Genetic structure of Citrus tristeza virus (CTV) populations from field Argentinian grapefruit isolates.

Abstract

Commercial pigmented grapefruit in the Northwest region of Argentina have been seriously affected by Citrus tristeza virus (CTV) and a great number of trees have died. The population structure of 12 grapefruit isolates was assessed by single-strand conformation polymorphism (SSCP) analysis of the p20 gene. SSCP patterns showed that most isolates contained only one predominant sequence variant. Global genetic diversity values were similar to those found for CTV isolates from other world citrus growing regions. However, the within-isolate genetic diversity, quantified by the parameter heterozygosity , showed values higher than those found for CTV isolates from other citrus production areas. The existence of several haplotypes in the same isolate may be due to continuous aphid inoculations with isolates having divergent haplotypes. In Argentina, the main aphid CTV vector, Toxoptera citricida , is endemic. The high heterozygosity of Argentinian isolates makes probable the presence of sequence variants that cause severe symptoms, which could predominate under certain conditions. Analysis of phylogenetic relationships, using p20 gene sequences from GenBank, showed an unrooted tree in which most Argentinian isolates grouped with isolates VT and SY568.