Densoviruses (DVs) are parvoviruses of arthropods and causative agents of natural epizootics in insects and crustaceans populations. Structurally simple, these small DNA viruses, display a large diversity of genomic sequences, structures and organizations. Such diversity, together with the diversity of their invertebrate hosts, from shrimps to mosquitoes and recently including sea stars, suggests that DVs are largely unknown and ubiquitous in the environment. Densoviruses are considered as a model of choice to study virus-host interactions and their evolution at different scales, from individuals to populations. This review summarizes the knowledge on densovirus biology obtained through mechanistic and global approaches. Finally, the potential use of these viruses as biological control agents against insect pests and disease-vectors are exposed.

La lutte microbiologique connait un regain d'interet suite aux problematiques soulevee spar les pesticides chimiques en agriculture. L'objectif de ces travaux de these est d'etudier le potentiel d'une ressource virale alternative, les Densovirus pathogenes de Lepidopteres,appartenant a la sous famille des Densovirinae, strictement infeodee aux Arthropodes. Nos travaux ont ete axes sur trois points (i) decrire le spectre d'hotes de deux Densovirus candidats et identifier des determinants de specificite pour comprendre l'evolution du spectre d'hotes ; (ii) caracteriser des mecanismes de transmission horizontale pour analyser la dynamique de l'infection et (iii) valider des outils de detection des Densovirinae afin d'etudier la diversite genetique virale dans la nature. Le premier axe nous a permis d'identifier (i) un Densovirus candidat qui ne semble etre virulent que pour des Lepidopteres ravageurs de culture, et (ii) des determinants de specificite a la surface de la capside impliques le franchissement de la barriere intestinale. Concernant la transmission, nos resultats montrent qu'un Densovirus se propage rapidement dans une population hote en induisant un comportement cannibale probablement developpe par les individus non infectes. Nous avons egalement caracterise deux mecanismes de transmission horizontale,par morsures et par un vecteur endoparasitoide. Enfin, pour etudier la prevalence virale,nous avons mis au point un protocole base sur le principe d'une PCR nichee que nous avons teste sur des insectes collectes dans la nature. Ceci nous a permis de dresser un inventaire de la diversite d'insectes, sans toutefois reveler a ce jour la presence de Densovirinae. Ces resultats constituent les premieres connaissances pour l'elaboration d'un modele epidemiologique qui nous permettra d'evaluer l'impact sur l'environnement d'utiliser un Densovirus en tant qu'agent de lutte microbiologique.

ABSTRACT Densoviruses are insect parvoviruses that are orally infectious for Lepidoptera . To assess the mechanisms underlying their specificity and their virulence, we investigated the role of eight candidate residues in the densovirus capsid. We showed that the substitutions of four amino acids were associated with decreased virulence due to a decreased ability to cross the host midgut epithelium, without an effect on viral replication in other tissues.

E-mail: nathalie.mondy@univ-lyon1.fr, fl oriane.chardonnet@orange.fr, emilie.delava@univ-lyon1.fr; vincent.foray@univ-lyon1.fr, ceciclia.multeau@supagro.inra.fr, aurelien.vigneron@insa-lyon.fr Accepté le 16 juin 2011 T present paper represents a synthesis of communications collected in the “16e Colloque Biologie de l’Insecte” which gathered about 160 researchers from France and other European countries at Lyon in October 2010 (on-line reports of the symposium http://www.insavalor.fr/CBI2010). Because insects are some of the most familiar organisms of global ecosystems and because of their diversity, they are suitable models to investigate a broad spectrum of questions, in genetics and ecology as well as on populations and physiological responses to environmental changes. Molecular ecology approaches, spurred on by the recent advances in genomics, were omnipresent during this congress and revealed their relevance for the study of specifi c proximal mechanisms. As a consequence, most of presentations were focused on functional and mechanistic biology rather than conceptual approaches, following several of the recent trends outlined below.

To evaluate densovirus potential against lepidopteran pests and their capacity to invade new hosts, we have characterised in vivo the infection and pathogenesis of the Junonia coenia densovirus (JcDNV) in the noctuid pest Spodoptera frugiperda. Here we show that infection starts with the ingestion of viral particles that cross the midgut epithelium without replicating. By quantitative PCR we established the kinetic and the route of infection, from virus ingestion to replication in visceral tracheae and hemocytes. JcDNV has a high particle-to-infection ratio mostly due to the barrier function of the midgut. Pathology and cytopathology suggested that infection of tracheal cells impairs oxygen delivery to demanding tissues leading to cytopathic effects in all the tissues. Finally, larval death results from several physiological shocks, including molting arrest and anoxia.