Population genetic structure of Plasmodium falciparum in the two main African vectors, Anopheles gambiae and Anopheles funestus

Abstract

We investigated patterns of genetic diversity of Plasmodium falciparum associated with its two main African vectors: Anopheles gambiae and Anopheles funestus. We dissected 10,296 wildcaught mosquitoes from three tropical sites, two in Cameroon (Simbock and Tibati, separated by 320 km) and one in Kenya (Rota, >2,000 km from the other two sites). We assayed seven microsatellite loci in 746 oocysts from 183 infected mosquito guts. Genetic polymorphism was very high in parasites isolated from both vector species. The expected heterozygosity (HE) was 0.79 in both species; the observed heterozygosities (HO) were 0.32 in A. funestus and 0.42 in A. gambiae, indicating considerable inbreeding within both vector species. Mean selfing (s) between genetically identical gametes was s ‫؍‬ 0.33. Differences in the rate of inbreeding were statistically insignificant among sites and between the two vector species. As expected, because of the high rate of inbreeding, linkage disequilibrium was very high; it was significant for all 21 loci pairs in A. gambiae and for 15 of 21 pairs in A. funestus, although only two pairwise comparisons were between loci on the same chromosome. Overall, the genetic population structure of P. falciparum, as evaluated by F statistics, was predominantly clonal rather than panmictic, a population structure that facilitates the spread of antimalarial drug and vaccine resistance and thus may impair the effectiveness of malaria control efforts. malaria ͉ epidemiology ͉ evolutionary genetics ͉ Cameroon ͉ Kenya M alaria is the most significant and widespread vectortransmitted human disease, accounting yearly for several hundred million clinical cases and Ͼ2 million deaths, mostly affecting young children and pregnant women in subSaharan Africa (1). Prevention and cure of the disease are major publichealth challenges that are tackled by using several strategies, among them vector control. The transmission of Plasmodium falciparum, the agent of malignant malaria, involves a complex vectorial system consisting of Ϸ10 Anopheles species, colonizing different ecoclimatic settings, regions, and seasons in strongly variable relative abundances (2-5).The two most important vectors of malignant malaria in Africa are Anopheles gambiae and Anopheles funestus because of their widespread distribution, highly anthropophilic and endophilic behavior, and long life spans (6, 7). A. gambiae is the most important vector throughout Africa and the most extensively studied Anopheles species (8). The effectiveness of malaria transmission emerges from the complementary ecoclimatic attributes and seasonal patterns of both species. A. funestus breeds in permanent larval sites that enable this species, in regions of seasonal transmission, to extend parasite transmission far into the dry season, after the temporary breeding pools of A. gambiae have dried out (7, 9-11).Differences in the biology and ecology of these two main vectors might entail a differential impact on the genetic composition of the P. falciparum populations they harbo...

New Insights into Clonality and Panmixia in Plasmodium and Toxoplasma

“…That selfing and inbreeding amount to clonality is a view shared by most authors working on Plasmodium (Anderson et al, 2000;Annan et al, 2007;Beck et al, 2009;Conway, 2007;Griffing et al, 2011;Mu et al, 2005;Mzilahowa et al, 2007;Nkhoma et al, 2013;Razakandrainibe et al, 2005;Volkman et al, 2012a) and Toxoplasma (Beck et al, 2009;Grigg and Sundar, 2009;Lehmann et al, 2004;Sibley and Ajioka, 2008;Su et al, 2012;Wendte et al, 2010). However, clonality does not limit itself to selfing and inbreeding and can have other origins (mitotic propagation and several cases of parthenogenesis, gynogenesis, and hybridogenesis).…”

“…Discussions of pathogen population structure have focused on the "clonality/sexuality debate" (Tibayrenc et al, 1990). Clonality is here understood as scarcity or absence of genetic recombination, a definition widely accepted for all kinds of pathogens (Tibayrenc and Ayala, 2012), including Plasmodium (Anderson et al, 2000;Annan et al, 2007;Beck et al, 2009;Conway, 2007;Griffing et al, 2011;Heitman, 2006;Mu et al, 2005;Nkhoma et al, 2013;Razakandrainibe et al, 2005;Volkman et al, 2012a) and Toxoplasma (Beck et al, 2009;Heitman, 2006;Khan et al, 2011;Rajendran et al, 2012;Sibley and Ajioka, 2008;Smith, 2009;Su et al, 2003Su et al, , 2010Wendte et al, 2011). Clonality does not amount to genetic monomorphism, an erroneous definition still used by some researchers (Tibayrenc and Ayala, 2012).…”

Effective size of the hierarchically structured populations of the agent of malaria: a coalescent-based model

“…Again, the few studies that have analysed the genetic variability of oocyst infrapopulations found strong genetic differentiation between them (Razakandrainibe et al, 2005;Annan et al, 2007). Several aspects of parasite biology or of their hosts may explain the amount of genetic differentiation observed between oocyst infrapopulations.…”

“…The few studies that have analysed it within natural oocyst infrapopulations reported high positive values (Paul et al, 1995;Razakandrainibe et al, 2005;Annan et al, 2007;Mzilahowa et al, 2007), meaning that infrapopulations harboured more homozygous individuals than expected if gametes had met at random. Two main hypotheses are generally proposed to explain a positive F IS (in our model, F IM ) in oocyst infrapopulations: selffertilization and/or Wahlund effects.…”

“…Therefore, it depends on both the mobility of humans and that of the vector. In regions of high transmission, the analysis of genetic differentiation among different villages or sites distant by several kilometres up to hundreds or thousands of kilometres have reported very small F VT (reported as F ST estimates corrected for the differentiation occurring at lower levels (between infrapopulations) in Annan et al (2007) and Prugnolle et al (2008) and as traditional F ST in Anderson et al (2000). In contrast, in regions of low transmission, where the local effective size is likely to be lower (due to lower parasite prevalence), other studies reported strong F VT even between close sites (Bogreau et al, 2006).…”