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Dynamics of deletion genotypes in an experimental insect virus population.

ABSTRACT: Defective viruses, that are deficient in certain essential genes, are maintained in the population by trans-complementation, exploiting the gene products of complete genotypes in co-infected cells. This process becomes prevalent only when cells are frequently infected by several virus particles, and only then will the fitness of defective viruses be subjected to frequency-dependent selection. Deletion variants that are not infectious per os are present in a multicapsid nucleopolyhedrovirus (SfMNPV, Baculoviridae) that infects the fall army worm, Spodoptera frugiperda. These variants enhance the pathogenicity and, therefore, the likelihood of transmission of the virus when co-infecting cells with complete genotypes, resulting in occlusion bodies (OBs) that may contain both genotypes co-occluded. Mixtures of complete (B) and defective (C) variants in ratios of 90% B+10% C, 50% B+50% C and 10% B+90% C were used to inoculate by injection S. frugiperda larvae. Viral OBs extracted from diseased insects were subjected to four or five successive rounds of per os infection. Following successive passages, genotype frequencies in all three experimental populations converged to a single equilibrium frequency comprising approximately 20% of deletion genotype C and approximately 80% of complete genotype B. This mirrors the relative proportions of deletion (22%) and complete (78%) genotypes observed in the wild-type SfMNPV population. The pathogenicity of experimental populations at the final passage was not significantly different from that of the wild-type isolate. In contrast, OBs of all genotype mixtures were significantly more pathogenic than OBs of genotype B alone. A population genetics model, in which virus populations were assigned linear frequency-dependent transmissibility values, was in remarkably close agreement to empirical data. Clearly, non-infectious deletion variants can profoundly affect the likelihood of transmission and the genetic structure and stability of virus populations.

SUBMITTER: Simon O

PROVIDER: S-EPMC1560231 | biostudies-literature | 2006 Apr

REPOSITORIES: biostudies-literature

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Dynamics of deletion genotypes in an experimental insect virus population.

Simón Oihane O Williams Trevor T Caballero Primitivo P López-Ferber Miguel M

Proceedings. Biological sciences 20060401 1588

Defective viruses, that are deficient in certain essential genes, are maintained in the population by trans-complementation, exploiting the gene products of complete genotypes in co-infected cells. This process becomes prevalent only when cells are frequently infected by several virus particles, and only then will the fitness of defective viruses be subjected to frequency-dependent selection. Deletion variants that are not infectious per os are present in a multicapsid nucleopolyhedrovirus (SfMN ...[more]

PMID: 16618670

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Project description:Tomato yellow leaf curl virus is a species in the genus Begomovirus and family Geminiviridae. Tomato yellow leaf curl virus (TYLCV) infection induces severe symptoms on tomato plants and causes serious yield losses worldwide. TYLCV is persistently transmitted by the sweetpotato whitefly, Bemisia tabaci (Gennadius). Cultivars and hybrids with a single or few genes conferring resistance against TYLCV are often planted to mitigate TYLCV-induced losses. These resistant genotypes (cultivars or hybrids) are not immune to TYLCV. They typically develop systemic infection, display mild symptoms, and produce more marketable tomatoes than susceptible genotypes under TYLCV pressure. In several pathosystems, extensive use of resistant cultivars with single dominant resistance-conferring gene has led to intense selection pressure on the virus, development of highly virulent strains, and resistance breakdown. This study assessed differences in TYLCV genomes isolated from susceptible and resistant genotypes in Florida and Georgia. Phylogenetic analyses indicated that Florida and Georgia isolates were distinct from each other. Population genetics analyses with genomes field-collected from resistant and susceptible genotypes from Florida and/or Georgia provided no evidence of a genetic structure between the resistant and susceptible genotypes. No codons in TYLCV genomes from TYLCV-resistant or susceptible genotypes were under positive selection, suggesting that highly virulent or resistance-breaking TYLCV strains might not be common in tomato farmscapes in Florida and Georgia. With TYLCV-resistant genotypes usage increasing recently and multiple tomato crops being planted during a calendar year, host resistance-induced selection pressure on the virus remains a critical issue. To address the same, a greenhouse selection experiment with one TYLCV-resistant and susceptible genotype was conducted. Each genotype was challenged with TYLCV through whitefly-mediated transmission serially 10 times (T1-T10). Population genetics parameters at the genome level were assessed at T1, T5, and T10. Results indicated that genomes from resistant and susceptible genotypes did not differentiate with increasing transmission number, no specific mutations were repeatedly observed, and no positive selection was detected. These results reiterate that resistance in tomato might not be exerting selection pressure against TYLCV to facilitate development of resistance-breaking strains. TYLCV populations rather seem to be shaped by purifying selection and/or population expansion.

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Dynamics of deletion genotypes in an experimental insect virus population.

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Dynamics of deletion genotypes in an experimental insect virus population.

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