Sequences of Citrus Tristeza Virus Separated in Time and Space Are Essentially Identical

Abstract

The first Citrus tristeza virus (CTV) genomes completely sequenced (19.3-kb positive-sense RNA), from four biologically distinct isolates, are unexpectedly divergent in nucleotide sequence (up to 60% divergence). Understanding of whether these large sequence differences resulted from recent evolution is important for the design of disease management strategies, particularly the use of genetically engineered mild (essentially symptomless)-strain cross protection and RNA-mediated transgenic resistance. The complete sequence of a mild isolate (T30) which has been endemic in Florida for about a century was found to be nearly identical to the genomic sequence of a mild isolate (T385) from Spain. Moreover, samples of sequences of other isolates from distinct geographic locations, maintained in different citrus hosts and also separated in time (B252 from Taiwan, B272 from Colombia, and B354 from California), were nearly identical to the T30 sequence. The sequence differences between these isolates were within or near the range of variability of the T30 population. A possible explanation for these results is that the parents of isolates T30, T385, B252, B272, and B354 have a common origin, probably Asia, and have changed little since they were dispersed throughout the world by the movement of citrus. Considering that the nucleotide divergence among the other known CTV genomes is much greater than those expected for strains of the same virus, the remarkable similarity of these five isolates indicates a high degree of evolutionary stasis in some CTV populations.

Molecular Virology and Pathogenicity of Citrus tristeza virus

“…This high genomic divergence between distinct genotypes confirmed the genetic variability found between strains composing the CTV isolates. In spite of this genetic variability, Albiach-Martí et al (2000b) reported that the genomes of the symptomless isolates T30 from Florida and T385 from Spain, which where separated geographically and in time, were essentially identical (Figure 3). Moreover, these authors demonstrated that the T30/T385 genotype was distributed around the world and it could have been stable at least 500 years, which suggests that the T30/T385 genotype is well adapted to the citrus environment.…”

“…Another problem is that natural mild CTV isolates may contain minor severe stem pitting variants that, upon aphid transmission, could become prevalent (Moreno et al, 1993;Velazquez-Monreal et al, 2009). Since only isolates within a closely related sequence group will cross-protect , naturally occurring mild T30-like isolates (Albiach-Martí et al, 2000b), would not protect against disease inducing isolates from other genotypes.…”

“…This genetic stasis has been explained as a consequence of strong selection and competition between the mutants that arise in each replication cycle, which creates equilibrium in the viral quasispecies distribution (Blok et al, 1987;Fraile et al, 1997). Therefore, if some CTV sequences tend to remain relatively stable over periods of years, sequence-based control strategies like transgenic plants based in PTGS or cross-protection using recombinant mild CTV strains, have a higher probability of success (Albiach-Martí et al, 2000bFolimonova et al, 2010).…”

“…Additionally, aphid or graft transmission and host passage could modify the composition of the D-RNAs population in the CTV isolates (Albiach-Martí et al, 2000a;Mawassi et al, 1995b) The sequencing of the genomes of nineteen CTV isolates from distant places in the planet, which represented a subset of its local CTV population, helped the understanding of the evolution of CTV and the complex structure of the actual CTV isolates. These genomic sequences are T36 and T30 from Florida, USA (Albiach-Martí et al, 2000b;Karasev et al, 1995); VT from Israel ; SY568R from California (Z.N. Vives et al, 2005); T385 and T318A from Spain (Ruiz-Ruiz et al, 2006;Vives et al, 1999); NuagA from Japan (Suastika et al, 2001); Qaha (AY340974) from Egypt; Mexican isolate (DQ272579); B165 from India ; NZ-M16, NZ-B18, NZRB-TH28, NZRB-TH30, NZRB-M12, NZRB-M17 and NZRB-G9 from New Zealand (Harper et al, 2009(Harper et al, , 2010; and HA16-5 and HA18-9 from Hawaii (Melzer et al, 2010).…”

The Complex Genetics of Citrus tristeza virus

“…Actually, there are twenty CTV genomic sequences available. These are T36 and T30 from Florida [2,27]; VT from Israel [28]; SY568R from California [29,30]; T385 and T318A from Spain [31,32]; NuagA from Japan [33]; Qaha (AY340974) from Egypt; Mexican isolate (DQ272579); B165 form India [34]; NZ-M16, NZ-B18, NZRB-TH28, NZRB-TH30, NZRB-M12, NZRB-M17 and NZRB-G9 from New Zealand [35,36]; HA16-5 and HA18-9 from Hawaii [37] and Kpg3 from China [38]. Genetic comparison of these CTV genomes revealed an extreme genomic divergence for genotypes of the same viral species (Figure 3).…”