P. humanus humanus is a small, wingless insect and an obligatory parasite of humans that thrives in conditions of economic hardship and poor hygiene, frequently appearing during periods of armed conflict and other humanitarian disasters. The evolution of this blood-sucking louse is closely related to that of the human head louse (P. humanus capitis), but the body louse lives in the folds and seams of clothing and develops its entire life cycle alongside the host organism. The international team of scientists behind the study published in the PNAS was directed by Barry R. Pittendrigh (University of Illinois) and Ewen F. Kirkness (J. Craig Venter Institute). In Spain, work was also carried out by teams from the Santiago de Compostela University Hospital and the University of Santiago de Compostela.
The smallest insect genome sequenced to date
The genome has some 100 megabases, making it "the smallest genome that has been sequenced to date in an insect", explains Barry R. Pittendrigh, coordinator of the international team behind the research. It is formed by five metacentric chromosomes and one telocentric chromosome, and contains 1% of transposons or mobile elements, a smaller proportion than found in other insects. P. humanus humanus has organs called mycetomes that house a bacterial endosymbiont Candidatus Riesia pediculicola, with which it has co-evolved. This prokaryote has a short linear chromosome and a circular plasmid that contains genes vital to the survival of the parasite (specifically, for the synthesis of panthotenic acid or vitamin B5).
The study published in the PNAS also describes the evolutionary relationship between the body louse and the bacterial endosymbiont. Evidence suggests that the genome of this louse does not contain prokaryotic genes, and that the endosymbiotic relationship between lice and bacteria - the emergence of which has been dated to 13-23 millions of years ago - is in fact comparatively recent on the evolutionary scale. More generally, the human body louse, a hemimetabolous insect, provides the most complete reference for studies of the complex holometabolous insects (which exhibit a complete metamorphosis, a key process in the evolutionary success of).
A pared-down but efficient genome
The UB team has made two important contributions to the study, using sophisticated bioinformatics tools: characterization of the major genes in the insulin signal transduction pathway, and analysis of the genes associated with the chemosensory system - specifically, the multigene families of odorant-binding proteins (OBP) and chemosensory proteins (CSP). The results indicate that the human body louse has only a minimal number of genes associated with functions such as the insulin signal transduction pathway and environmental sensing - in the latter case, only fives genes for the OBPs and seven for the CSPs have been identified, which is much smaller than the number found in other insects. "This parasite also has the smallest number of detoxification enzymes observed in any insect", explain John Clark, from the University of Massachusetts Amherst, and Si Hyeock Lee, from Seoul National University, who directed this part of the study. For Pittendrigh, "the pared-down list of detoxifying enzymes makes it an attractive organism for the study of resistance to insecticides or other types of chemical defence". University of Illinois entomology professor and department head May Berenbaum and former graduate student Reed Johnson contributed to this effort.
For Julio Rojas, the study has shown that "the louse's genome is very small but apparently functional: the different biological processes operate with only a minimal number of genes. In the case of the insulin signal transduction pathway, there is only one copy of each important gene, unlike in other insects, which generally have more copies of certain genes". Rojas explains that, "this genome reduction in the human body louse is a global phenomenon indicative of a specific parasite that has shed many non-essential genes, is well adapted to an extremely homogeneous environment, is totally dependent on the host for its survival and has a highly restricted diet that is supplemented by the contributions of its bacterial endosymbionts".
Identifying genes to design control strategies
The team from the UB's
Department of Genetics has worked on other international genome sequencing projects focused on insect species (the
Drosophila fly and the pea aphid, among others). For experts, obtaining the genome sequence of the human body louse is a major achievement that will enable them to design therapeutic strategies based on genetic targets specific to the parasite. "Mapping the genome is important for identifying the target that should be acted on to prevent the spread of the parasite without affecting the host environment. The important thing is to act directly on the louse: if we can identify the genes that contribute to host recognition, we will be able to target the parasite directly. Given that the bacterial endosymbiont contains genes essential to the survival of the parasite, we are probably talking about another potential genetic target in the fight against these lice", concludes Julio Rozas.
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