Tranosema rostrate ichnovirus repeat element genes display distinct transcriptional patterns in caterpillar and wasp hosts

The endoparasitic wasp Tranosema rostrale transmits an ichnovirus (IV) to its lepidopteran host, Choristoneura fumiferana, during parasitization. As shown for other IVs, the segmented dsDNA genome of the T. rostrale virus (TrIV) features several multi-gene families, including the repeat element (rep) family whose products display no known similarity to non-IV proteins; their functions remain unknown. Here, transcript abundance for all 17 TrIV rep ORFs was assessed by q-PCR in parasitized and virus-injected C. fumiferana larvae, as well as in T. rostrale ovaries and head-thorax complexes. Although transcripts were detected for most rep ORFs in infected caterpillars, two of them, F1-1 and F1-2, clearly outnumbered the others in whole larvae, with levels dropping significantly over time after infection in both parasitized and injected hosts. No correlation was observed between transcript levels of individual rep genes and the relative abundance of the genome segments bearing them in TrIV DNA extracts. TrIV rep genes also showed differences in the relative abundance of their transcripts in specific tissues; some transcripts displayed higher levels in cuticular epithelium and fat body than in haemocytes or midgut, while others were present in higher proportions in the latter two tissues, suggesting tissue-specific roles for individual members of this gene family. Significantly, no rep transcripts were detected in 71 rostrale head-thorax complexes whereas some were abundant in ovaries. There, the transcriptional pattern was completely different from that observed in infected caterpillars, suggesting that some rep genes have wasp-specific functions.