Mapping Louse DNA Could Help Fight The Parasite

A louse is rarely a welcome visitor. And if one louse is bad, a load of lice is, well, lousy. Not only do they cause itching, but body lice can be responsible for the spreading of disease.

To combat an enemy, it's best to know its weaknesses. That's why a team of scientists decided to sequence the genome of the human body louse. Their findings now give them valuable insight into what makes a louse tick.

The Biology Of Lice

Before we get to the genetics, let's go over some basic louse biology. First of all, without you and me, there would be no human body louse.

"It absolutely has to have the human host in order to survive," says Barry Pittendrigh, a louse geneticist at the University of Illinois, Urbana-Champaign.

Pittendrigh is currently in Turkey where he just attended the Fourth International Congress on Phthiraptera -- the technical name for lice. If you already knew how to spell Phthiraptera before you read this story, you've probably won a spelling bee at some point in your life.

According to Pittendrigh, both body and head lice live on human beings because they get most of their nutrients from human blood. But, because lice can't get vitamin B5 by themselves, they have to rely on a bacterial endosymbiont -- an organism that lives inside another organism and helps it out.

"In the absence of this endosymbiont, the body louse does not get the vitamins it needs to survive," says Pittendrigh.

So a louse with a healthy endosymbiont and access to human blood is one happy louse.

Human lice come in two varieties -- head lice and body lice. While head lice are harmless, body lice can spread diseases like relapsing fever, epidemic typhus and trench fever, says John M. Clark, an entomologist and pesticide toxicologist at the University of Massachusetts. "These [diseases] have killed millions and millions of people."

Decoding The Genome

A few years ago, Clark and Pittendrigh decided it would be a good idea to learn more about the louse genome, so they sent samples of louse DNA to a colleague of theirs in Texas.

Clark says his colleague called them back extremely excited because the louse genome turned out to be only 108 megabases -- millions of letters of DNA -- quite a small number for insects.

The scientists recently reported in the journal Proceedings of the National Academy of Sciences that they've now sequenced the complete louse genome, made up of 108 million letters of DNA.

Why So Simple?

That louse genome contains a mere 11,000 genes, according to May R. Berenbaum, the head of the entomology department at University of Illinois, Urbana-Champaign. "The [genome] is so reduced," she says, "it's sort of the basic stripped-down instructions for making an insect."

Lice can make do with a small number of genes because they've had what Berenbaum calls a "cushy existence" over their evolutionary association with humans. Because lice don't have to hunt for food and can simply look to their host, their genomes lack many genes that other insects would need to sense their surroundings. Lice also lack a lot of the genes needed to break down toxins.

"There's only a dozen of these genes that deal with toxins, which is half of what honeybees have and a third of what the fruit fly has," says Berenbaum. "They don't need an elaborate system for sort of breaking down chemical poisons because over their evolutionary association with humans, they haven't had to deal with them until relatively recently."

As humans use toxins more frequently to treat lice infestations, the lice are developing resistance to some of these methods. But, Berenbaum says that knowing the louse genome sequence should help scientists design new ways to shoo lice away.

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