Multi-locus investigation of Anopheles-mediated selective pressure on Plasmodium falciparum in Africa.

BACKGROUND

The high burden of malaria in Africa is largely due to the presence of competent and adapted Anopheles vector species. With invasive Anopheles stephensi implicated in malaria outbreaks in Africa, understanding the genomic basis of vector-parasite compatibility is essential for assessing the risk of future outbreaks due to this mosquito. Vector compatibility with P. falciparum arises from ancient coevolution and involves genes such as Pfs47 in P. falciparum and P47Rec in Anopheles. Questions remain about whether sub-continental vector variation is a selective pressure on current Plasmodium populations.

METHODS

We analyzed the genetic diversity in parasite-vector interaction genes in P. falciparum and An. gambiae from 9 and 15 countries in Africa, respectively. Specifically, we looked for evidence of malaria vector-mediated selection within three P. falciparum genes (Pfs47, Pfs16, Pfs37) and conducted association analyses with occurrence probabilities of prominent malaria vectors.

RESULTS

Higher protein haplotype diversities of Pfs47 and Pfs16 were associated with the probability of occurrence of An. arabiensis and An. funestus together. Only Pfs16 carried a signature of positive selection consistently (average Tajima's D = -2.96), which was associated with the probability of occurrence of An. funestus. These findings support vector-mediated selection on the basis of vector species diversity that may be occurring within Africa. We also employed phylogenetic analyses of An. gambiae interaction genes (P47Rec, APN1, HPX15) to identify significant subspecies diversity as a prerequisite to vector-population-mediated selection. Anopheles gambiae HPX15 revealed significant within-species differentiation (multiple branches bootstrap > 70) compared with absence of variation in P47Rec, suggesting that further investigation into subspecies-mediated selection on the basis of HPX15 is needed. Finally, we observed five amino acid changes at P47Rec in invasive An. stephensi compared with dominant African Anopheles species, calling for further investigation of the impact these distinct P47Rec variants might have on local African P. falciparum Pfs47 diversity.

CONCLUSIONS

Overall, these findings suggest that vector variation within Africa could influence P. falciparum diversity and lay a genomic framework for future investigation of invasive An. stephensi's impact on African malaria.