Allelic diversity in the merozoite surface protein 1 gene of Plasmodium falciparum on Palawan Island, the Philippines.

Allelic diversity of the Plasmodium falciparum merozoite surface protein 1 gene (msp1) is mainly generated by meiotic recombination at the mosquito stage. We investigated recombination-based allelic diversity of msp1 in a P. falciparum population from Palawan Island, the Philippines, where malaria transmission is moderate. We identified the 5' recombinant types, 3' sequence types and msp1 haplotypes (unique combinations of 5' recombinant type and 3' sequence type), and compared them with those of P. falciparum from the Solomon Islands, where malaria transmission is high. The mean number of 5' recombinant types per patient in Palawan was 1.44, which is comparable to the number for the Solomon Islands (1.41). The Palawan parasite population had 15 msp1 haplotypes, whereas the Solomon Islands population had only 8 haplotypes. The Palawan population showed strong linkage disequilibrium between polymorphic blocks/sites within msp1, which is comparable to the results for the Solomon Islands. These findings support our hypothesis that the extent of allelic diversity of msp1 is determined not only by the transmission intensity but also by the number of msp1 alleles prevalent in the local parasite population and the extent of mixed-allele infections. Contribution of a high prevalence of the chloroquine (CQ)-sensitive allele of P. falciparum CQ resistance transporter (pfcrt) to the relatively high msp1 diversity in the Palawan population is discussed.