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Genome seqüenciation of Pediculus humanus

An international team sequenced the genome of the human body louse (Pediculus humanus humanus), a parasite that feeds on the blood of its host and a vector of pathogens causing diseases such as epidemic typhus, relapsing fever and fever the trenches. The work of this team is published in Proceedings of the National Academy of Sciences (PNAS) and has the participation of a group led by UB professors Rozas and Montserrat Aguadé July, experts from the Consolidated Research Group Molecular Evolutionary Genetics and members of the Research Institute of Biodiversity (IRBio), and researchers Filipe G. Vieira, Sara Guirao-Rico-Ponce and David Alvarez. 

P. humanus humanus is a small wingless insects, obligate parasite of humans. This louse, closely related evolutionarily to the head louse (P. humanus capitis), specializes in the suction of the blood, lives in the folds and seams of clothing and develop throughout their life cycle in the host. In close dependent relationship with the man, thrives in conditions of poverty and poor hygiene, and is usually present during episodes of armed conflicts, humanitarian disasters, etc.. The science team has released the complete genome sequence of this parasite in PNAS is led at the international level by Barry R. Pittendrigh (University of Illinois) and Ewen F. Kirkness (J. Craig Venter Institute), and on the Spanish participation, has also involved teams of the University Hospital and the University of Santiago de Compostela.

The smallest genome sequenced so far in insects

The genome has about 100 megabases, so that, as emphasized by Barry R. Pittendrigh, coordinator of the international team that analyzed the sequence, "is the smallest genome has been sequenced so far in an insect." It consists of five metacentric and one telocentric chromosomes, and contains 1% of transposons or mobile elements, a proportion well below that of other insects. Fr humanus humanus also presents structures (mycetoma) that harbor endosymbiotic bacteria, named Candidatus Riesi pediculicola, with whom he has co-evolved. This prokaryote is equipped with a linear chromosome circular plasmid containing genes vital for parasite survival (in particular for the synthesis of pantothenic acid or vitamin B5). To study the evolutionary relationship between the louse and the endosymbiont bacteria is also one of the objectives of the PNAS article. Everything indicates that the genome of this louse is not home prokaryotic genes, and moreover, the endosymbiotic relationship between lice and bacteria-dated at about 13-23 million years-is relatively recent. With a more general perspective, the body louse, which is an insect hemimetabolous extends the framework known so far to study the genome of insects more complex, the holometabolous (complete metamorphosis with a key process for the evolutionary success of insects in the world).

A small but efficient genetic endowment

The UB team has participated in this study with a double scientific contribution: first, to characterize the most important genes of the insulin pathway, and secondly, to study the genes related to insect chemoreception system, in particular multigene families of odorant binding proteins (OBP) and chemosensory (CSP). These two tasks have been done with bioinformatics tools. The results indicate that the human body louse has a minimum allocation of genes, both the insulin pathway to the uptake of environmental stimuli. In the latter case, only five genes have been identified for OBP and seven for the CSP, a number of genes significantly lower than that observed in other insects. "This parasite also has the smallest number of enzymes with antitoxic function has been observed so far in an insect 'experts explained John Clark, University of Massachusetts Amherst, and Si Hyeock Lee of Seoul National University, which addressed this part of the study. To Pittendrigh, "having a limited sample of antitoxic enzymes of this parasite makes an attractive model for studying resistance to insecticides or other chemical defense mechanisms." In this part of the work, have worked Entomology Professor May Berenbaum of the University of Illinois, and his collaborator Reed Johnson. For Julio Rojas, the study has revealed that 'the genetic endowment of the louse is very small but it seems functional various biological processes operate with a minimum number of genes. In the case of the insulin pathway, only one copy of each gene important, and this differs from other insects, where some genes have more copies. " It also states: "This is a global phenomenon of reduction of the genetic heritage of the louse, which fits the profile of specific parasite that has lost many genes are not essential, is well suited to a very homogeneous, is totally dependent on the host and has a highly restricted diet is supplemented with contributions from endosymbiotic bacteria. "

Knowing the genes for designing control strategies

The scientific team from the Department of Genetics, UB has a distinguished career in international efforts to sequence genomes of insects (Drosophila, aphids, etc.). For the experts, knowing the genome of the human body louse is a scientific milestone that will allow design therapeutic strategies using specific genetic targets of the parasite. "Knowing the genes is important to know what target can act to prevent the spread of the insect without affecting the environment. What matters is to act directly on the Scale: If you know the genes involved in host recognition, we can act on these parasites. As the endosymbiotic bacteria have genes that are essential for parasite survival, we face another potential genetic target to combat lice, "concludes July Rozas.