The body louse, a plague to humans and our ancestors for millions of years, subsists exclusively on our unwitting hospitality. Scientists have now parsed the modern human body louse's (Pediculus humanus humanus) genome, revealing a deep evolutionary dependence on humans and "remarkable completeness," despite being the shortest yet decoded in the insect group, the researchers wrote in a study published online June 21 in Proceedings of the National Academy of Sciences.

"The ecology of lice is very, very simple," entomologist Barry Pittendrigh, of the University of Illinois, Urbana, and study coauthor, said in a prepared statement. "It either lives in your hair or on your clothing, and it has one type of meal, and that's blood." The body louse is distinct from head lice (Pediculus humanus capitis) and public lice (or "crabs" Phthirus pubis), both of which also depend on human hosts.

Compared with more free-ranging louse relatives, the body louse has a much more limited range—both biologically and genetically. The body louse's genome contains just 108 million DNA base pairs, compared with the more than 3 billion base pairs in the human genome.

"Most of the genes that are responsible for sensing or responding to the environment are very much reduced," Pittendrigh said. The body louse was found to have "significantly fewer genes" for smell and taste, as well as minimal genes responsible for a "simple visual system," the study authors wrote. They found just 10 genes to code for odor receptors. The researchers speculated that by relying only on one type of host (humans) and one type of meal (blood), the lice could make do with very limited environmental cues. Additionally, the lice have fewer genes responsible for detoxification, perhaps due to their limited contact with other, non-human environments.

The researchers also dug into the lice to sequence the bacterium that lives symbiotically in the insects' guts, Candidatus Riesia pediculicola . The bacterium manufactures and supplies the louse with the essential vitamin pantothenate, B5, which the insect cannot synthesize on its own.

"The genome should also help us develop better methods of controlling both head and body lice," Pittendrigh said. Throughout recent history, the body louse has been responsible for spreading epidemic typhus and trench fever, among other diseases, and it continues to plague many people who live in less sanitary conditions. The researchers noted that targeting the Riesia bacterium, on which the lice depend, or the insect's limited detoxification or olfactory capabilities, are possible strategies for future control.

"Beyond its importance in the context of human health, the body louse genome is of considerable importance to understanding insect evolution," May Berenbaum, head of the University of Illinois's entomology department and study coauthor, said in a prepared statement.

These new findings support the theory that the human louse diverged from the chimpanzee louse some five million to seven million years ago, the researchers noted. And human body lice are thought to have evolved from the head louse only after humans started to wear clothing (where the body lice lay their eggs). Analysis of body lice DNA earlier this year pegged the origin of clothing a little before 190,000 years ago, although some propose clothing originated with human ancestors closer to 650,000 years ago.

But the evolutionary implications stretch farther back than human emergence. "It is only the second genome sequenced to date of an insect with gradual development," in that it does not have sudden anatomical changes, as in from an egg to larva, Berenbaum added. "Gradual metamorphosis is the older developmental program," she said. "The body louse genome can provide a baseline for understanding how complete metamorphosis, a key to insect domination of the planet, came to evolve."

Image of body louse courtesy of CDC/Frank Collins