Variability of p27 (dCP) and p25 (CP) genes among Argentine citrus tristeza virus isolates

GAGO-ZACHERT, SELMA P.; ROBLEDO, GERMÁN; COSTA, NORMA; GRAU, OSCAR; SEMORILE, LILIANA C.

Sydney

Congreso; XIth International Congress of Virology; 1999

International Union of Microbiological Societies.

Citrus tristeza virus (CTV) isolates are complex populations of different genomic and defective RNAs that vary greatly in biological characteristics such as aphid transmissibility or symptoms caused in host species. The genome regions involved in the expression of any of these symptoms are presently unknown. However, most virus isolates consist of a master sequence that usually makes up the majority of the population. CTV has a positive sense RNA genome of 19.3 nts encapsidated in flexuous, rod-shaped particles with two capsid proteins: a 25 kDa coat protein covering about 95% of the particle length, and a diverged 27 kDa coat protein that covers one end of the particle forming a rattlesnake structure. SSCP analysis is a method for detecting sequence differences in ssDNA by non-denaturing PAGE. In this work we applied this technique to evaluate sequence heterogeneity in p27 (dCP) and p25 (CP) genes of field Argentine CTV isolates. Eleven isolates from different geographical regions and citrus hosts were selected and biologically characterized by inoculation in five indicator citrus species. Each Held CTV isolate was assigned to one of the five biogroups defined for this virus. For each isolate, both genes were reverse transcribed using the same 3´primer, amplified by PCR and ten clones of each viral gene were analyzed. A variable number of SSCP patterns and percentage of the major clone were obtained for each gene of each isolate. From the results achieved, three interesting observations were made: i) p25 (CP) gene showed a higher heterogeneity than p27 (dCP) gene suggesting that a different selective pressure is acting in their evolution, ii) viral population structure for both genes was more homogeneous in grapefruit isolates than in orange ones and iii) in two of the most severe grapefruit CTV isolates p27 gene were highly homogeneous.