Summary auto-generated
Cotesia congregata is a parasitoid wasp that carries polydnavirus (CcPDV), which is essential for successful parasitism of tobacco hornworm larvae. This study investigated how viral DNA sequences are maintained and transmitted in the wasp. Researchers analyzed the EP1 gene, which encodes an early parasitism protein. Using molecular techniques, they discovered that EP1 sequences exist in wasp DNA in two forms: as circular molecules (like in virus particles) and integrated into the wasp genome. The integrated sequences can excise from the chromosome in ovarian cells, producing circular viral DNA. Analysis of the junction regions revealed direct repeats flanking the EP1 sequences that resemble binding sites for Hin recombinase, a bacterial site-specific recombinase. The research suggests that EP1 sequences excise through a mechanism related to Conservative Site-Specific Recombination (CSSR), a process typically found in prokaryotes. An 'empty locus' lacking the viral sequence was identified, representing the chromosomal location after excision. These findings provide insight into how polydnavirus sequences are managed and transmitted vertically through the wasp germline.
Key findings
- EP1 polydnavirus sequences exist in both circular and integrated forms in Cotesia congregata wasp DNA, with circular forms produced by excision of integrated sequences
- The EP1 integrated locus is flanked by direct repeats containing potential binding sites resembling Hin family recombinase recognition sites from bacteria
- Excision of EP1 sequences appears to occur via a mechanism related to Conservative Site-Specific Recombination (CSSR), generating an 'empty locus' with the viral DNA removed
- Excision occurs preferentially in ovarian tissue that supports virus replication, suggesting tissue-specific activation of the process
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Abstract
Cotesia congregata polydnavirus (CcPDV) is essential for successful parasitism of Manduca sexta larvae by the braconid wasp Cotesia congregata. To determine the molecular mechanisms for the vertical transmission of CcPDV in the wasps, we analysed the different forms of the virus sequences containing the gene encoding the early parasitism- specific protein 1 (EP1). By a detailed molecular analysis, we demonstrated that the EP1 sequences are present in wasp DNA in two forms: a circular form as seen in the virus particles and a form integrated into the wasp genome. Moreover, we showed that the integrated form of the EP1 sequences is able to excise in the ovary cells. A fragment corresponding to an EP1 'empty locus' (without the viral sequence) was PCR-amplified from ovarian DNA. Comparison of the sequences isolated from the EP1 circle, the integrated form and the empty locus revealed that the extremities of the EP1 genomic sequences constitute a direct repeat. Strikingly, these sequences contain a potential binding site for a recombinase of the Hin family located in close vicinity to the position where the DNA strand exchange occurs. Thus, the data bear upon the possibility that the bracovirus circles are excised via a mechanism related to the Hin mediated Conservative Specific-Specific Recombination (CSSR) of prokaryotes.