Population dynamics of a Florida Citrus tristeza virus isolate and aphid-transmitted subisolates: identification of three genotypic groups and recombinants after aphid transmission.

Tristeza is an important citrus disease affecting the viability and productivity of citrus worldwide. The causal agent, Citrus tristeza virus (CTV), usually occurs as a mixture of genotypes in nature, with one of the genotypes often dominating the population. CTV has a monopartite, positive-sense RNA genome of approximately 19.3 kb and exhibits over 30% diversity in the 5' half and less than 10% in the 3' half among different genotypes. A Florida CTV isolate, FS627, was selected for this study. Isolate FS627 was analyzed by reverse-transcription polymerase chain reaction (RT-PCR) using primers to three regions: 788-bp region in the 5' (697 to 1,484 nucleotides), open reading frame (ORF)1a, 696 or 718 bp from the overlapping region of the RdRp (ORF1b) and p33 (ORF2) gene, and a 672-bp major coat protein gene (ORF7) in the 3' end of the CTV genome. The presence of T36, T30, and VT genotypes in isolate FS627 was confirmed utilizing the genotype specific overlapping region of RdRp primer pairs for RT-PCR amplification followed by cloning and sequence analysis. Analysis of single-strand conformational polymorphisms and sequences of RT-PCR-amplified products of the above regions were used to determine the presence of genotypes in both the parent and aphid-transmitted (AT) subisolates. Although the parent isolate had T36 as the major genotype, T30 was the major genotype in most of the AT subisolates. Some intermediate genotypes were detected that differed from the parental or AT subisolates. These intermediate genotypes were considered to be recombinants of the T30 and T36 genotypes and also were observed in the second level of AT subisolates generated from the of first-level AT subisolates of CTV-FS627. This work provides advance information on the population dynamics in CTV mixtures and the generation of virus recombinants after aphid transmission.