Experimental design.

<p>Three CMV strains were inoculated in five Cen-1 <i>Arabidopsis</i> plants each. Each of these 15 plants represented the origin of a lineage of vertically transmitted viruses indicated by blue arrows (left column), another lineage of horizontally transmitted viruses indicated by red arrows (upper row), and a third group of lineages in which vertical and horizontal transmission modes were alternated (diagonal).</p>