Bedbugs (Cimex lectularius) have received a lot of attention lately for their role as a human parasite. But it turns out that bedbugs are involved with another interesting ecological interaction – their mutualism with the bacterium Wolbachia. And now a collaborative study with researchers from multiple institutions in Japan has unravelled the mechanism for how a bacterial infection can end up being mutually beneficial.
The Wolbachia genus of bacteria infects a broad array of invertebrates, including nematodes, insects and spiders. Usually, it acts as a parasite, causing reproductive problems for its host. But in the bedbug, Wolbachia infection has positive effects on growth and reproductive output. Vertebrate blood (the bedbug's favourite meal) is low in B vitamins and bedbugs rely on their bacterial partner to synthesize them. Naruo Nikoh and his colleagues set out to investigate the origin of the relationship between these two odd bedfellows by examining the genome of the strain of Wolbachia, called ‘wCle’, that infects bedbugs.
To test whether wCle was necessary for synthesis of vitamin B7 (biotin) and the development of bedbugs, the researchers raised bedbug nymphs on a diet of rabbit blood that was either supplemented with antibiotics (to produce wCle-free bedbugs) or unsupplemented (to produce bedbugs with a healthy population of wCle). In addition, they supplemented the diets of some of the nymphs with vitamin B7. Investigating the impact of each diet on the bedbugs and their bacterial lodgers, the team found that bedbugs that had a healthy wCle population had higher concentrations of vitamin B7 while bedbugs that had been cured of wCle infection and raised on rabbit blood without vitamin B7 were much less likely to survive to adulthood.
After sequencing and assembling the wCle genome, the researchers examined the genes responsible for synthesizing B vitamins. They found that while most Wolbachia species have genes that can synthesize B2, as well as some of the genes necessary to synthesize vitamins B6 and B9, wCle can also synthesize vitamin B7. Also, the genes involved in vitamin B7 synthesis were extremely similar to those involved in the synthesis in other bacteria that live as endosymbionts. The researchers concluded that wCle may have received the genes through lateral gene transfer from one of these other species while infecting the same animal.
Finally, they compared the genes of proteins involved in vitamin B7 synthesis in wCle with those in a Wolbachia species that infects the bedbug's closely related sister species Cimex japonicus, the bat bug. They found that the two sets of genes were almost identical, suggesting that the biotin synthesis genes were transferred to a Wolbachia strain and that it then infected the common ancestor of the bedbug and bat bug.
Taken together, the authors think that the lateral transfer of vitamin B7 synthesis genes is the event that permitted wCle and bedbugs to strike up their symbiotic relationship. Although understanding that wCle makes bedbugs stronger might not be good news for humans worried about bedbugs sharing our homes, knowing how that relationship came about could help us understand the ecology of symbiosis a little better.
