SEQUENCE COMPARISON AND CLASSIFICATION OF BEET LUTEOVIRUS ISOLATES

Three distinct sequence groups were found among partial nucleotide seq uences of 38 isolates of beet western yellows virus (BWYV) and beet mi ld yellowing virus (BMYV) from Europe, Iran and the USA. The first gro up contains both sugar beet and oilseed rape specific isolates, and th e differentiating characteristic linked to this host range specificity are 2 single base pair changes in a 1 200 nucleotide region of the ge nome. It is proposed that the European BWYV strains that can be transf erred at low frequency between rape and sugar beet belong to this grou p. Also belonging to this group are the published BWYV sequences of Ve idt et al. [59] and of the Californian BWYV-ST9 isolate [4]. The secon d group contains mostly rape-derived isolates which have an intergenic region highly distinct from that of group-1 isolates but similar poly merase and coat protein regions. It is proposed that the rape-specific BWYV isolates which cannot be transmitted to sugar beet belong to thi s group. The third group contains mostly beet-specific isolates from S outhern Europe and Iran, and may be adapted to the Mediterranean clima te and flora. It is distinct from groups 1 and 2 in all three genome r egions investigated and its polymerase and intergenic regions are as m uch related to those of potato leafroll virus (PLRV) and curcurbit aph id borne yellows virus (CABYV) as they are to those of group-1 and gro up-2. On the basis of sequence similarities and established nomenclatu re it is proposed to use BWYV for groups 1 and 2 (BWYV-1 and BWYV-2 re spectively) and to use BMYV for group-3 isolates, which are distinct e nough from the other two groups to merit a separate nomenclature.