Route of a multipartite (nano)virus across the body of its aphid vector

Vector transmission plays a primary role in the life cycle of viruses and insects are the most common vectors. An important mode of vector transmission, reported only for plant viruses, is the circulative non-propagative transmission where the virus cycles within the body of its insect vector, from gut to salivary glands and saliva, without replicating. This mode of transmission has been extensively studied in the viral families Luteoviridae and Geminiviridae and is also reported for Nanoviridae The biology of viruses within these three families is different and whether they have evolved similar molecular/cellular virus-vector interactions is unclear. In particular, nanoviruses have a multipartite genome organization and how the distinct genome segments encapsidated individually transit through the insect body is unknown. Here, using a combination of fluorescent in situ hybridization and immuno-fluorescence, we monitor distinct proteins and genome segments of the nanovirus Faba bean necrotic stunt virus (FBNSV) during transcytosis through the gut and salivary gland cells of its aphid vector Acyrthosiphon pisum FBNSV specifically transits through cells of the anterior midgut and principal salivary gland cells, a route similar to geminiviruses but distinct from luteoviruses. Our results further demonstrate that a large number of virus particles enter every single susceptible cells, so that distinct genome segments always remain together. Finally, contrasting with the two other viral families mentioned here, we confirm that the success of nanovirus-vector interaction depends on a non-structural helper component, the viral protein NSP, which is shown to be mandatory for viral accumulation within gut cells.ImportanceAn intriguing mode of vector-transmission described only for plant viruses is the circulative non-propagative transmission, where the virus passes through the gut and salivary glands of the insect vector without replicating. Three plant virus families are transmitted this way, but details of the molecular/cellular mechanisms of the virus-vector interaction are missing. This is striking for nanoviruses that are believed to interact with aphid vectors in ways similar to luteoviruses or geminiviruses but for which empirical evidence is scarce. We here confirm that nanoviruses follow a within-vector route similar to gemini- but distinct from luteoviruses. We show that they produce a non-structural protein mandatory for viral entry into gut cells, a unique phenomenon for this mode of transmission. Finally, nanoviruses being multipartite viruses, we demonstrate that a large amount of viral particles penetrate susceptible cells of the vector, allowing distinct genome segments to remain together.