Effects of graft and aphid transmission on the genetic diversity and population structure of Turkish citrus tristeza virus isolates

Citrus tristeza virus (CTV) causes the most destructive viral disease of citrus and is transmitted by grafting and multiple aphid vectors. The mode of transmission is important for the epidemiology, genetic diversity, and evolution of viruses. Therefore, the effects of graft and aphid transmission on the population structure of Turkish CTV isolates were evaluated in this study. First, molecular, and biological characteristics of original field isolates (OR) collected from five different citrus growing regions were determined. Then, they were all grafted onto Madam Vinous sweet orange and the 24 graft-transmitted (GT) isolates were obtained and maintained. When 24 GT isolates were transmitted to Mexican lime seedlings by Aphis gossypii, 30 sub-isolates (AT) were obtained from 15 aphid-transmissible isolates. The CP genes of all OR isolates their GT and AT sub-isolates were amplified by RT-PCR, cloned, and sequenced and compared with each other and well-characterized type strains. The changes in population structure of isolates and sub-isolates were analyzed. While genetic diversity was increased by graft transmission, it was reduced to the level of the original isolates after aphid transmission. Comparisons of the phylogenetic grouping and the genotype profiles of the OR isolates and their GT and AT sub-isolates showed a partial or complete genotype shift in some isolates. The results suggested that mode of transmission may play a role in changes in the genetic diversity of CTV isolates.