Abstract
“Far-West” Africa is known to be a secondary contact zone between the two major malaria vectors Anopheles coluzzii and A. gambiae. We investigated gene-flow and potentially adaptive introgression between these species along a west-to-east transect in Guinea Bissau, the putative core of this hybrid zone. To evaluate the extent and direction of gene flow, we genotyped site 702 in Intron-1 of the para Voltage-Gated Sodium Channel gene, a species-diagnostic nucleotide position throughout most of A. coluzzii and A. gambiae sympatric range. We also analyzed polymorphism in the thioester-binding domain (TED) of the innate immunity-linked thioester-containing protein 1 (TEP1) to investigate whether elevated hybridization might facilitate the exchange of variants linked to adaptive immunity and Plasmodium refractoriness. Our results confirm asymmetric introgression of genetic material from A. coluzzii to A. gambiae and disruption of linkage between the centromeric "genomic islands" of inter-specific divergence. We report that A. gambiae from the Guinean hybrid zone possesses an introgressed TEP1 resistant allelic class, found exclusively in A. coluzzii elsewhere and apparently swept to fixation in West Africa (i.e. Mali and Burkina Faso). However, no detectable fixation of this allele was found in Guinea Bissau, which may suggest that ecological pressures driving segregation between the two species in larval habitats in this region may be different from those experienced in northern and more arid parts of the species’ range. Finally, our results also suggest a genetic subdivision between coastal and inland A. gambiae Guinean populations and provide clues on the importance of ecological factors in intra-specific differentiation processes.
Highlights
Anopheles gambiae (Giles) and A. coluzzii (Coetzee & Wilkerson sp. n.)
The introgression of the TEP1r1 allele from A. coluzzii to A. gambiae in Guinea Bissau (Fig 1) shows that hybridization can promote the transfer of potentially adaptive immune-resistant alleles from a 'donor' (A. coluzzii) to a 'recipient' (A. gambiae) vector species
It is tempting to speculate that the observed absence of fixation and/or relatively low frequency of the ‘novel’ TEP1r1 acquired by A. gambiae in Guinea Bissau could be due to a lower adaptive benefit of this allele to the 'recipient' species in this region, where hybridization between A. gambiae and A. coluzzii is highest and stable
Summary
Anopheles gambiae (Giles) and A. coluzzii (Coetzee & Wilkerson sp. n.) Experimental infections demonstrated that laboratory-reared A. gambiae individuals homozygous or heterozygous for TEP1ÃR1 [18] and TEP1rB [17] alleles are significantly more resistant to Plasmodium and bacterial infections than mosquitoes carrying other TEP1 alleles. Given the relatively low rates and intensities of natural Plasmodium infection in both mosquito species, it was speculated that the most likely source of pathogen-mediated selection for resistance came from larval habitat [17]. The longer-lasting and more biotically diverse aquatic milieu exploited by A. coluzzii, presumably harboring richer pathogen populations than temporary breeding sites exploited by A. gambiae, would exert higher selective pressures on the immune system of the former species [17, 20]
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