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International Team Sequences Body Louse Genome

By Andrea Anderson

NEW YORK (GenomeWeb News) – In a paper appearing in the early, online edition of the Proceedings of the National Academy of Sciences yesterday, an international research team reported that they have sequenced the genome of the human body louse, Pediculus humanus humanus.

In the process, the researchers also generated sequence that helped them piece together the genome of a bacterial symbiont residing in the louse. Together, the two genomes are providing clues about louse adaptations to obligate parasitism on the human body. In addition, those involved say the work may ultimately provide insights needed to pave the way for more targeted control measures.

"Understanding the genome should ultimately open up doors to better understanding how to deal with louse populations," co-senior author Barry Pittendrigh, an entomologist at the University of Illinois at Urbana-Champaign, told GenomeWeb Daily News.

The body louse sequencing group drew up a white paper about six years ago calling for P. h. humanus genome sequencing, he explained. "We originally pushed for a genome project simply because it had such a small genome size."

The body louse is also of interest to researchers because of its role in transmitting important human diseases, including typhus, relapsing fever, and trench fever. "Certainly the public health aspects of it are very, very important," Pittendrigh added.

In a mitochondrial genome study appearing in Genome Research last spring, a trio of researchers from the body louse sequencing effort provided evidence that body lice and some other blood-feeding louse species carry multiple fragmented mitochondrial genomes.

For the current paper, the team used Sanger sequencing to sequence the 108 million base pair body louse genome to around 8.5 times average coverage.

They also got around 50 times coverage of the 574,526 base pair genome of Candidatus Riesia pediculicola — an endosymbiont thought to carry out some biological processes that are essential to lice survival. For example, Riesia appears to produce the vitamin B5 used by the human body louse.

During their subsequent analyses of the louse and bacterial endosymbiont genomes, the researchers identified 10,773 protein-coding genes, 161 transfer RNAs, and 57 microRNAs in the louse genome.

The louse's gene content was consistent with its simply ecology, Pittendrigh noted, explaining that body lice alternate between feeding on human blood and reproducing in clothing.

For instance, the genome contained fewer genes involved in responding to xenobiotic or chemical challenges than previously characterized insect genomes. The body louse also appears to have reductions in receptor, odorant, and other chemosensory gene families compared with other insects.

"They have a very limited number and a reduced set of genes associated with both sensing their environment and then responding to that environment," Pittendrigh said. "It all makes sense within the ecology of the insect. They have a very simple lifestyle — actually much simpler than most other insect species I know of."

On the other hand, the genome did contain many of the genes used to accomplish other physiological processes associate with free-living insects.

"Contrary to the expectations of reductive evolution common in obligate parasites, the body louse has retained a remarkably complete repertoire of both protein-coding and non-protein-coding genes," the team noted.

The researchers also tracked down some genes that may help explain louse resistance to pesticides, including cytochrome p450. Even so, P. h. humanus appears to carry only a dozen CYP3 xenobiotic metabolism-related genes — far fewer than either the fruit fly, Drosophila melanogaster, or the honey bee, Apis mellifera.

The symbiont genome, meanwhile, contained 557 open reading frames, 33 tRNAs, six ribosomal RNAs, and at least one structural RNA. Along with nearly 240 core genes shared with other characterized bacteria, Riesia carries two dozen genes not found in other bacterial species so far.

And while Riesia's nuclear genome did not carry genes coding for enzymes involved in vitamin B5 production, these genes did turn up on a plasmid carried by the endosymbiont.

"Having these genes on a multicopy plasmid could represent a mechanism that reduces the risk of genome degradation and increases expression levels to secure synthesis of [vitamin B5] at required amounts," the researchers speculated.

Finally, because the endosymbiont is necessary for body louse survival but appears to lack antibiotic resistance genes, the team suggested it might be feasible to control lice by directly targeting Riesia.

In the future, members of the body louse sequencing team are keen to pursue a range of research questions stemming from current genome analyses — from functional studies of lice and their symbionts to population genetic studies of head and body lice.

The researchers also hope to sequence additional louse species in order to understand the genetic and genomic features linked to hosts and/or body part specialization.

"There are probably other species that would be very, very interesting because of their divergent ecology," Pittendrigh said, explaining that the genomes of lice from different ecological niches should offer clues about louse evolution and adaptation.

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