Selective pressure on the merozoite surface protein-1 genes of Plasmodium vivax, P. knowlesi and P. cynomolgi

ABSTRACT

Background: The merozoite surface protein 1 (msp-1) of malarial parasites is a promising target for malaria vaccine development. Because antigenic polymorphism of this protein occurs in both Plasmodium falciparum and P. vivax, it is important to know the extent of sequence variation of this gene in nonhuman primate malarias that can infect humans. Objective: To determine the complete nucleotide sequences of the msp-1 genes of Plasmodium knowlesi (Pkmsp-1) and P. cynomolgi (Pcymsp-1) and their evolutionary history comparing with an orthologous gene of P. vivax (Pvmsp-1). Method: The msp-1 genes of P. knowlesi and P. cynomolgi were amplified by the polymerase chain reaction and their nucleotide sequences were determined directly from the amplified products. Sequences were aligned using the Clustal X program. The extent of polymorphism is expressed in terms of nucleotide diversity (π) and the average number of nucleotide substitutions per site between populations (Dxy) in pair-wise comparison. The ratio of nucleotide substitutions at synonymous and nonsynonymous sites was computed by the McDonald and Kreitman test to detect evidence of departure from neutrality. The phylogenetic tree was constructed by the neighbor-joining method using the maximum composite likelihood with 1,000 bootstrap iterations. Results: The deduced amino acid sequences of the msp-1 of P. knowlesi, P. cynomolgi and P. vivax exhibit sequence similarity, ranging from 56.2 to 60.8 %. Variable blocks detected in Pkmsp-1 and Pcymsp-1 are located in regions corresponding to variable blocks of Pvmsp-1. Sliding window plots of Dxy along the entire coding regions of the msp-1 genes gave similar profiles for each pair of comparison. Pvmsp-1 is more closely related to Pcymsp-1 than that of Pkmsp-1 based on a phylogenetic analysis. However, the Tajima’s relative rate test suggests that the merozoite surface protein 1 (msp-1) of each species has not evolved at the same rate. The McDonald-Kreitman test detects positive selection on regions equivalent to blocks 1 and 9 of Pvmsp-1. Conclusion: The msp-1 genes of P. knowlesi, P. cynomolgi and P. vivax exhibit a similar pattern of sequence variation. Regions equivalent to conserved blocks 1 and 9 of Pvmsp-1 have evolved under positive selective pressure, suggestive of functional importance probably as targets for host immune responses.