Posterior midgut and hindgut are both sites of acquisition of Cucurbit aphid-borne yellows virus in Myzus persicae and Aphis gossypii

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Abstract
Members of the family Luteoviridae (‘luteovirids’) rely strictly on aphid vectors for plant-to-plant transmission. This interaction operates according to a persistent and circulative manner, which implies that the virions are being endocytosed and exocytosed across two epithelial barriers (alimentary tract and accessory salivary glands) in the vector's body. In several luteovirid–aphid vector species combinations, the route of virions in the insect has been investigated ultrastructurally by transmission electron microscopy (TEM). Here, we used TEM to follow the route of Cucurbit aphid-borne yellows virus (CABYV; genus Polerovirus) in its two efficient vector species, Myzus persicae and Aphis gossypii. We demonstrated that CABYV particles are acquired from the gut lumen to the haemocoel through two different sites in both aphid species, i.e. the posterior midgut (as for Beet western yellows virus in M. persicae) and the hindgut (as for Barley yellow dwarf virus complex in cereal aphids). This ‘dual’ tissue specificity of CABYV represents an original situation among viruses in the family Luteoviridae examined so far by TEM. A variety of virion-containing structures (e.g. clathrin-coated and tubular vesicles, endosome-like bodies) are found in intestinal cells of both types in both aphids. Release of virus particles from midgut and hindgut cells into the haemolymph was confirmed by immunotrapping using CABYV-specific antibodies. In accessory salivary glands, transport of CABYV virions across the cells was similar in each aphid species, and occurred by a transcytosis mechanism involving formation of tubular and coated vesicles before release of free virions in the salivary canal.