Fig. 1: Scheme of the Continuum hypothesis adapted to plant viruses from the model developed by ref. ³⁸.

a Relationship between the virus transmission mode (triangles) and virulence (dark red line) leading to a parasitism-commensalism-mutualism continuum (light red line). The width of the triangles represents the relative importance of vertical and horizontal transmission for virus fitness. Mostly vertically transmitted viruses evolve lower virulence to maximize plant fitness and the production of virus-infected seeds. Therefore, these viruses establish more frequently commensalist or mutualistic relationships with the host. In contrast, mostly horizontally transmitted viruses do not require the host to produce progeny and will evolve higher virulence, thus being more often on the parasitism side of the continuum. b Illustration of the two extremes of the mutualism-commensalism-parasitism continuum. Left: a strictly vertically transmitted virus will evolve to be avirulent (commensalist) or to confer a benefit to the plant (mutualist), allowing greater production of viable seed and achieving perfect transmission to the plant progeny. Right, a strictly horizontally transmitted virus will maximize virulence (leading to plant castration in the most extreme cases) and, therefore within-host multiplication, which is positively associated with horizontal transmission rate (here represented by the aphid vector). Such viruses will establish a parasitic relationship with the host plant and will not be vertically transmitted.