Wolbachia is a group of bacteria, perhaps the most abundant reproductive parasite in the world. I first heard about them in my final year studies: we were talking about how bacteria are able to manipulate the hosts they infect for their own benefit, and how Wolbachia will dictate whether mating between their insect hosts is successful or not. This was first discovered in the 70s, when scientists at UCLA found that the sperm of infected mosquitos killed the female’s eggs. The phenomenon is called cytoplasmic incompatibility because the bacteria reside in the cytoplasm (the material within a cell). This helps the bacteria to spread through a population of insects, advancing at a rate of up to 100km per year.

Indeed, Wolbachia probably rely on their hosts for survival, as you can’t grow them by themselves. They live in symbiosis with an amazing range of insects and other invertebrates. That is to say, they have an intimate and often interdependent relationship with their hosts. They fit into this story because they also live in symbiosis with the filarial worms that cause elephantiasis. They exist within the parasites that infect humans, creating a bizarre Russian doll situation. As a side note, there is fairly recent evidence that Wolbachia rely on a virus inserted within their own genetic code, to bring about the cytoplasmic incompatibility mentioned previously. Four levels of nested interaction make for a pretty intricate system, by anyone’s estimation.

Regardless, the worms themselves rely heavily on Wolbachia: without their symbionts they very quickly become sterile and die. In studies where antibiotics were used to kill the bacteria, filarial embryos were severely deformed, suggesting that the bacteria are essential in the early development of worm offspring.

They are also needed for proper regulation of apoptosis, a kind of controlled cell death. Without Wolbachia there is a dramatic increase in apoptosis(and sterility), demonstrating that host and symbiont have coevolved so that this particular aspect of regulation is outsourced (with dire consequences if the symbiont is absent). This is both true for cells of the body, and those of the germline (the cells that pass on their genetic material to offspring). In fact, Wolbachia actually live inside the germline cells of their hosts, and have the amazing peculiarity of being transmitted by the host female germline to offspring, like mitochondria are in humans.

Through this process, called vertical transmission, the Wolbachia have become master manipulators of their hosts to make sure they’re passed on to the next generation. The worms depend on Wolbachia in the germline for two reasons: they maintain a level of dormancy, ensuring a steady stream of egg production for many years, and they encourage the cells to proliferate. This is a rare example of developmental symbiosis, but the obvious application of this knowledge is to target the bacteria and kill the worms indirectly, something I will come onto next.