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

¹ Université Laval;
² Laurentian Forestry Centre;
³ Laurentian Forestry Centre NRCan-CFS

⁴ E-mail: michel.cusson{at}nrcan.gc.ca

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, except for a homolog encoded by the genome of the granulovirus HearGV; their functions remain unknown. Here, we applied "linear regression of efficiency" (LRE) analysis to the qPCR quantification of transcript abundance for all 17 TrIV rep ORFs, 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 clearly outnumbered the others in whole larvae, with a tendency for levels to drop over time after infection. The genome segments bearing the three most highly expressed rep genes in parasitized caterpillars were present in higher proportions than other rep-bearing genome segments in TrIV DNA, suggesting a possible role for gene dosage in the regulation of transcription level. TrIV rep genes also showed important differences in the relative abundance of their transcripts in specific tissues (cuticular epithelium, fat body, haemocytes, midgut), implying tissue-specific roles for individual members of this gene family. Significantly, no rep transcripts were detected in T. 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.

Received 7 November 2008; accepted 2 February 2009.