**7. Virus preservation in seeds**

RNA viruses have high mutation rate [127] that contributes to rapid evolution dynamics which may ensure adaptation to new host plants or to stressful and fluctuating environments [128, 129]. Seed transmission may comprise a genetic bottleneck, which may reduce population size and induce the immergence of new virus strains [130]. Analysis of the effect of the vertical transmission bottleneck on *Zucchini yellow mosaic virus* (ZYMV), for example, showed that although high variability was observed in the 5′ untranslated region, the regions necessary for vector transmission were unchanged, indicating their contribution to virus spread [131]. Genetic bottlenecks may also cause reduction in virus virulence as suggested to occur in PSbMV seed transmission [132]. Interestingly, experiments on virus seed transmission carried out with CMV infecting Arabidopsis thaliana, under controlled conditions, showed that there is a reduction in virus virulence after several vertical passages [133]. Virus accumulation was reduced as well although seed transmission rate increased. These changes of the virus were concomitant to adaptation of the host plant to the evolving virus, showing an increase in vertical transmission. However, this host adaptation which favors a theory of coevolution of plants and viruses in vertical transmission exposes the cells to high virulence of non-evolved horizontally transmitted viruses [133].
