Molecular evolution and intragenic recombination of the merozoite surface protein MSP-3alpha from the malaria parasite Plasmodium vivax in Thailand.

The merozoite surface antigens of malaria parasites are prime anti-morbidity/mortality vaccine candidates. However, their highly polymorphic nature requires extensive surveys of parasite populations to validate vaccine designs. Previous studies have found 3 molecular types (A, B and C) of the Plasmodium vivax merozoite surface protein 3a (PvMSP-3alpha) among parasite field populations. Here we analysed complete PvMSP-3alpha sequences from 17 clinical P. vivax isolates from Thailand and found that the nucleotide diversity was as high as that from samples widely separated by time and space. The polymorphic sites were not randomly distributed but concentrated in the N-terminal Ala-rich domain (block 2A), which is partially deleted in type B and C sequences. The size variations among type A sequences were due to small indels occurring in block 2A, whereas type B and C sequences were uniform in length with each type having a different large deletion. Analysis of synonymous and non-synonymous substitutions suggested that different selection forces were operating on different regions of the molecule. The numerous recombination sites detected within the Ala-rich domain suggested that intragenic recombination was at least partially responsible for the observed genetic diversity of the PvMSP-3alpha gene. Phylogenetic analysis failed to link any alleles to a specific geographical origin, even when different domains of PvMSP-3alpha were used for analysis. The highly polymorphic nature and lack of geographical clustering of isolates suggest that more systematic investigations of the PvMSP-3alpha gene are needed to explore its evolution and vaccine potential.