Population genetic analysis of lettuce big-vein disease viruses and their vector fungi Olpidium virulentus in Ankara province, Turkey

Abstract

Lettuce big-vein disease (LBVD) is caused by either Mirafiori lettuce big-vein virus (MiLBVV) alone, or co-infected with Lettuce big-vein associated virus (LBVaV). Both viruses are transmitted by the fungal vector Olpidium spp. In the present study, population genetics of these two viruses and their fungal vector, O. virulentus, were analyzed, using the nucleotide sequences of the virus coat protein (CP) partial genes and the internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) of the vector fungi. The analysis was conducted along with all previously reported LBVD viruses and Olpidium spp. populations. We used conventional PCR techniques to identify the viruses in lettuce samples and their fungal vector in soil samples collected from the root zone of field-grown lettuce plants in the Ankara province of Turkey. Phylogenetic analyses showed that LBVD viruses had at least two evolutionary lineages, and Ankara LBVD isolates were more closely related to the European populations. Also, we found that there is a strong negative selection pressure in the partial CP genes of LBVD viruses isolated from Ankara, due to the effective terrestrial climate. The negative selection pressure indicates balancing selection within Turkish isolates and other populations. The population of O. virulentus isolates from Ankara and other geographic regions were branched into five sub-clades, and there was no geographical separation in these isolates. Our analysis also showed Olpidium spp. to be monophyletic, and showed speciation among the isolates.