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. INTRODUCTIONPolydnaviruses represent a unique type of virus which is found only in parasitic wasps of the families Ichneumonidae and Braconidae; accordingly they are classified as ichnoviruses and bracoviruses (Webb et al., 2000). These viruses are formed in the calyx cells of the wasp's ovary and are injected along with the eggs into the host insects, mostly lepidopteran larvae or eggs (Webb, 1998). Polydnaviruses do not replicate in the lepidopteran host but are essential for survival of the wasp larva within the host; they play an important role in abrogation of the host's immune reaction (Schmidt et al., 2001) and in various aspects of host regulation (Lawrence & Lanzrein, 1993). We are working with the egg-larval parasitoid Chelonus inanitus (Braconidae) and its natural host Spodoptera littoralis (Noctuidae). In this system the bracovirus prevents encapsulation of the parasitoid larva (Stettler et al., 1998), affects feeding and nutritional physiology of the last instar host (Kaeslin et al., 2005) and induces a developmental arrest of the host in the prepupal stage (Soller & Lanzrein, 1996) by interference with the ecdysteroid system (Grossniklaus-Bürgin et al., 1998). Ichnoviruses and bracoviruses have different evolutionary origins (Whitfield & Asgari, 2003). The bracovirus-braconid association arose approximately 74 million years ago in a monophyletic lineage (microgastroid braconids) that consists of over 17 000 species (Whitfield, 2002); much less ...

When parasitic wasps hijacked viruses: genomic and functional evolution of polydnaviruses

“…Like all Cheloninae, C. inanitus oviposits into the eggs of lepidopteran hosts, whereas the other BV-associated wasps inject their eggs inside immune competent larvae (figure 1) [14]. Parasitism strategies involving the oviposition into host eggs versus larvae, exposing the wasp larvae to different physiological contexts, could explain the different panel of virulence proteins found in chelonines [83,84] compared with other wasps.…”

“…Specific genes or gene families could reflect how physiological interactions with different hosts have modelled these genomes. For example, despite originating from the same ancestral nudivirus, no common virulence genes have been identified to date between CiBV and other BV genomes [83]. Like all Cheloninae, C. inanitus oviposits into the eggs of lepidopteran hosts, whereas the other BV-associated wasps inject their eggs inside immune competent larvae (figure 1) [14].…”

Functional endogenous viral elements in the genome of the parasitoid wasp Cotesia congregata : insights into the evolutionary dynamics of bracoviruses

“…All proviral segments from all BVs analysed to date are terminated by direct repeats at both extremities, termed DRJs [12,13,16,29,30,32]. Bracovirus circles contain a unique sequence (circle junction) produced from a recombination event between these DRJs [12,33].…”

“…Previous studies using molecular approaches [12,13] and in situ hybridization on wasp chromosomes [14] showed that the proviral segments analysed were clustered together. This led to the hypothesis that all proviral segments might be organized in the wasp genome in tandem arrays constituting a macrolocus [14,15].…”

Exogenous JH and ecdysteroid applications alter initiation of polydnaviral replication in an endoparasitoid wasp, Cotesia plutellae (Braconidae: Hymenoptera)

“…The conserved PDV excision site sequence obtained from previous related PDVs (8,44) was used to locate sites in different CpBV segments (NCBI accession number): S2 (DQ075354), S3 (DQ075355), S4 (DQ075356), S5 (DQ075357), S8 (DQ0753 58), S9 (DQ075359), S10 (EF067319), S11 (DQ075360), S14 (EF067320), S16 (EF067321), S21 (EF067322), S22 (EF067323), S27 (DQ067324), S28 (AY651829), S30 (AY651828), S33 (AY 651830), S35 (EF067325), S36 (EF067326), S37 (EF067327), S38 (EF067328), S41 (EF067329), S48 (EF067330), S50 (EF06 7331), and S51 (EF067332). Multiple alignment of these terminal CpBV repeats was performed using the DNAstar program (Version 5.02, DNAstar Inc., Madison, WI, USA).…”