Experimental evolution, genetic analysis and genome re-sequencing reveal the mutation conferring artemisinin resistance in an isogenic lineage of malaria parasites
© Hunt et al; licensee BioMed Central Ltd. 2010
Received: 10 March 2010
Accepted: 16 September 2010
Published: 16 September 2010
Classical and quantitative linkage analyses of genetic crosses have traditionally been used to map genes of interest, such as those conferring chloroquine or quinine resistance in malaria parasites. Next-generation sequencing technologies now present the possibility of determining genome-wide genetic variation at single base-pair resolution. Here, we combine in vivo experimental evolution, a rapid genetic strategy and whole genome re-sequencing to identify the precise genetic basis of artemisinin resistance in a lineage of the rodent malaria parasite, Plasmodium chabaudi. Such genetic markers will further the investigation of resistance and its control in natural infections of the human malaria, P. falciparum.
A lineage of isogenic in vivo drug-selected mutant P. chabaudi parasites was investigated. By measuring the artemisinin responses of these clones, the appearance of an in vivo artemisinin resistance phenotype within the lineage was defined. The underlying genetic locus was mapped to a region of chromosome 2 by Linkage Group Selection in two different genetic crosses. Whole-genome deep coverage short-read re-sequencing (Illumina® Solexa) defined the point mutations, insertions, deletions and copy-number variations arising in the lineage. Eight point mutations arise within the mutant lineage, only one of which appears on chromosome 2. This missense mutation arises contemporaneously with artemisinin resistance and maps to a gene encoding a de-ubiquitinating enzyme.
This integrated approach facilitates the rapid identification of mutations conferring selectable phenotypes, without prior knowledge of biological and molecular mechanisms. For malaria, this model can identify candidate genes before resistant parasites are commonly observed in natural human malaria populations.
The molecular basis of drug resistance in malaria parasites and its evolution in time and space can be investigated, and possibly controlled, once the genes and specific mutations involved have been identified. However, despite intense investigations of human malaria parasites, few mutations have been unambiguously linked to drug resistance phenotypes. The most direct evidence for novel genetic markers comes from classical genetic studies that require no prior knowledge regarding mode of action or resistance . For example, linkage analysis of a genetic cross in Plasmodium falciparum identified the locus containing pfcrt[2, 3], the determinant of chloroquine resistance. This facilitated the characterisation of selective sweeps  driven by the world-wide use of chloroquine, and generated insights into the molecular basis of resistance [5, 6]. Genetic studies have been extended to consider smaller gene effects, such as those underlying quinine susceptibility, by quantitative trait loci analysis . However, there are experimental and ethical factors that constrain the use of P. falciparum for genetic studies. For example, it has sometimes proved difficult to generate drug-resistant P. falciparum mutant clones in vitro and in these cases it has been necessary to await the appearance of resistant parasites in field samples. Also, although three genetic crosses between P. falciparum parasites have been performed and analysed [9–11] for genes involved in drug resistance or erythrocyte invasion, these require primate hosts and are therefore expensive and demand stringent ethical approval. Fortunately, the experimental tractability of the rodent malaria P. chabaudi presents a number of advantages, as follows.
In order to map genes conferring selectable phenotypes, P. chabaudi has also been used to develop Linkage Group Selection (LGS) , where recombinant parasites from genetic crosses are analysed en masse; i.e. without the need to investigate the genotypes and phenotypes of individually cloned recombinant parasites. LGS scans the genome for 'selection valleys'; regions of selection where the proportion of alleles from a drug-sensitive parent are reduced in the uncloned drug-treated progeny of a genetic cross, relative to those in an untreated population. This approach has been used to map genes underlying pyrimethamine resistance  and strain-specific immunity [19, 20] in P. chabaudi, and growth rate in P. yoelii.
Now, the advent of next-generation sequencing technology  makes possible whole-genome re-sequencing of mutant clones [23, 24]. Because an annotated P. chabaudi AS-sens whole-genome assembly has recently been made available , a comprehensive whole genome re-sequencing of clones of the AS-lineage (Figure 1) will define all of the genetic variation arising within the lineage, identify mutations in selection valleys and, thereby, advance our understanding of the relationships between genetic mutation, drug response and selection.
This paper integrates these evolutionary, genetic and genomic approaches to specify the specific mutation underlying in vivo resistance to artemisinin. We demonstrate the evolution of artemisinin resistance in the P. chabaudi AS lineage, use LGS to map an underlying gene in two independent genetic crosses and re-sequence the complete genomes of the wild-type AS parasite and two artemisinin resistant mutants. We conclude that a single point mutation in the gene ubp1, encoding a de-ubiquitinating enzyme, confers artemisinin resistance in this P. chabaudi lineage.
Experimental evolution of artemisinin resistance in P. chabaudi
Whole-genome genetic analysis of artemisinin resistance
Because apparent genetic associations can arise by chance, we used two statistical approaches to test the significance of this selection valley (Methods, Additional File 2). Firstly, the deepest selection valley from each of 500 simulations (Additional File 3) of the experimental design were used to estimate the probability (null hypothesis) that the depth of an observed selection valley was the result of random processes working in combination with selection at minor loci for resistance (p-value for no major gene = 0.026, Figure 3A, Additional Files 4, 5). Secondly, the non-parametric Mann-Whitney U-test was used to evaluate the probability that the allele frequency observed under drugs is significantly different from the random scatter of allele frequencies observed in the absence of drugs. The stringency of the statistical analysis was increased incrementally by reducing the AJ allele frequencies (AF-reduction) in untreated parasites (p < 0.001 at 50% AF-reduction, Additional File 6). Even so, the Mann-Whitney U-test underestimates the significance of the chr02 selection valley because the selection valley extended beyond the 5 consecutive loci used to compute significance (7 linked assays gave low AJ proportions, 4 of which give p < 0.05). Also, chr02 was the most dominant selection valley when a different genetic cross (AS-15MF × AJ) was selected with 100 mg kg-1 ART (data not shown) and the profile of selection on chr02 was similar to that of the AS-30CQ × AJ cross (Figure 3B). In this case, the AJ frequency of marker pcpf01-0197 was reduced from 54.2% to 1.0% (Mann-Whitney, p < 0.001 at 25% AF-reduction).
Whole genome re-sequencing of ART resistant parasites
Solexa genome re-sequencing - summary statistics.
Number of lanes
Read length (bases)
Nominal fold coverage
% reads mapped to reference sequence
- fold coverage
Point mutations common to both AS-30CQ and AS-15MF.
AS-30CQ and AS-15MF
This analysis of high-quality SNPs also predicted three point mutations specific to AS-30CQ and one point mutation specific to AS-15MF, all on chromosomes other than chr02 (data not shown). The complete set of lineage-specific mutations is therefore extremely limited suggesting that a small number of mutations account for the evolution of multiple drug resistance phenotypes in this lineage. Their contribution to other drug-resistance phenotypes (such as chloroquine resistance) will be evaluated in future investigations.
Five further putative point mutations (in AS-30CQ) were identified by SSAHA2 but not by MAQ (data not shown). These putative mutations have markedly lower quality scores and low relative coverage. One mapped to a gene without a P. falciparum orthologue and was negated by di-deoxy sequencing. The other four mapped to sub-telomeric or unassigned contigs which have reduced sequence complexity, increased mapping ambiguity and inferior reference sequence quality. We were therefore unable to amplify products from these regions for di-deoxy sequencing.
Previous sequence analysis of the orthologues of four candidate genes (atp6, crt, mdr1, tctp) showed that these were not mutated in AS-ART . As expected, the genome re-sequencing data presented here showed that there were no mutations in these genes in AS-30CQ or AS-15MF either.
A major effect gene conferring artemisinin resistance was mapped to chr02 by LGS analysis, as described in the previous section. If the V2728F ubp1 point mutation is the sole mutation identified on chr02 and if that mutation arose between the isogenic sensitive and resistant parasite, it must be considered the critical mutation conferring the resistance phenotype. It was therefore important to evaluate the possibility that a chr02 mutation had been missed in our analysis.
Unique mapping of reads to AS-WTSI reference sequence.
% of genome sequence (WTSI) mapped
in silico (unique)
> = 5
> = 10
> = 5
> = 10
AS-30CQ genome coverage by chromosome
% coverage >10
Total (chrs only)
ubp1 +/-200 kb on chr02
Identification of insertions, deletions and copy number variants by Solexa re-sequencing
We applied MAQ and SSAHA2 to detect candidate insertions/deletions (indels) and copy number variants (CNVs). Full details of this analysis are given in Additional Files 7, 9. To summarise, we used SSAHA2 to predict small (≤ 3 bp) insertions and deletions (indels), but no such mutations were confirmed. Analysis of decreased or increased read coverage can be used to predict larger deletions and CNVs, respectively. A 34 bp deletion in an intergenic region of chr07 was confirmed by di-deoxy sequencing. A potential large (~1 kb) deletion at the right-hand end of chr05 within a rodent malaria-specific gene (PCHAS_051920) was also identified but unconfirmed because appropriate DNA fragments could not be amplified and sequenced, probably because of low-complexity genomic sequence. Specifically, there were no predictions of indels or CNVs on chr02, confirming that these mutation classes are unlikely to confer artemisinin resistance in AS-30CQ.
Ubp1 mutation (V2728F) confers ART resistance
Various lines of evidence confirm that the V2728F ubp1 mutation confers artemisinin resistance. The ubp1 mutation mapped to the bottom of the selection valley in both genetic crosses analysed (Figure 3B). Solexa re-sequencing shows that this mutation is the only genetic variation between resistant and sensitive clones on chr02. Sanger dideoxy-sequencing confirmed that the wild-type V2697, V2728 ubp 1 haplotype is carried by all four of the artemisinin-sensitive parasites (AS-sens, AS-PYR, AS-3CQ and AJ) while V2728F (in AS-30CQ, AS-ART and AS-15MF) or V2697F (in AS-ATN, formerly V739F) mutations  are borne by parasites with artemisinin resistance (Additional File 10 in this lineage of isogenic parasites. Of the eight point mutations identified within the AS lineage, those in ubp1 were the only ones acquired along with the artemisinin resistance phenotype (i.e. between AS-3CQ and AS-30CQ, AS-15MF, or AS-ATN).
The V2728F ubp1 mutation in the AS lineage, was previously identified  but its association with artemisinin resistance was not demonstrated because (i) the ubp1 mutation in AS-ART arose during the generation of AS-30CQ under chloroquine selection, and could not underpin the increase in artemisinin resistance in AS-ART relative to AS-30CQ, (ii) the selection valley on chr02 (in the AS-ART × AJ cross) was not sufficiently dominant with a 25 mg kg-1 artemisinin selection, and (iii) the possibility that additional mutations in genes linked to ubp1 on chr02 could not be excluded. The present data resolve these ambiguities. Firstly, all clones (including AS-30CQ, AS-15MF, AS-ATN and AS-ART) with ubp1 mutations have artemisinin-resistant phenotypes (100 mg kg-1, 3 d). Secondly, the chr02 locus is strongly selected in two different additional independent crosses by 100 mg kg-1 artemisinin. Three independent genetic crosses (using one of the artemisinin-resistant clones AS-30CQ, AS-15MF (both in this report) or AS-ART ) have now mapped the gene underlying artemisinin resistance to chr02. Thirdly, whole-genome re-sequencing confirmed that the point mutation in ubp1 is the only mutation on chr02.
The cloned parasite AS-ATN (Figures 1, 2) also has an artemisinin resistance phenotype but contains a different ubp1 mutation (V2697F) . We propose that the V2697F ubp1 mutation underlies artemisinin resistance in AS-ATN too. Formal confirmation of this would require an LGS analysis to identify a chr02 selection valley in an AS-ATN × AJ cross (with respect to artemisinin, or artesunate), and genome re-sequencing of AS-ATN to confirm that there was no other mutation on chr02. We believe that both ubp1 mutations arose (in different parasites) during the production of the uncloned line AS-15CQ  from AS-3CQ under chloroquine selection (Additional File 11). This line was subsequently subjected to further selection by chloroquine , mefloquine  or artesunate . Chloroquine and mefloquine are thought to have selected (to fixation, or near fixation) V2728F ubp1 parasites, while artesunate is thought to have selected V2697F ubp1 parasites. A full discussion of these issues is included elsewhere (Additional File 11)
The structural consequences of the V2728F ubp1 mutation in AS-30CQ (and V2697F ubp1 in AS-ATN) were predicted to be a reduction in de-ubiquitinating activity . This would increase the proportion of ubiquitinated substrate, which may be destined for degradation through the 26 S proteasome. We suggest that reduced parasite susceptibility to artemisinin may involve increased turnover or altered trafficking of a specific protein or class of proteins (such as membrane transporters like MDR1, regulators of cell cycle etc.), or non-specific effects such as a general increased turnover of proteins damaged by oxidative stress
The model - its generality, application and future development
The experimental integration of mutant selection, genetic linkage analysis using LGS, and genome re-sequencing is a powerful framework for the elucidation of genetic determinants without prior knowledge of the underlying biology of the phenotype. The model exploits the co-incidence between genetically defined selection valleys and isolated mutations identified by deep genome re-sequencing. Importantly, it may also confirm the lack of dominant contributions from additional genes elsewhere in the genome. The approach complements reverse genetic validation (transfection) of gene candidates which may be restricted to limited numbers of pre-defined genes and gene mutations (allelic replacements). Unfortunately, a well-developed transfection system is currently not available in P. chabaudi.
The approach is applicable to many recombining organisms for which genome sequence data and experimentally tractable selectable phenotypes are available. In P. falciparum, there remain difficulties in reliably generating genetically stable drug-resistant mutants and the practicality of performing genetic crosses . Nevertheless, the approach described here may find applications in the study of human malaria. For example, a whole-genome tiling array was used to identify a large amplification event in an in vitro selected fosmidomycin-resistant P. falciparum mutant clone . The amplified fragment included the gene encoding 1-deoxy-D-xylulose-5-phosphate reductoisomerase, the putative target of fosmidomycin. Deep re-sequencing technologies can add further resolution to such investigations. Importantly, in addition, genetic analyses such as LGS also map the functional relationship between gene locus and phenotype; critical when elucidating resistance to drugs for which the mode of action is not well understood.
Pyrosequencing assays were used here to estimate the proportions of AS and AJ parental alleles and to detect loci under selection. However, we envisage the future exploitation of deep short-read sequencing to rapidly define and quantitate parental SNPs in pooled recombinant parasites (from genetic crosses) before and after selection. We expect this to improve the detection and resolution of selection valleys in LGS experiments. It will also avoid the time required to develop pre-mapped quantitative parental markers, and permit the use of a greater range of species and strains.
The genetic basis of multi-drug resistance
The comprehensive genome re-sequencing described here identified a small number of (mainly non-synonymous) mutations arising in the drug-resistance lineage of P. chabaudi. These data suggest that complex multi-drug resistant phenotypes evolved with the accumulation of a relatively small number of mutations, contrary to some previous speculations . The question arises: are all of these mutations selected (to fixation or near fixation) by drugs, or are some mutations neutral and randomly fixed during cloning by stochastic processes? Future studies will address this question and evaluate the extent to which ubp1 and other mutations affect the responses of these parasites to other drugs such as chloroquine.
Public health implications
The prediction of genes involved in drug resistance phenotypes has important public health implications, especially when candidate markers are proposed before resistance is commonly observed in the field, as is the case for the current study. For example, molecular genetic markers of resistance are required to monitor strategies used to contain the spread of resistant parasites. At present, we are vulnerable to the evolution of artemisinin resistance. Indeed, although artemisinin combination therapies continue to be widely effective, there have been a number of recent reports documenting reduced susceptibilities of malaria parasites to artemisinin derivatives in vivo and in vitro, especially in parts of South-east Asia, such as Western Cambodia [34–36]. Although genes such as the multi-drug resistance locus, mdr1[37, 38], or pfatp6, encoding an ATP-ase (SERCA-type), have been proposed to contribute to artemisinin resistance, either these markers have yet to be confirmed or it is generally believed that they mediate small effects or reflect responses to the non-artemisinin partner drug [40, 41]. At present therefore, there are no generally accepted validated markers for artemisinin resistance-mediated treatment failure.
The ubp1 mutations are the first genetic determinants to be shown to underlie a distinct change in in vivo artemisinin responses. The relevance of mutations or polymorphisms in the ubp1 orthologue (PFA0220w on P. falciparum chr01) to in vivo and in vitro artemisinin responses of P. falciparum parasites can now be addressed by characterising natural infections and by transfection studies in the laboratory. For example, a recent study of parasites from Western Cambodia  showed that there were no corresponding mutations in ubp1 in artemisinin-resistant P. falciparum parasites.
The data presented here show that, in P. chabaudi, a single point mutation confers a distinct shift in artemisinin responses. However, whether further mutations in ubp1 or other genes may confer additional shifts in artemisinin response is still uncertain. These possibilities can be addressed by the generation of multiple independently selected resistant clones of P. chabaudi, or by selecting mutants with higher levels of resistance.
Interestingly, artemisinin-resistance and V2728F ubp1 mutations arose in the P. chabaudi isogenic lineage before the parasites were exposed to artemisinin itself. Thus, ubp1 mutations may prove to play a role in multi-drug resistance, as is the case for mdr1. These relationships are the subject of ongoing investigations. Although a distinct change in artemisinin response is described here, observations that in vitro IC50s of artemisinin derivatives in P. falciparum are often correlated with those of structurally unrelated aminoquinolines [44–46] may indicate that similar processes occur in human malaria parasites.
The identification of ubp1 mutations will augment our understanding of artemisinin-resistance mechanisms by opening up new experimental approaches, such as the identification of proteins interacting with ubp1. This may clarify whether artemisinin resistance is mediated by the modulation of pre-existing drug resistance mechanisms or by interference with the pathway of artemisinin action.
Combination therapies have been proposed to reduce the probability of selecting (multi-) resistant parasites [47, 48]. This rationale depends upon choosing component drugs with different mechanisms of resistance, and upon the absence of pre-existing resistance to one (or both) components. The future choice of optimal combinations may be guided by knowledge delivered using a predictive model such as that presented here.
Experimental evolution of drug resistance, a genome-wide genetic linkage analysis (LGS) and whole genome deep re-sequencing have been used to define the mutation conferring artemisinin resistance in the malaria parasite, P. chabaudi. This system can be used for the rapid identification of candidate genetic markers of in vivo drug resistance, before resistance arises in the field. These data may be used to monitor, understand and prevent the evolution of drug resistance in the field and to choose component drugs in combination treatments designed to maximise drug longevity.
Parasite lines, maintenance, parasite preparation and DNA extraction
A number of P. chabaudi clones of the AS-lineage (Figure 1) were used [12–16]. They were routinely inoculated, passaged in CBA mice (4-6 weeks) and cryopreserved as previously described . Parasites were prepared and DNA extracted as previously described , ensuring that host white cells were removed by CF11 cellulose (Whatman, UK) and Plasmodipur filters (Eurodiagnostica, Netherlands).
Phenotyping, drug tests
Artemisinin drug tests for P. chabaudi parasites have been previously described [16, 27]. Here we used 3 day treatments on day 1-3 post-inoculation (p.i.) (1 × 106 parasites) at the defined doses in CBA mice. Parasitaemias were estimated from a drop of blood from the tail in Giemsa stained thin blood smears as follows. Infected and uninfected RBCs were counted in up to a minimum of 2 representative microscope fields (approx 100 RBC/field). Counting of additional fields was stopped when > 20 infected RBCs had been counted (maximum 5 fields).
The generation of recombinant progeny arising from genetic crosses between drug-sensitive (AJ) and drug-resistant (AS lineage) parasites has been previously described [13, 15, 18, 27]. In the present experiments, back crosses of both AS-30CQ × AJ and AS-15MF × AJ against the sensitive AJ background were performed in order to maximise the numbers of independent recombinant parasites and to optimise the resolution of selection valleys. Previously generated cryopreserved cross progeny [14, 15] were selected with different drug concentrations (including untreated) and the surviving parasites at day 5 to day 11 p.i. mixed in approximately equal proportion with sensitive (AJ) parasites prior to the genetic backcross procedure. 1 × 105 mixed parasites were inoculated into C57 mice for production of gametocytes. Parasites were transmitted on day 5 p.i. or day 6 p.i. through Anopheles stephensi mosquitoes, maintained as described previously . Sporozoites arising in the mosquitoes were recovered by intraperitoneal injection, as previously described [13, 15, 18, 27]. Asexual red blood cell forms were collected, pooled and used to inoculate CBA mice used for drug selection (LGS) experiments.
Quantitative analysis of genetic crosses
We used pyrosequencing (Biotage) to determine the proportions of parental alleles at many polymorphic sites across the genome as previously described  and according to the manufacturer's methodology. In this study, which pre-dated the release of the full genome assembly and annotation, the position of individual markers within the P. chabaudi genome was defined as follows. The required position of markers within the P. chabaudi genome was mapped to orthologous genes in the P. falciparum genome using the rodent malaria synteny maps . Suitable P. falciparum protein sequences were used to define P. chabaudi sequences in orthologous genes by TBLASTN analysis, with validation of adjacent genes in both species. SNPs between AS and AJ parasites were identified by amplification of nucleotide sequences from AS and AJ parasites, and assays developed using high-quality sequence from both clones. Details of the genome-wide set of assays are given in Additional File 1.
Statistical analysis of genome-wide scans
Overview We developed and used two independent approaches to assess the statistical significance of putative selection valleys. The first is based on simulations of the experimental design and tests the null-hypothesis that resistance is caused by many loci of small effects and can therefore not be attributed to one or a few dominant selection valleys. The second approach compares observed and corrected allele frequencies in experiments with and without drug treatment using the non-parametric Mann-Whitney U-test. Both approaches have complementary strengths and weaknesses. Our main goal is to identify a statistically significant selection valley. The principles, methodologies, results, model and validations are detailed in Additional Files 2, 3, 4, 5, 6, 12.
Genome re-sequencing analysis by the Illumina® (Solexa) platform
Clones AS-sens, AS-15MF and AS-30CQ were re-sequenced by the Illumina® (Solexa) platform with 36 - (AS-sens, AS-15MF) or 50 base (AS-30CQ) single reads (nominal coverage x20, x30, x30 fold respectively) (Table 1). For AS-30CQ, the final 9 base calls were universally of poor quality and were removed before mapping the remaining 41 base sequence strings. Individual sequence strings (reads) from each clone were aligned against an isogenic AS reference sequence (AS-WTSI, September 2009 assembly, obtained from the publicly available ftp site of the P. chabaudi AS genome sequencing project website at the Wellcome Trust Sanger Institute; ftp://ftp.sanger.ac.uk/pub/pathogens/P_chabaudi/) using two different software packages; MAQ (Mapping and Assembly with Quality)  and SSAHA2 (Sequence Search and Alignment by Hashing Algorithm) .
For SSAHA2 analysis, Solexa reads were mapped against AS-WTSI using the "pileup" command for single end reads with the appropriate options as described in ftp://ftp.sanger.ac.uk/pub/zn1/ssaha_pileup/ssaha_pileup-readme. Three output files (*.snp, *.ins and *.del) calling SNPs, insertions or deletions were combined into a single Artemis  - compatible file (*.gff) using a custom script that further selected SNPs based on a minimum read coverage that was set at "3" for all the clones, in this study. A second script was developed to compare the *.gff files of either of the two mutant clones against the output obtained for AS-sens. In this way, only mutations appearing within the AS-lineage were retained. Furthermore, a third custom script was used to filter out all the remaining SNPs that had ambiguous (or heterozygous) base calls, where the 'minority' base was represented in ≥20% of the total number of reads. The filtered files were then imported into Artemis which allowed the inspection, identification and evaluation of these mutations on the fully annotated genome. For example, mutations could be immediately identifed as intergenic or intragenic, synonymous or non-synonymous etc.
MAQ was used to call point mutations and compared to those made by SSAHA2. MAQ analysis was performed using the "easyrun" command in the MAQ software suite with default parameters, as described in http://maq.sourceforge.net. This generates a list of candidate SNPs for each clone as a 'cns.final.snp' file. These SNPs were subsequently filtered using custom scripts, as follows (see also Additional File 7). A minimum coverage of 3 reads was applied to call SNPs in all clones. Proposed SNPs with ambiguous calls were removed. The SNPs obtained for mutant clones were then filtered to remove those SNPs defined for AS-sens. The remaining SNPs are candidates for mutations within the AS lineage.
The identification of insertion/deletion mutations (indels) and CNVs was performed with SSAHA2 using two approaches, namely SSAHA2's internal algorithm for the identification of small indels (≤ 3bp) and a "comparative coverage" analysis, both described in detail in the Additional File 7. Visualisation of individual reads and coverage which is available in MAQ was used to confirm indels and CNVs identified by SSAHA2, giving increased confidence.
We thank Richard Carter and the late David Walliker for their contributions to malaria genetic and drug-resistance analysis upon which the present work depends, Sandie Cheesman, Kathryn Degnan for pyrosequencing assistance and Gisela Henriques for technical support. We thank members of the Pathogen Genomics group at the Wellcome Trust Sanger Institute and WTSI core sequencing and informatics groups for making the P. chabaudi genome sequence, assembly and annotation data publicly available. We thank the Wellcome Trust for funding the P. chabaudi genome sequencing project. Artemisinin was a generous gift of Dafra Pharma.
The research was supported by funding from the Medical Research Council (MRC), UK (G0400476) and the Fundacao para a Ciencia e a Tecnologia (FCT), Portugal. For personal support, we thank the MRC (PH, AC, RF), FCT (AM, SB, LR, PC), the Darwin Trust (KM), the Natural Environment Research Council (DB, SK), European Union 6th Framework Program grant to the BioMalPar Consortium (TDO), the Wellcome Trust (AP), the Scottish Universities' Life Science Alliance (UT) and the School of Biological Sciences, University of Edinburgh (MB, MT). LL was supported by The Centre for Systems Biology at Edinburgh is a Centre for Integrative Systems Biology (CISB) funded by BBSRC and EPSRC, reference BB/D019621/1. PH was additionally supported by the Wellcome Trust (Centre in Development in Immunity, Infection and Evolution, 082611/Z/07/Z).
- Su X, Hayton K, Wellems T: Genetic linkage and association analyses for trait mapping in Plasmodium falciparum. Nature Rev Genet. 2007, 8: 497-506. 10.1038/nrg2126.PubMedView Article
- Su X, Kirkman L, Fujioka H, Wellems T: Complex polymorphisms in an ~330kDa protein are linked to chloroquine-resistant P. falciparum in Southeast Asia and Africa. Cell. 1997, 91: 593-603. 10.1016/S0092-8674(00)80447-X.PubMedView Article
- Fidock D, Nomura T, Talley A, Cooper R, Dzekunov S, Ferdig MT, Ursos LM, Sidhu AB, Naudé B, Deitsch KW, Su XZ, Wootton JC, Roepe PD, Wellems TE: Mutations in the P. falciparum digestive vacuole transmembrane protein PfCRT and evidence for their role in chloroquine resistance. Mol Cell. 2000, 6: 861-871. 10.1016/S1097-2765(05)00077-8.PubMed CentralPubMedView Article
- Wootton J, Feng X, Ferdig M, Cooper R, Mu J, Baruch DI, Magill AJ, Su XZ: Genetic diversity and chloroquine selective sweeps in Plasmodium falciparum. Nature. 2002, 418: 320-322. 10.1038/nature00813.PubMedView Article
- Lehane A, Kirk K: Chloroquine resistance-conferring mutations in pfcrt give rise to a chloroquine-associated H+ leak from the malaria parasite's digestive vacuole. Antimicrob Ag Chemother. 2008, 52: 4374-4380. 10.1128/AAC.00666-08.View Article
- Martin RE, Marchetti RV, Cowan AI, Howitt SM, Bröer S, Kirk K: Chloroquine transport via the malaria parasite's chloroquine resistance transporter. Science. 2009, 325: 1680-1682. 10.1126/science.1175667.PubMedView Article
- Ferdig MT, Cooper RA, Mu J, Deng B, Joy DA, Su XZ, Wellems TE: Dissecting the loci of low-level quinine resistance in malaria parasites. Mol Microbiol. 2004, 52: 985-997. 10.1111/j.1365-2958.2004.04035.x.PubMedView Article
- Nzila A, Mwai L: In vitro selection of Plasmodium falciparum drug-resistant parasite lines. J Antimicrob Chemother. 2010, 65: 390-398. 10.1093/jac/dkp449.PubMed CentralPubMedView Article
- Walliker D, Quakyi IA, Wellems TE, McCutchan TF, Szarfman A, London WT, Corcoran LM, Burkot TR, Carter R: Genetic analysis of the human malaria parasite Plasmodium falciparum. Science. 1987, 236: 1661-1666. 10.1126/science.3299700.PubMedView Article
- Wellems TE, Panton LJ, Gluzman IY, do Rosario VE, Gwadz RW, Walker-Jonah A, Krogstad DJ: Chloroquine resistance not linked to mdr-like genes in a Plasmodium falciparum cross. Nature. 1990, 253: 253-255. 10.1038/345253a0.View Article
- Hayton K, Gaur D, Liu A, Takahashi J, Henschen B, Singh S, Lambert L, Furuya T, Bouttenot R, Doll M, Nawaz F, Mu J, Jiang L, Miller LH, Wellems TE: Erythrocyte binding protein PfRH5 polymorphisms determine species-specific pathways of Plasmodium falciparum invasion. Cell Host Microbe. 2008, 4: 40-51. 10.1016/j.chom.2008.06.001.PubMed CentralPubMedView Article
- Carlton J, Hayton K, Cravo P, Walliker D: Of mice and malaria mutants: unravelling the genetics of drug resistance using rodent malaria models. Trends Parasitol. 2001, 17: 236-242. 10.1016/S1471-4922(01)01899-2.PubMedView Article
- Rosario V: Genetics of chloroquine resistance in malaria parasites. Nature. 1976, 261: 585-586. 10.1038/261585a0.PubMedView Article
- Padua R: Plasmodium chabaudi: genetics of resistance to chloroquine. Exp Parasitol. 1981, 52: 419-426. 10.1016/0014-4894(81)90101-6.PubMedView Article
- Cravo PV, Carlton JM, Hunt P, Bisoni L, Padua RA, Walliker D: Genetics of mefloquine resistance in the rodent malaria parasite Plasmodium chabaudi. Antimicrob Agents Chemother. 2003, 47: 709-718. 10.1128/AAC.47.2.709-718.2003.PubMed CentralPubMedView Article
- Afonso A, Hunt P, Cheesman S, Alves AC, Cunha CV, do Rosário V, Cravo P: Malaria parasites can develop stable resistance to artemisinin but lack mutations in candidate genes atp6 (encoding the sarcoplasmic and endoplasmic reticutlum Ca2+ ATPase), tctp, mdr1 and cg10. Antimicrob Agents Chemother. 2006, 50: 480-489. 10.1128/AAC.50.2.480-489.2006.PubMed CentralPubMedView Article
- Carter R, Hunt P, Cheesman S: Linkage group selection - a fast approach to the genetic analysis of malaria parasites. Int J Parasitol. 2007, 37: 285-293. 10.1016/j.ijpara.2006.11.013.PubMedView Article
- Culleton R, Martinelli A, Hunt P, Carter R: Linkage Group Selection: rapid gene discovery in malaria parasites. Genome Research. 2005, 15: 92-97. 10.1101/gr.2866205.PubMed CentralPubMedView Article
- Martinelli A, Cheesman S, Hunt P, Culleton R, Raza A, Mackinnon M, Carter R: A genetic approach to the de novo identification of targets of strain-specific immunity in malaria parasites. Proc Natl Acad Sci. 2005, 102: 814-819. 10.1073/pnas.0405097102.PubMed CentralPubMedView Article
- Pattaradilokrat S, Cheesman S, Carter R: Linkage group selection: towards identifying genes controlling strain-specific protective immunity in malaria. PLoS One. 2007, 12: e857-10.1371/journal.pone.0000857.View Article
- Pattaradilokrat S, Culleton R, Cheesman S, Carter R: Gene encoding erythrocyte binding ligand linked to blood stage multiplication rate phenotype in Plasmodium yoelii yoelii. Proc Natl Acad Sci. 2009, 106: 7161-7166. 10.1073/pnas.0811430106.PubMed CentralPubMedView Article
- Bentley D: Whole-genome resequencing. Curr Opin Genet Dev. 2006, 16: 545-552. 10.1016/j.gde.2006.10.009.PubMedView Article
- Hillier LW, Marth GT, Quinlan AR, Dooling D, Fewell G, Barnett D, Fox P, Glasscock JI, Hickenbotham M, Huang W, Magrini VJ, Richt RJ, Sander SN, Stewart DA, Stromberg M, Tsung EF, Wylie T, Schedl T, Wilson RK, Mardis ER: Whole-genome sequencing and variant discovery in C. elegans. Nat Methods. 2008, 5: 183-188. 10.1038/nmeth.1179.PubMedView Article
- Srivatsan A, Han Y, Peng J, Tehranchi AK, Gibbs R, Wang JD, Chen R: High-precision, whole-genome sequencing of laboratory strains facilitates genetic studies. PLoS Genet. 2008, 4: e1000139-10.1371/journal.pgen.1000139.PubMed CentralPubMedView Article
- The Plasmodium chabaudi genome project (Sept 2009 assembly). [http://www.sanger.ac.uk/Projects/P_chabaudi/]
- Cheesman S, Creasey A, Degnan K, Kooij T, Afonso A, Cravo P, Carter R, Hunt P: Validation of pyrosequencing for accurate and high throughput estimation of allele frequencies in malaria parasites. Mol Biochem Parasitol. 2007, 152: 213-219. 10.1016/j.molbiopara.2006.12.008.PubMedView Article
- Hunt P, Afonso A, Creasey A, Culleton R, Sidhu AB, Logan J, Valderramos SG, McNae I, Cheesman S, do Rosario V, Carter R, Fidock DA, Cravo P: Gene encoding a deubiquitinating enzyme is mutated in artesunate- and chloroquine-resistant rodent malaria parasites. Mol Microbiol. 2007, 65: 27-40. 10.1111/j.1365-2958.2007.05753.x.PubMed CentralPubMedView Article
- Maq: Mapping and Assembly with Qualities. [http://maq.sourceforge.net/]
- SSAHA2: Sequence Search and Alignment by Hashing Algorithm. [http://www.sanger.ac.uk/resources/software/ssaha2/]
- Kooij TW, Carlton JM, Bidwell SL, Hall N, Ramesar J, Janse CJ, Waters AP: A Plasmodium whole-genome synteny map: indels and synteny breakpoints as foci for species-specific genes. PLoS Pathogens. 2005, 1: e44-10.1371/journal.ppat.0010044.PubMed CentralPubMedView Article
- Cheng Q, Saul A: The dihydrofolate reductase domain of rodent malarias: point mutations and pyrimethamine resistance. Mol Biochem Parasitol. 1994, 65: 361-363. 10.1016/0166-6851(94)90087-6.PubMedView Article
- Cowman AF, Lew AM: Chromosomal rearrangements and point mutations in the DHFR-TS gene of Plasmodium chabaudi under antifolate selection. Mol Biochem Parasitol. 1990, 42: 21-30. 10.1016/0166-6851(90)90109-Y.PubMedView Article
- Dharia NV, Sidhu AB, Cassera MB, Westenberger SJ, Bopp SE, Eastman RT, Plouffe D, Batalov S, Park DJ, Volkman SK, Wirth DF, Zhou Y, Fidock DA, Winzeler EA: Use of high-density tiling microarrays to identify mutations globally and elucidate mechanisms of drug resistance in Plasmodium falciparum. Genome Biology. 2009, 10 (R21): 1-13.
- Noedl H, Se Y, Schaecher K, Smith BL, Socheat D, Fukuda MM, Artemisinin Resistance in Cambodia 1 (ARC1) Study Consortium: Evidence of artemisinin-resistant malaria in Western Cambodia. N Engl J Med. 2008, 359: 2619-2620. 10.1056/NEJMc0805011.PubMedView Article
- Denis MB, Tsuyuoka R, Poravuth Y, Narann TS, Seila S, Lim C, Incardona S, Lim P, Sem R, Socheat D, Christophel EM, Ringwald P: Surveillance of the efficacy of artesunate and mefloquine combination for the treatment of uncomplicated falciparum malaria in Cambodia. Trop Med Int Health. 2006, 11: 211-219. 10.1111/j.1365-3156.2005.01557.x.View Article
- Dondorp AM, Nosten F, Yi P, Das D, Phyo AP, Tarning J, Lwin KM, Ariey F, Hanpithakpong W, Lee SJ, Ringwald P, Silamut K, Imwong M, Chotivanich K, Lim P, Herdman T, An SS, Yeung S, Singhasivanon P, Day NP, Lindegardh N, Socheat D, White NJ: Artemisinin resistance in Plasmodium falciparum malaria. N Engl J Med. 2009, 361: 455-467. 10.1056/NEJMoa0808859.PubMed CentralPubMedView Article
- Reed M, Saliba K, Caruana S, Kirk K, Cowman A: Pgh1 modulates sensitivity and resistance to multiple antimalarials in Plasmodium falciparum. Nature. 2000, 403: 906-909. 10.1038/35002615.PubMedView Article
- Sidhu AB, Uhlemann AC, Valderramos SG, Valderramos JC, Krishna S, Fidock DA: Decreasing pfmdr1 copy number in Plasmodium falciparum malaria heightens susceptibility to mefloquine, lumefantrine, halofantrine, quinine and artemisinin. J Infect Dis. 2006, 194: 528-535. 10.1086/507115.PubMed CentralPubMedView Article
- Uhlemann AC, Cameron A, Eckstein-Ludwig U, Fischbarg J, Iserovich P, Zuniga FA, East M, Lee A, Brady L, Haynes RK, Krishna S: A single amino acid residue can determine the sensitivity of SERCAs to artemisinins. Nat Struct Mol Biol. 2005, 12: 628-629. 10.1038/nsmb947.PubMedView Article
- Price RN, Uhlemann AC, van Vugt M, Brockman A, Hutagalung R, Nair S, Nash D, Singhasivanon P, Anderson TJ, Krishna S, White NJ, Nosten F: Molecular and pharmacological determinants of the therapeutic response to artemether-lumefantrine in multidrug-resistant Plasmodium falciparum malaria. Clin Inf Dis. 2006, 42: 1570-1577. 10.1086/503423.View Article
- Rogers WO, Sem R, Tero T, Chim P, Lim P, Muth S, Socheat D, Ariey F, Wongsrichanalai C: Failure of artesunate-mefloquine combination therapy for uncomplicated Plasmodium falciparum malaria in southern Cambodia. Malaria J. 2009, 8: 10-10.1186/1475-2875-8-10.View Article
- Imwong M, Dondorp AM, Nosten F, Yi P, Mungthin M, Hanchana S, Das D, Phyo AP, Lwin KM, Pukrittayakamee S, Lee SJ, Koecharoen K, Nguon C, Day NPJ, Socheat D, White NJ: Exploring the contribution of candidate genes to artemisinin resistance in Plasmodium falciparum. Antimicrob Agents Chemother. 2010, 54: 2886-2892. 10.1128/AAC.00032-10.PubMed CentralPubMedView Article
- Valderammos S, Fidock D: Transporters involved in resistance to antimalarial drugs. Trends Pharmacol Sci. 2006, 27: 594-601. 10.1016/j.tips.2006.09.005.View Article
- Lim P, Chim P, Sem R, Nemh S, Poravuth Y, Lim C, Seila S, Tsuyuoka R, Denis MB, Socheat D, Fandeur T: vitro monitoring of Plasmodium falciparum susceptibility to artesunate, mefloquine, quinine and chloroquine in Cambodia 2001-2002. Acta Trop. 2005, 93: 31-40. 10.1016/j.actatropica.2004.09.002.PubMedView Article
- Tinto H, Rwagacondo C, Karema C, Mupfasoni D, Vandoren W, Rusanganwa E, Erhart A, Van Overmeir C, Van Marck E, D'Alessandro U: In-vitro susceptibility of Plasmodium falciparum to monodesethylamodiaquine, dihydroartemisinin and quinine in an area of high chloroquine resistance in Rwanda. Trans R Soc Trop Med Hyg. 2006, 100: 509-14. 10.1016/j.trstmh.2005.09.018.PubMedView Article
- Ferreira ID, Lopes D, Martinelli A, Ferreira C, do Rosário VE, Cravo P: In vitro assessment of artesunate, artemether and amodiaquine susceptibility and molecular analysis of putative resistance-associated mutations of Plasmodium falciparum from São Tomé & Príncipe. Trop Med Int Health. 2007, 12: 353-62. 10.1111/j.1365-3156.2006.01789.x.PubMedView Article
- White N, Olliaro P: Strategies for the prevention of anitmalarial drug resistance: rationale for combination chemotherapy for malaria. Parasitol Today. 1996, 12: 399-401. 10.1016/0169-4758(96)10055-7.PubMedView Article
- White N: Preventing antimalarial drug resistance through combinations. Drug Resist Update. 1998, 1: 3-9. 10.1016/S1368-7646(98)80208-2.View Article
- Walliker D, Carter R, Sanderson A: Genetic studies on Plasmodium chabaudi: recombination between enzyme markers. Parasitology. 1975, 70: 19-24. 10.1017/S0031182000048824.PubMedView Article
- Grech K, Martinelli A, Pathirana S, Walliker D, Hunt P, Carter R: Numerous, robust genetic markers for Plasmodium chabaudi by the method of amplified fragment length polymorphism. Mol Biochem Parasitol. 2002, 123: 95-104. 10.1016/S0166-6851(02)00142-1.PubMedView Article
- Rutherford K, Parkhill J, Crook J, Horsnell T, Rice P, Rajandream MA, Barrell B: Artemis: sequence visualisation and annotation. Bioinformatics. 2000, 16: 944-945. 10.1093/bioinformatics/16.10.944.PubMedView Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.