Genome organization of the Chelonus inanitus polydnavirus: excision sites, spacers and abundance of proviral and excised segments

Abstract

Polydnaviruses are only found in symbiotic association with parasitic wasps within the families Ichneumonidae and Braconidae (ichnoviruses and bracoviruses). They have a segmented genome consisting of circular double-stranded DNA. In the proviral linear form they are integrated in the wasp's genome; in two bracoviruses, segments were found to be clustered. Proviral segments have direct terminal repeats. Segment excision has been proposed to occur through juxtaposition of these repeats by formation of a loop and recombination; one copy of the repeat then ends up in the circular segment and one in the rejoined DNA. Here we analysed the excision/circularization site of four segments of the Chelonus inanitus bracovirus (CiV) and found that they are similar to the two already known sites; on the basis of the combined data an extended excision site motif was found. Analyses of segment flanking sequences led to the first identification of one complete and several partial spacers between proviral segments in a polydnavirus. The spacer between the proviral segments CiV14 and CiV22.5 has a length of 2065 bp; the terminal repeats of CiV14 and CiV22.5 were seen to have an opposite orientation and from this a model on the spacial organization of the loops of the proviral cluster is proposed. Through various approaches it was shown that spacers are not excised or injected into the host. Measurement of relative abundances of various segments in proviral and excised form indicates for the first time that abundant segments are present in multiple copies in the proviral form.