Analysis of Plasmodium falciparum diversity in natural infections by deep sequencing

methods for the large-scale analysis of genetic variation in Plasmodium falciparum by deep sequencing of parasite DNA obtained from the blood of patients with malaria, either directly or after short-term culture. Analysis of 86,158 exonic single nucleotide polymorphisms that passed genotyping quality control in 227 samples from Africa, Asia and Oceania provides genomewide estimates of allele frequency distribution, population structure and linkage disequilibrium. By comparing the genetic diversity of individual infections with that of the local parasite population, we derive a metric of within-host diversity that is related to the level of inbreeding in the population. An open-access web application has been established for the exploration of regional differences in allele frequency and of highly differentiated loci in the P. falciparum genome. The genetic diversity and evolutionary plasticity of P. falciparum are major obstacles for malaria elimination. New forms of resistance against antimalarial drugs are continually emerging 1,2 , and new forms of antigenic variation are a critical point of vulnerability for future malaria vaccines. Effective tools are needed to detect evolutionary changes in the parasite population and to monitor the spread of genetic variants that affect malaria control. Here we describe the use of deep sequencing to analyse P. falciparum diversity, using blood samples from patients with malaria. The P. falciparum genome has several unusual features that greatly complicate sequence analysis, such as extreme AT bias, large tracts of nonunique sequence and several large families of intensely polymorphic genes 3 . Our aim was therefore not to determine the entire genome sequence of individual field samples—which would be prohibitively expensive with current technologies—but to define an initial set of single nucleotide polymorphisms (SNPs) distributed across the P. falciparum genome, whose genotype can be ascertained with confidence in parasitized blood samples by deep sequencing. An additional complication in the analysis of P. falciparum genome variation is that the billions of haploid parasites that infect a single individual can be a complex mixture of genetic types. Previous studies 4–8 have largely focused on laboratory-adapted parasite clones, but the within-host diversity of natural infections is of fundamental biological interest. Parasites in the blood replicate asexually, but when they are taken up in the blood meal of an Anopheles mosquito they undergo sexual mating. If the parasites in the blood are of diverse genetic types, this process of sexual mating can generate novel recombinant forms. Deep sequencing provides new ways of investigating within-host diversity and the role of sexual recombination in parasite evolution.