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1 October 2000 STRAIN TYPING OF TOXOPLASMA GONDII: COMPARISON OF ANTIGEN-CODING AND HOUSEKEEPING GENES
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Abstract
Molecular characterization of Toxoplasma gondii isolates is central for understanding differences in disease transmission and manifestations. Only 3 subgroups (lineages) have been discerned with subtle within-lineage variation, permitting low-resolution classification of isolates. Because proteins, coding sequences, and especially antigen-coding genes have been used extensively in previous studies, we focused on sequence variation in introns of housekeeping genes, which may be more informative for phylogenetic analysis because they evolve under lower selection. We compared sequence variation in introns of 5 housekeeping genes with 2 antigen-coding genes. Introns of housekeeping genes were slightly more polymorphic than coding and noncoding regions of antigen-coding genes and only the former showed intralineage variation. Intragenic linkage disequilibrium was complete, but intergenic linkage, although highly significant, was incomplete, suggesting that genes are partially uncoupled. Six of 7 substitutions found within the region coding for the tachyzoite surface antigen, SAG2, were nonsynonymous, indicating that diversifying selection acts on this locus. Typing isolates on the basis of housekeeping and antigen-coding genes was consistent, but the phylogenetic relationships among the resulting groups was inconsistent. A cougar isolate typed as lineage II using a restriction fragment length polymorphism assay possessed multiple unique polymorphisms, suggesting that it represents a new lineage. We concluded that introns of housekeeping genes are preferred markers for phylogenetic study, and that multilocus genotyping is preferred for typing parasites, especially from feral or unstudied environments.
T. Lehmann, C. R. Blackston, S. F. Parmley, J. S. Remington and J. P. Dubey "STRAIN TYPING OF TOXOPLASMA GONDII: COMPARISON OF ANTIGEN-CODING AND HOUSEKEEPING GENES," Journal of Parasitology 86(5), (1 October 2000). https://doi.org/10.1645/0022-3395(2000)086[0960:STOTGC]2.0.CO;2
Received: 1 November 1999; Accepted: 1 March 2000; Published: 1 October 2000
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