Molecular variability of geographically distinct isolates of Rice yellow mottle virus in Africa.

The coat protein gene (ORF4) and the 3' untranslated region of a sample of 40 isolates of Rice yellow mottle virus (RYMV), 32 from West Africa and 8 from East Africa, have been sequenced. Five major strains were differentiated, three from West Africa (S1, S2, S3) and two from East Africa (S4, S5), with a spatial overlap of the strains within each of these two regions. Nucleotide and amino-acid divergence between strains was up to 11%. Although more isolates from West African were sequenced, variability was twofold lower than among East African isolates. Variability in ORF4 and in ORF2 coincided. Within strain and within isolate variations in nucleotide sequences were low. Bipartite nuclear targeting motif, Ca2+ binding sites and at least two stretches of amino-acids were conserved among the 40 RYMV isolates and the other sobemoviruses. Variants associating sequence motifs characteristic of different strains have been found, possibly resulting from recombination events. Differences in pathogenicity among isolates were associated with changes of amino-acids in the bipartite nuclear targeting motif of the R domain of the capsid protein, and around conserved positions 151-154 of the S domain. We hypothesise that the observed pattern of variation of RYMV reflects the effect of spatial isolation between East and West Africa coupled with adaptive changes associated to the original virus reservoirs of the different strains.