Models for virus vertical transmission.

N1 (respectively N2) is the number of PSbMV variant 1 (DPD1-R) (respectively variant 2 (DPD1)) particles entering a given seed of a given plant and Nc is a critical threshold for the infection of the seedling issued from this seed. Note that for Nc = 0 the models M1, M2 and M3 are identical to model M4.aWhen variant-variant interactions occur, two cases were distinguished depending on whether, or not, virus variants are interchangeable whatever their type. Variants are interchangeable for seedling infection if the contribution to seedling infection of a virus particle of one variant does not depend on the density of virus particles of the other variant.