Differential sequence diversity at merozoite surface protein-1 locus of Plasmodium knowlesi from humans and macaques in Thailand

- Pkmsp-1 sequences contain five conserved and four variable domains.
- Most nucleotide substitutions in conserved domains of Pkmsp-1 are dimorphic.
- Diversity in Pkmsp-1 is influenced by purifying selection and intragenic recombination.
- Pkmsp-1 sequences from humans exhibit more sequence diversity than those from macaque natural hosts.
- Network analysis displays two distinct clusters of P. knowlesi.

To determine the genetic diversity and potential transmission routes of Plasmodium knowlesi, we analyzed the complete nucleotide sequence of the gene encoding the merozoite surface protein-1 of this simian malaria (Pkmsp-1), an asexual blood-stage vaccine candidate, from naturally infected humans and macaques in Thailand. Analysis of Pkmsp-1 sequences from humans (n = 12) and monkeys (n = 12) reveals five conserved and four variable domains. Most nucleotide substitutions in conserved domains were dimorphic whereas three of four variable domains contained complex repeats with extensive sequence and size variation. Besides purifying selection in conserved domains, evidence of intragenic recombination scattering across Pkmsp-1 was detected. The number of haplotypes, haplotype diversity, nucleotide diversity and recombination sites of human-derived sequences exceeded that of monkey-derived sequences. Phylogenetic networks based on concatenated conserved sequences of Pkmsp-1 displayed a character pattern that could have arisen from sampling process or the presence of two independent routes of P. knowlesi transmission, i.e. from macaques to human and from human to humans in Thailand.