Evidence of non-neutral polymorphism in Plasmodium falciparum gamete surface protein genes Pfs47 and Pfs48/45.

Targeted disruption of particular members of the Plasmodium 6-cys protein gene family, including Ps47, Ps48/45 and Ps230, is known to dramatically affect parasite fertility. Because loci critical to fertility in many eukaryote species have been shown to be under strong positive selection, we examined sequence variation in four members of the 6-cys protein gene family in Plasmodium falciparum (Pfs36, Pfs38, Pfs47 and Pfs48/45) to determine whether genetic variation in these loci may be of functional significance. Sequence polymorphism among 11 laboratory isolates of P. falciparum was compared with divergence from the respective orthologues in the closely related species P. reichenowi, showing an almost significant skew towards within-species non-synonymous polymorphism in Pfs47 and Pfs48/45 (by the McDonald-Kreitman test) but clearly non-significant results for Pfs36 and Pfs38. A preliminary analysis of Pfs47 sequence polymorphism in field isolates of P. falciparum showed exceptionally high fixation indices (F(ST)) among geographically distinct populations, similar to results seen previously for Pfs48/45. Therefore, both Pfs47 and Pfs48/45 were further analysed by sequencing polymorphic parts of the genes from a Tanzanian population sample of oocysts (a means of analysing diploid genotypes). Both genes displayed higher inbreeding coefficients (F(IS)) compared with the average of 11 unlinked microsatellite loci. These results suggest that allelic variation in these two genes may be functionally significant in influencing mating interactions, a hypothesis that could be tested by fertilization experiments with targeted allelic replacement.

Fragmented population structure of plasmodium falciparum in a region of declining endemicity.

BACKGROUND: The population genetic structure of Plasmodium falciparum differs between endemic regions, but the characteristics of a population recently fragmented by effective malaria control have been unknown. METHODS: Genotypic analysis of 10 microsatellite loci widely separated in the parasite genome was conducted on 288 P. falciparum isolates from 8 foci in Malaysian Borneo, a region in which malaria incidence has been progressively reduced. RESULTS: Within all P. falciparum foci, moderate levels of allelic diversity were found, but levels of multilocus linkage disequilibrium were extremely variable. The population with the highest proportion of mixed-clone infections also had the highest allelic diversity and nonsignificant linkage disequilibrium. In contrast, several populations showed evidence of clonal expansion, and one offshore island population had exceptionally high levels of linkage disequilibrium. Genetic differentiation between many populations was very high and strongly associated with the geographical distance between them. CONCLUSIONS: High levels of differentiation and contrasting population structure among P. falciparum populations in Malaysian Borneo indicate that they are genetically independent. This supports the feasibility of individually eradicating the remaining P. falciparum foci.