Evidence for clonal propagation in natural isolates of Plasmodium falciparum from Venezuela

We have analyzed 75 isolates of Plasmodium falciparum. collected in Venezue la during both the dry (November) and rainy (May-July) seasons, with a rang e of genetic markers including antigen genes and 14 random amplified polymo rphic DNA (RAPD) primers. Thirteen P, falciparum stocks from Kenya and four other Plasmodium species are included in the analysis for comparison. Cros s-hybridization shows that the 14 RAPD primers reveal 14 separate regions o f the parasite's genome. The P, falciparum isolates are a monophyletic clad e, significantly different from the other Plasmodium species. We identify t hree RAPD characters that could be useful as "tags" for rapid species ident ification, The Venezuelan genotypes fall into two discrete genetic subdivis ions associated with either the dry or the rainy season; the isolates colle cted in the rainy season exhibit greater genetic diversity. There is signif icant linkage disequilibrium in each seasonal subsample and in the full sam ple. In contrast, no linkage disequilibrium is detected in the African samp le. These results support the hypothesis that the population structure of P . falciparum in Venezuela, but not in Africa, is predominantly clonal, Howe ver, the impact of genetic recombination on Venezuelan P, falciparum seems higher than in parasitic species with long-term clonal evolution like Trypa nosoma cruzi, the agent of Chagas' disease. The genetic structure of the Ve nezuelan samples is similar to that of Escherichia coli. a bacterium that p ropagates clonally. with occasional genetic recombination.