Abstract
BackgroundThe merozoite surface protein-1 (MSP-1) is a candidate target for the development of blood stage vaccines against malaria. Polymorphism in MSP-1 can be useful as a genetic marker for strain differentiation in malarial parasites. Although sequence diversity in the MSP-1 locus has been extensively analyzed in field isolates of Plasmodium falciparum and P. vivax, the extent of variation in its homologues in P. ovale curtisi and P. ovale wallikeri, remains unknown.Methodology/Principal FindingsAnalysis of the mitochondrial cytochrome b sequences of 10 P. ovale isolates from symptomatic malaria patients from diverse endemic areas of Thailand revealed co-existence of P. ovale curtisi (n = 5) and P. ovale wallikeri (n = 5). Direct sequencing of the PCR-amplified products encompassing the entire coding region of MSP-1 of P. ovale curtisi (PocMSP-1) and P. ovale wallikeri (PowMSP-1) has identified 3 imperfect repeated segments in the former and one in the latter. Most amino acid differences between these proteins were located in the interspecies variable domains of malarial MSP-1. Synonymous nucleotide diversity (πS) exceeded nonsynonymous nucleotide diversity (πN) for both PocMSP-1 and PowMSP-1, albeit at a non-significant level. However, when MSP-1 of both these species was considered together, πS was significantly greater than πN (p<0.0001), suggesting that purifying selection has shaped diversity at this locus prior to speciation. Phylogenetic analysis based on conserved domains has placed PocMSP-1 and PowMSP-1 in a distinct bifurcating branch that probably diverged from each other around 4.5 million years ago.Conclusion/SignificanceThe MSP-1 sequences support that P. ovale curtisi and P. ovale wallikeri are distinct species. Both species are sympatric in Thailand. The low level of sequence diversity in PocMSP-1 and PowMSP-1 among Thai isolates could stem from persistent low prevalence of these species, limiting the chance of outcrossing at this locus.
Highlights
About half of the world’s population resides in areas at risk of contracting malaria, one of the leading causes of morbidity and mortality, accounting for more than 200 million cases and more than 600,000 deaths per annum [1]
Identical sequences were observed in 3 isolates belonging to P. ovale curtisi (PO-4, PO-8 and PO-9) and 2 isolates identified as P. ovale wallikeri (PO-2 and PO-3)
Our recent polymerase chain reaction (PCR)-based diagnosis of malaria species distribution in Thailand involving 5,044 malaria patients during 2006–2007 and 2008–2009 in major endemic areas have shown that malaria caused by P. ovale contributed to 1.03% and 0.13%, respectively, of all Plasmodium identified [15,16]
Summary
About half of the world’s population resides in areas at risk of contracting malaria, one of the leading causes of morbidity and mortality, accounting for more than 200 million cases and more than 600,000 deaths per annum [1]. Six species in the genus Plasmodium are known to cause human malaria under natural transmission [2,3,4]. Only malaria caused by Plasmodium falciparum and Plasmodium vivax have been extensively studied, whereas relatively little is known about the less prevalent malaria parasites. In several malaria endemic areas, P. ovale has been found to be sympatric with the major malaria species, P. falciparum and P. vivax. The low parasite densities of P. ovale in infected individuals and its morphological resemblance to P. vivax have hampered efficient microscopy detection, especially when they co-exist with other malaria species in circulation. Sequence diversity in the MSP-1 locus has been extensively analyzed in field isolates of Plasmodium falciparum and P. vivax, the extent of variation in its homologues in P. ovale curtisi and P. ovale wallikeri, remains unknown
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