Genetic diversity of beet necrotic yellow vein virus

Beet necrotic yellow vein virus (BNYVV) is a member of the genus Benyvirus in the family Benyviridae with multipartite positive-sense single-stranded RNA genomes encapsidated in rigid rod-shaped particles. The members of Benyvirus including four species are similar to those of the family Virgaviridae, in respect of viral particle assembly, movement, and plasmodiophorid transmissibility. Recent studies revealed that ancestors and/or relatives of benyviruses may have infected a wide range of hosts such as plants, insects, algae, and fungi. For phylogenetic analyses of BNYVV genes, worldwide BNYVV isolates form four clades, A-I, A-II, A-III, and B, from which at least ten subgroup isolates (strains) have derived. These original BNYVV types and their progeny strains might have existed in East Asia, and each source had introduced infection to cultivated sugar beet plants and might have spread worldwide only in the last half century. Along with the growth of resistant varieties in rhizomania-infested areas since the 1980s, strong selection pressure has been imposed on the RNA3-encoded p25 gene, and, consequently, resistance-breaking variants that have single amino acid changes in the p25 protein have been generated. RNA5-encoded p26 gene is also associated with resistance breaking as well as symptom severity in sugar beet roots.