism of cytoplasmic incompatibility. The Wolbachia strain wMel, which provides mosquitoes with anti-viral protection was introduced into Cairns, Australia, and now most mosquitoes in the area are less capable of transmitting dengue. But in this case Wolbachia were transferred artificially, by injecting Wolbachia into mosquito eggs in the laboratory. How is it that Wolbachia became so widespread in insects if it’s transmitted from a mother to her offspring?
A recent study from a collaboration between PEARG and Californian researchers documents the rapid spread of Wolbachia (wRi-like) into eight different species of Drosophila flies around the world. Some of these species are quite distant relatives, with their common ancestor being at least 10 million years in the past. But the Wolbachia they are infected with are much more closely related, diverging no more than 30 thousand years ago. Wolbachia host shifts were previously thought to occur very rarely, but in the case of these Drosophila species and wRi Wolbachia have jumped from fly species to fly species on multiple occasions and in a very short period of a few thousand years.
In nature, Wolbachia can be transferred from one species to another in two main ways. The first is though inter-breeding, but Wolbachia can only be transferred in this way when two species are closely related. This new study also finds evidence of horizontal transfer between distantly related Drosophila. In this case, the mechanism behind the host shift is unclear, but there are a few ways in which Wolbachia can jump to another species. The bacteria could hitch-hike via mites or parasitoids. Wolbachia might even be transferred through cannibalism and predation. However this was expected to be an extremely rare process occurring only every few million years. In contrast to the rapid spread of Wolbachia between species, the authors find no evidence of horizontal transfer occurring within species. This means that each infected individual of a species likely originated from a single infected female.
Another surprising finding of this research is that despite their genetic similarity, the wRi-like Wolbachia strains have different effects in different hosts. In Drosophila anomalata, Wolbachia induces cytoplasmic incompatibility but its counterpart in Drosophila suzukii has no effect on fly reproduction. This means that Wolbachia can still invade a species even without this reproductive advantage, though Wolbachia must induce some beneficial effects for the infection to persist.
wRi-like Wolbachia strains seem to be particularly good at spreading throughout populations once they are introduced to a novel host, and could therefore have applications for disease and pest control if they are introduced artificially into other insects. The wRi strain from Drosophila simulans has already been introduced into the dengue vector mosquito Aedes aegypti as a potential biological control agent. Although this strain doesn’t block dengue transmission very effectively, wRi has minimal negative effects on the mosquito and induces very strong cytoplasmic incompatibility. This could make it very useful for introducing anti-viral genes into areas where dengue is endemic, and could even be used for suppressing or eradicating mosquito populations through cytoplasmic incompatibility.