How A Microbe Became A Living Supplement For A Tiny Vampire

Bedbugs have been sucking our blood for millennia and after a brief retreat following World War II, they are back and more numerous than ever. Infestations are rising, hotels are worried, and people are very, very itchy. But the bedbug isn’t solely responsible for its success. It has an accomplice.

Mammal blood is an unusual meal. It’s loaded with proteins and fats, but bereft of some vital nutrients like B vitamins. If you’re going to make a living off vampirism, you need to somehow supplement your diet. Bedbugs do it with bacteria. Scientists have suspected as much since the 1920s when they discovered that bedbugs have a pair of organs called bacteriomes that are full of bacteria. But it wasn’t till 2010 that Takema Fukatsu from the National Institute of Advanced Industrial Science and Technology in Japan finally identified the species that lives in these organs.

It’s called Wolbachia. Fukatsu found it in the bacteriomes of every bedbug he examined, and showed that females pass it to their offspring when they are still embryos. Wolbachia provides the insect with the B vitamins that it misses in its diet; kill the bacterium, and the bedbug grows slowly and can’t reproduce.

Wolbachia is arguably one of the most successful bacteria on the planet, colonising two out of every three species of insect. It’s not surprising to find it in a bedbug, but its role as a living supplement is stranger. Biologists typically see Wolbachia as a parasite. Since it always passes from mother to offspring, it has no need for males and has evolved many strategies for getting rid of them, including killing them outright or converting them to females.

But in the bedbug, it does nothing of the kind. Instead, this selfish “master manipulator” has become an important ally. It’s more body part than puppet master. How did it evolve that way? Once again, Fukatsu’s team has the answer: it borrowed a package of genes from other bacteria.

The team, including Naruo Nikoh, Takahiro Hosokawa and Minoru Moriyama, sequenced the genomes of Wolbachia from several bedbugs in Japan and Australia. Inside, they found an unusual cluster of six genes that allow these microbes to make vitamins B7 (biotin) and B1 (thiamine).

These genes are vital. If the team killed off the Wolbachia strains with antibiotics, the bedbugs couldn’t grow normally or reproduce unless they were fed on blood that was fortified with B vitamins. And when the team specifically took biotin out of this supplemental cocktail, the insects suffered again. The bedbugs depend on biotin and, by extension, on the microbes that make it.

The biotin cluster isn’t a feature of Wolbachia in general; the team only found it in strains that infect bedbugs (and the closely related batbugs). But it also exists in many other bacteria that infect insects. This strongly suggests that this sextet of genes isn’t an invention, but a loan. The ancestor of the bedbug’s Wolbachia picked it up from another microbe that infected the same host. “These genes are rampantly moving across diverse bacterial lineages,” says Fukatsu.

This is evolution in sixth gear: fast and dramatic. Wolbachia picked up the right genes from another species, and its bedbug host suddenly gained the ability to supplement its diet. That event may well have sealed the partnership between the insect and the microbe.

The partnership must be a recent one. The genomes of bacteria that live inside insect cells—endosymbionts—always waste away with time, abandoning any genes that they once needed for a free-living existence. But the bedbug’s Wolbachia still has a relatively intact genome, which means that it hasn’t been an endosymbiont for very long.

What will happen to it as the millennia tick by? We can find clues by looking at other insects. Aphids have similar problems to bedbugs. They drink a fluid—plant sap—that lacks a number of important nutrients, and they rely on an endosymbiont called Buchnera to supplement their diet.  These two have been co-evolving since the dawn of the dinosaurs. If you chart the family tree of Buchnera strains, it looks almost identical to the family tree of aphids. And while killing Wolbachia harms a bedbug, killing Buchnera kills its aphid host too.

Plant sap lacks more nutrients than blood, so the symbionts of sap-suckers tend to be more indispensible than those of blood-feeders. Still, it’s conceivable that the bedbug’s Wolbachia could head down the same evolutionary path as the aphid’s Buchnera.

A bedbug, then, is a continuously changing concept. One evolutionary moment, it’s an insect. The next, it’s an insect with a bacterial partner. Perhaps in the future, the two will become so inextricably linked that it makes no sense to talk about them as parts of a whole—just as a whole. After all, there’s at least one insect with Wolbachia’s entire genome inside its own.

Reference: Nikoh, Hosokawa, Moriyama, Oshima, Hattori & Fukatsu. 2014. Evolutionary origin of insect–Wolbachia nutritional mutualism. http://dx.doi.org/pnas.org/cgi/doi/10.1073/pnas.1409284111