Tropism, compartmentalization and retention of banana bunchy top virus (Nanoviridae) in the aphid vector Pentalonia nigronervosa.

Plant viruses of the families Luteoviridae and Geminiviridae rely on hemipteran vectors for the infection of their hosts. Several lines of evidence have revealed that these viruses are transmitted by competent vectors in a circulative manner, involving entry into the vector's body and the crossing of epithelial tissues forming the alimentary tract and the salivary glands. Similar to luteovirids and geminiviruses, a third family of plant viruses, the family Nanoviridae, have also been reported to be transmitted by aphids in a circulative manner. However, there is limited direct evidence of a possible path of translocation through the aphid vectors. Here, we used time-course experiments and transmission assays coupled with real-time PCR and immunofluorescence assays on dissected tissues to examine the translocation, compartmentalization and retention of banana bunchy top virus (BBTV) into the aphid vector Pentalonia nigronervosa. Our results indicate that BBTV translocates rapidly through the aphid vector; it is internalized into the anterior midgut in which it accumulates and is retained at concentrations higher than either the haemolymph or the principal salivary glands. Despite the large increase in viral concentration, we have failed to detect BBTV transcripts with RT-PCR. When tissues were not permeabilized, BBTV localized as distinct puncta in the proximity of the basal surface of the cells forming the anterior midgut and principal salivary glands, suggesting an on-going process of virion escape and internalization, respectively. Interestingly, we document that those organs can have direct contact within the aphid body, suggesting a possible haemolymph-independent translocation path.