A cryptic subgroup of Anopheles gambiae is highly susceptible to human malaria parasites

Abstract

Population subgroups of the African malaria vector Anopheles gambiae have not been comprehensively characterized owing to the lack of unbiased sampling methods. In the arid savanna zone of West Africa, where potential oviposition sites are scarce, widespread collection from larval pools in the peridomestic human habitat yielded a comprehensive genetic survey of local A. gambiae population subgroups, independent of adult resting behavior and ecological preference. A previously unknown subgroup of exophilic A. gambiae is sympatric with the known endophilic A. gambiae in this region. The exophilic subgroup is abundant, lacks differentiation into M and S molecular forms, and is highly susceptible to infection with wild Plasmodium falciparum. These findings might have implications for the epidemiology of malaria transmission and control.

Figure 1. Comprehensive sampling of A. gambiae populations in the West African Sudan Savanna zone reveals unexpected genotypes

Histograms compare populations of indoor-resting and larval pool collections of A. gambiae ss. Larval pools are a limiting resource in the arid zone, and larval pool sampling represents an unbiased survey of all population subgroups, while indoor house collections sample behaviorally endophilic mosquitoes. Frequencies of: A. molecular form M and S diagnostic SNP alleles (MFDS), and B. 2La inversion alleles are significantly different in larval pool and indoor-resting mosquito collections (for MFDS, χ²=89.669, p=3E-20; for 2La inversion, χ²=178.533, p=1.7E-39). The wild-type 2La+ chromosome allele and hybrids of the MFDS occur at unexpectedly high frequency in larval collections as compared to indoor-resting mosquito collections, suggesting population genetic substructure and behavioral partitioning. All samples are A. gambiae ss by molecular and cytogenetic species assays, collected ≤1km from human houses at 3 independent geographic sites across a 400 km transect in Burkina Faso (map, Figure S1) during 2007 and 2008 malaria transmission seasons.

Figure 2. Bayesian cluster assignments divide A. gambiae ss Sudan-Savanna populations into two genetically and behaviorally differentiated subgroups

Mosquitoes from indoor-resting and larval pool collections were clustered by multilocus genotypes using STRUCTURE. The indoor-resting mosquitoes consistently belonged to a single cluster, while the matched larval pools consistently contained two genetically differentiated subgroups: i) individuals that cluster robustly with the indoor-captured mosquitoes and thus are the source of the behaviorally endophilic subgroup (yellow), and ii) a distinct subgroup that are not captured among indoor-resting mosquitoes and thus are behaviorally exophilic (blue; LnPD statistic, Figure S4). Individuals are depicted as colored vertical bars (n=679). Samples to the right of the black line are indoor-captured mosquitoes, and to the left are larval pool collections.

Figure 3. GOUNDRY and ENDO subgroups of A. gambiae ss display distinct genetic and phenotypic composition

Larval site collections were genotyped to determine, A. ENDO and GOUNDRY relative population abundance, B. MFDS and the kdr-w pyrethroid resistance locus, C. the chromosome 2La inversion, and D. susceptibility to local P. falciparum parasites (n=439). Genetic characteristics of the ENDO subgroup are consistent with the canonical West African A. gambiaess ( <1% hybrid rate for MFDS, 2La inverted allele virtually fixed, kdr-w resistant allele strongly associated with molecular S form SNPs). In contrast, the GOUNDRY subgroup displays unexpected genotypes: the MFDS segregate as neutral molecular variation, the 2La+ inverted chromosome segregates at 68%, and the kdr-w alleles segregate independently of MFDS. Phenotypically, after feeding on blood with wild P. falciparum gametocytes, 58% of GOUNDRY and 35% of ENDO mosquitoes were permissive for parasite infection. For all features measured, frequencies are significantly different between ENDO and GOUNDRY subgroups (GOUNDRY greater relative abundance in 75% of collections, n=20, Rank Sum Test p=0.003; MFDS, χ²=113.8, p=1.9E-25; kdr associated with M-form SNPs, χ²=73.473, 1.1E-16; kdr in S-form SNPs, χ²=37.26, 1E-8; 2La inversion, χ²=257.045, p=1.5E-56; infection prevalence, χ²=23.1, p=1.5E-6).