The Complex Genetics of Citrus tristeza virus

The 2000 x 11 nm long bipolar flexuous filamentous particles of Citrus tristeza virus (CTV) (genus Closterovirus, family Closteroviridae) (Figure 1) contain a single-stranded positive-sense RNA genome of 19.3 kb, which is encapsidated in two different capsid proteins that coat the opposite ends of the virions [1, 2]. CTV is the largest identified RNA virus infecting plants and the second largest worldwide after the animal Coronaviruses. The virus is phloem limited and it is transmitted by aphids (Hemiptera: Aphididae) (Figure 1), and mechanically by graft propagation of virus-infected plant tissues. CTV isolates from different hosts and areas display great variability either biologically or genetically. There are wild CTV isolates that consist basically of a main genotype and its quasispecies, but others could contain a mixture of strains (groups of viral variants with similar sequence) that differ in symptomology and in viral transmission efficiency by aphids. These CTV strains could bear divergent CTV genotypes. Additionally, wild isolates are also composed by a population of defective RNAs (D-RNAs) that could change by aphid or graft transmission or by host passage [3].