Transcriptional analysis of polydnaviral genes in the course of parasitization reveals segment-specific patterns

Polydnaviruses are symbiotic viruses of endoparasitic wasps, which are formed in their ovary and injected along with the eggs into the host. They manipulate the host in a way to allow successful parasitoid development. A hallmark of polydnaviruses is their segmented genome consisting of several circles of double-stranded DNA. We are studying the solitary egg-larval parasitoid Chelonus inanitus (Braconidae) parasitizing Spodoptera littoralis (Noctuidae). The polydnavirus of Chelonus inanitus (CiV) protects the parasitoid larva from encapsulation by the host's immune system, slightly modifies host nutritional physiology, and induces a developmental arrest of the host in the prepupal stage. Here we present data on newly identified CiV genes and their expression patterns in the course of parasitization. None of these genes has similarity to other genes and so far no gene families could be found. A rough estimation of transcript quantities revealed that even the most highly expressed CiV genes reach maximal values, which are 250 times lower than actin. This indicates that the CiV-induced alterations of the host are brought about by a concerted action of low levels of transcripts. In an overview, we show the expression patterns of all CiV genes analysed up to now; they indicate that several genes with similar expression patterns (early, persistent, intermediate, or late) are grouped together on the same segment. This is the first observation of this type. It suggests that one function of the segmentation of the polydnavirus genome may be the grouping together of genes, which are regulated in a similar manner.