Contributions of genetic drift and negative selection on the evolution of three strains of wheat streak mosaic tritimovirus - Brief report

Genome sequences of three Wheat streak mosaic virus (WSMV) strains were com pared. The Type and Sidney 81 strains of WSMV from the American Great Plain s were closely related, with sequence identities of 97.6% (nucleotide) and 98.7% (amino acid). In contrast, the El Batan 3 strain from central Mexico was divergent, and shared only 79.2-79.3% (nucleotide) and 90.3-90.5% (amin o acid) sequence identity with Type and Sidney 81. All three WSMV strains w ere serologically related, however the El Batan 3 capsid protein (CP) had 1 5 fewer amino acid residues. Phylogenetic analysis of the CP cistron indica ted that Type, Sidney 81, and nine other American isolates of WSMV were clo sely related and distinct from the El Batan 3 sequence. Nucleotide substitu tions among the WSMV strains were not randomly distributed across the genom e with more variation within P1, HC-Pro, and CP and less within P3. One 400 -nucleotide region of the genome, corresponding to the 3'-end of P3, was st rikingly deficient in silent substitutions. Nonetheless, the ratio of synon ymous to non-synonymous substitutions throughout the genome was essentially the same for all three WSMV strains. Collectively, our data indicate that both genetic drift and negative selection have contributed to the evolution of WSMV strains.