PHENOTYPE AND GENOTYPE DIVERSITY IN THE CIRCUMSPOROZOITE PROTEINS OF PLASMODIUM-VIVAX IN THAILAND

Two phenotypes of the circumsporozoite (CS) protein of the human malar ia parasite Plasmodium vivax occur in Thailand, each of which has a ch aracteristic nonamer repeat: GDRA(AID)GQPA for VK210-type and ANGAG-NQ PG for VK247-type. We have sequenced the repetitive domains and flanki ng regions from 17 specimens collected from a small area, some of whic h had given ambiguous results in allele-specific hybridization or enzy me-linked immunosorbent assays (ELISA). Base substitutions occurred in non-random, limited patterns that suggest the dissemination of mutati ons by both unequal crossing-over and gene conversion; most substituti ons were silent and phenotypic variation was relatively minor. Sequenc e variation and number of repeat units were much more variable in VK21 0-type clones than in those of VK247-type. Each VK210-type isolate wit h a poor ELISA response contained at least one clone with one of five residue substitutions not found in normally responsive isolates. The a bsence of obvious hybrid sequences between the two alleles suggests th at most successful recombination may have been between sister chromati ds, and the limited phenotypic variation suggests that CS antibody doe s not exert selective pressure on evolution.