Virus Evolution Workgroup: 1999 Workshop Abstract

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Genetic diversity of natural populations of plant viruses in the family Closteroviridae

Luis Rubio, Ping Kong and Bryce W. Falk
1 Shields Ave. Department of Plant Pathology, University of California, Davis, CA 95616.
Email: Luis@UCDavis.Edu or bwfalk@ucdavis.edu.

VIRUS POPULATION STUDIES
The family Closteroviridae is composed of single-stranded positive sense RNA plant viruses of the genera Closterovirus and Crinivirus. We studied the genetic variation of different genome regions for isolates from natural populations of the viruses: citrus tristeza closterovirus (CTV), cucurbit yellow stunting disorder crinivirus (CYSDV), beet pseudo-yellows crinivirus (BPYV) and sweet potato chlorotic stunt crinivirus (SPCSV) (Table 1).

BPYV and SPCSV isolates formed homogeneous populations (Table 1). CYSDV isolates were separated into two homogeneous subpopulations (genetic diversity within subpopulations was less than 0.006). CTV CP analysis of isolates from different origins showed a complex divergence pattern, but no correlation between genetic relationship of CTV isolates and their geographic origin was found. Most CTV Californian isolates showed nucleotide distances less than 0.02 with the Spanish isolate T385 (Genebank accession Y184201). The low nucleotide distance, between geographically distant BPYV, CYSDV and CTV isolates, seems to indicate a recent divergence and a rapid expansion of these isolates. The international traffic of virus-infected plant material could be partly responsible for this. For some Californian CTV isolates the nucleotide distance was drastically different for different genome regions, suggesting the possibility of recombination events in the origin of some CTV isolates.

Table 1. Genetic diversity of different genome regions of Closteroviridae populations

For BPYV, CYSDV and most CTV isolates, each isolate was composed of a population of closely related variants (haplotypes) with one predominant, as predicted by the quasispecies model. However, some Californian CTV isolates had a high intra-isolate genetic variation. For example, in the genome region A, the isolate 65 contained two haplotypes (65A and 65 B) with nucleotide distance of 0.1297 between them. Haplotype 65A showed a nucleotide distance of 0.0159 with CTV isolate 107 and haplotype 65B showed a nucleotide distance of 0.0041 with CTV isolate 190. These data suggest that isolate 65 might have originated from a mixed infection of a single plant by previously diverged isolates similar to 107 and 190. This is not unlikely since CTV hosts are perennial and can be infected several times by successive aphid-borne-CTV inoculations.

Abstract - Presented at the Virus Evolution Workshop
Ardmore, OK
October 21 - 24th, 1999

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To contact the organizers:
e-mail: mroossinck@noble.org

Dr. Marilyn Roossinck
Plant Biology Division
The Noble Foundation
P.O. Box 2180
Ardmore, OK 73402

phone: 580 224-6630