The results of the sequencing and analysis of the human body louse genome, which were published on June 21 in the Proceedings of the National Academy of Sciences (PNAS), offer new insights into the intriguing biology of this disease-vector insect. The project involved more than 70 international scientists led by Professor Evgeny Zdobnov at the University of Geneva Medical School and the SIB Swiss Institute of Bioinformatics, with Professor Barry Pittendrigh at the University of Illinois and Professor Ewen Kirkness at the J. Craig Venter Institute.
The human body louse, Pediculus humanus humanus, is an obligate human parasite responsible for the transmission of bacteria that cause relapsing fever, trench fever, and epidemic typhus. The sequencing and comparative analysis of the body louse genome with other sequenced species revealed many features that will enhance our understanding of the relations between disease-vector insects, the pathogens they transmit, and the affected human hosts.
Zdobnov's team at the University of Geneva - Dr Daniel Gerlach, Dr Evgenia Kriventseva, and Dr Robert Waterhouse - focused on the identification of microRNA genes and the comparative analysis of the protein-coding gene repertoire using computational approaches. These studies revealed that despite having the smallest known insect genome (108Mb) and being an obligate parasite, the body louse has retained a remarkably complete "basal insect" repertoire of 10,773 protein-coding genes and 57 microRNAs. The compactness of the louse genome greatly helped to accurately predict the encoded gene repertoire, which includes relatively few genes associated with sensing or responding to the environment - consistent with the body louse's relatively stable habitat offered by the human host. According to Zdobnov, "The key phylogenetic position of the body louse, together with the completeness and accuracy of its gene repertoire, mean that this genome will provide an invaluable evolutionary reference point for future studies of all other sequenced insect species, especially for the characterisation of key requirements for insect growth and development".
The human body louse usually lives in clothing and therefore infestations are associated with wearing unwashed clothes for prolonged periods such as during wartime, natural disasters or the often poor personal hygiene of homeless people or refugees. As well as irritations from infestations with body lice or the closely-related human head lice, the body louse may carry harmful bacteria such as Rickettsia prowazekii that cause epidemic typhus and are classified as a category B bioterrorism agent. As body and head lice are becoming increasingly resistant to traditional pesticides, the sequencing of the body louse genome will greatly help in the important search for new control methods facilitated by detailed molecular studies. With this in mind, the genome of the obligatory louse endosymbiont, Candidatus Riesia pediculicola, was also sequenced. Targeting the Reisia bacteria could offer novel louse control methods as Riesia are essential to the body louse because they encode the genes required for the production of vitamin B5, which is deficient in the louse diet of human blood.
"As the first sequenced genome of a permanent vertebrate ectoparasite", says Zdobnov, "these studies will help to understand the molecular mechanisms underlying the evolution of extreme specialization for life on a single host species". The compact yet complete body louse genome provides a robust outgroup for comparative studies with other insects, and the three-way interactions between the human host, the body louse parasite, and the Riesia endosymbiont offer numerous opportunities to gain greater insights into host-parasite-symbiont tripartite coevolution and speciation.