title: Symbiotic virus at the evolutionary intersection of three types of large DNA viruses; iridoviruses, ascoviruses, and ichnoviruses.
abstract: BACKGROUND
  The ascovirus, DpAV4a (family Ascoviridae), is a symbiotic virus that markedly increases the fitness of its vector, the parasitic ichneumonid wasp, Diadromus puchellus, by increasing survival of wasp eggs and larvae in their lepidopteran host, Acrolepiopsis assectella. Previous phylogenetic studies have indicated that DpAV4a is related to the pathogenic ascoviruses, such as the Spodoptera frugiperda ascovirus 1a (SfAV1a) and the lepidopteran iridovirus (family Iridoviridae), Chilo iridescent virus (CIV), and is also likely related to the ancestral source of certain ichnoviruses (family Polydnaviridae).
  
  
  RESULTS
  To clarify the evolutionary relationships of these large double-stranded DNA viruses, we sequenced the genome of DpAV4a and undertook phylogenetic analyses of the above viruses and others, including iridoviruses pathogenic to vertebrates. The DpAV4a genome consisted of 119,343 bp and contained at least 119 open reading frames (ORFs), the analysis of which confirmed the relatedness of this virus to iridoviruses and other ascoviruses.
  
  
  CONCLUSIONS
  Analyses of core DpAV4a genes confirmed that ascoviruses and iridoviruses are evolutionary related. Nevertheless, our results suggested that the symbiotic DpAV4a had a separate origin in the iridoviruses from the pathogenic ascoviruses, and that these two types shared parallel evolutionary paths, which converged with respect to virion structure (icosahedral to bacilliform), genome configuration (linear to circular), and cytopathology (plasmalemma blebbing to virion-containing vesicles). Our analyses also revealed that DpAV4a shared more core genes with CIV than with other ascoviruses and iridoviruses, providing additional evidence that DpAV4a represents a separate lineage. Given the differences in the biology of the various iridoviruses and ascoviruses studied, these results provide an interesting model for how viruses of different families evolved from one another.
authors.affiliation: Génétique, Immmunothérapie, Chimie et Cancer, UMR CNRS 6239, Université François Rabelais de Tours, UFR des Sciences et Techniques, Parc de Grandmont, Tours, France. bigot@univ-tours.fr
authors.forename: Yves
authors.initials: Y
authors.lastname: Bigot
authors.forename: Sylvaine
authors.initials: S
authors.lastname: Renault
authors.forename: Jacques
authors.initials: J
authors.lastname: Nicolas
authors.forename: Corinne
authors.initials: C
authors.lastname: Moundras
authors.forename: Marie-Véronique
authors.initials: MV
authors.lastname: Demattei
authors.forename: Sylvie
authors.initials: S
authors.lastname: Samain
authors.forename: Dennis K
authors.initials: DK
authors.lastname: Bideshi
authors.forename: Brian A
authors.initials: BA
authors.lastname: Federici
mesh_terms: D005075:Biological Evolution; D004267:DNA Viruses / Q000235:genetics*; D016679:Genome, Viral; D008969:Molecular Sequence Data; D016366:Open Reading Frames; D010802:Phylogeny; D013559:Symbiosis