Genetic variability in the coat protein genes of lettuce big-vein associated virus and Mirafiori lettuce big-vein virus

Summary. Available data suggests that lettuce big-vein disease is caused by the ophiovirus Mirafiori lettuce big-vein virus (MLBVV) but not by the varicosavirus Lettuce big-vein-associated virus (LBVaV), although the latter is frequently associated with the disease. Since the disease occurs worldwide, the putative coat protein (CP) open reading frames of geographically distinct isolates of MLBVV and LBVaV were sequenced. Comparison of both nucleotide and amino acid sequences showed a high level of sequence similarity among LBVaV isolates. Phylogenetic analysis of LBVaV CP nucleotide sequences showed that most of the Spanish isolates clustered in a phylogenetic group whereas English isolates were more similar to the USA isolate. An Australian isolate was closely related to the Dutch isolate. Genetic diversity among MLBVV CP nucleotide sequences was higher ranging from 0.2% to 12%. Phylogenetic analysis of MLBVV CP nucleotide sequences revealed two distinct subgroups. However, this grouping was not correlated with symptom development on lettuce or the geographic origin of the MLBVV isolates. Finally, a quick method based on RFLP analysis of RT-PCR amplicons was developed for assigning MLBVV isolates to the two subgroups.