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Contribution of recombination and selection to molecular evolution of Citrus tristeza virus

Susana Martín^1,2,,

Adrián Sambade^1,,

¹ Centro de Protección Vegetal y Biotecnología, Instituto Valenciano de Investigaciones Agrarias, Moncada, 46113 Valencia, Spain
² Instituto de Biología Molecular y Celular de Plantas (CSIC-UPV), 46022 Valencia, Spain

Correspondence
Pedro Moreno
moreno_pedgom{at}ivia.gva.es

The genetic variation of Citrus tristeza virus (CTV) was analysed by comparing the predominant sequence variants in seven genomic regions (p33, p65, p61, p18, p13, p20 and p23) of 18 pathogenically distinct isolates from seven different countries. Analyses of the selective constraints acting on each codon suggest that most regions were under purifying selection. Phylogenetic analysis shows diverse patterns of molecular evolution for different genomic regions. A first clade composed of isolates that are genetically close to the reference mild isolates T385 or T30 was inferred from all genomic regions. A second clade, mostly comprising virulent isolates, was defined from regions p33, p65, p13 and p23. For regions p65, p61, p18, p13 and p23, a third clade that mostly included South American isolates could not be related to any reference genotype. Phylogenetic relationships among isolates did not reflect their geographical origin, suggesting significant gene flow between geographically distant areas. Incongruent phylogenetic trees for different genomic regions suggested recombination events, an extreme that was supported by several recombination-detecting methods. A phylogenetic network incorporating the effect of recombination showed an explosive radiation pattern for the evolution of some isolates and also grouped isolates by virulence. Taken together, the above results suggest that negative selection, gene flow, sequence recombination and virulence may be important factors driving CTV evolution.

These authors contributed equally to this work.

Present address: Instituto de Biología Molecular y Celular de Plantas (CSIC-UPV), Campus UPV CPI 8E, Ingeniero Fausto Elio s/n, 46022 Valencia, Spain.

Present address: Department of Cell and Developmental Biology, John Innes Centre, Norwich NR4 7UH, UK.

The nucleotide sequences reported in this study have been assigned GenBank/EMBL/DDBJ accession numbers FM955890–FM956002.

Supplementary tables of primer sequences and FEL data are available with the online version of this paper.