(last updated 8/21/2014)
Kevin Deitz, Michael Reddy, Michel Slotman (PI) [and others]
We have undertaken a detailed study into the population genetic structure of Anopheles melas, a salt water breeding species of the Anopheles gambiae complex. This species has a patchy distribution along the West Coast of Africa, stretching from Senegal to Angola. Although not generally considered a primary malaria vector because of its limited distribution, An. melas is an important vector on Bioko Island, Equatorial Guinea, as well as other locations along the West African coast. On Bioko Island up to 90% of the Anopheline mosquitoes collected in certain sentinel sites are An. melas and this species contributes considerably to malaria transmission, with entomological inoculation rates reaching as high as 264 infective bites/year.
Because Anopheles melas is not considered a primary vector, little attention has been devoted to this species. In fact, few publications specifically dealing with this species exist and nothing was known about its population structure. We have remedied this by examining the genetic structure of An. melas throughout its range. We have analyzed 11 populations for 15 microsatellite loci and appr. 1,150 bp of the mitochondrial ND4 and ND5 genes. To avoid potential problems with null alleles, we developed species-specific, highly polymorphic microsatellite markers for An. melas by re-sequencing markers originally developed for its sibling species, An. gambiae s.s.
In this study we paid particular attention to populations on Bioko Island. The island is a potential candidate for a malaria elimination campaign, and understanding the probability of the re-introduction of malaria parasites and/or vectors after a successful elimination effort is important for continued success.
Our results of both the microsatellite (Fig 4 and 6) and mtDNA (Fig 5) data sets indicate that little or no migration exists from mainland An. melas populations to Bioko Island. However, An. melas mosquitoes do migrate in the opposite direction, with An. melas mosquitoes from Bioko Island populations moving into Tiko, Cameroon, the closest mainland population. Additionally, mainland populations are clearly divided into two clusters with an astonishing degree of genetic divergence between them. The level of genetic differentiation is on par with that found between species in the An. gambiae complex (fig 6), an raises the question whether these two clusters are in fact previously unrecognized species.
Approximate Bayesian Computation analysis has provided considerable insight into the history of the three population clusters. An. melas West and South were once a single connected population that diverged through vicariance event between 110k and 450k ybp. This vicariance event may be due to presence of Mount Cameroon, that is located near the point of geographic isolation between An. melas West and South. Similarly, Bioko Island populations of An. melas were once connected to An. melas West on the mainland, and separated through a vicariance event around 110k ypb. This timing corresponds closely to glacial interlude in which sea levels rose, physically separating Bioko Island from the mainland.
Figure 6A: Bayesian clustering analysis of An. melas populations using STRUCTURE showing very high level of isolation between An. melas Bioko (blue), West (green) and South (red). From Deitz et al. (2012b)
Figure 6B: Bayesian clustering analysis of An. gambiae (red) and An. arabiensis (green) populations from Mali and Cameroon, shown here as an example of a species level assignment plot.
(From Slotman et al 2006).
We have extended our work on An. melas using a genomics approach as part of the 16 Anopheles genomes project. We sequenced the genomes of pooled samples from one population each from the three forms of An. melas. This showed that fixed differences are present between the three forms extending across the genome and that genome-wide Fst values excede those observed between An. gambiae and An. melas. Together these data support a re-evaluation of the current taxonomic ranking of these clusters as a single species within the An. gambiae complex. A paper describing these results is in preparation.
Publications from this work:
Genome-wide divergence in the West-African Malaria Vector Anopheles melas (2016) Deitz KC, G Athrey, M Jawara, HJ Overgaard, A Matias, MA Slotman. G3: Genes, Genomes, Genetics.
Genetic Isolation within the Malaria Mosquito Anopheles melas (2012b) Deitz KC, G Athrey, MR Reddy, HJ Overgaard, A Matias, J Musa, A della Torre, J Pinto, AE Kiszewski, C Costantini, A Caccone and MA Slotman. Molecular Ecology 21, 4498-4513.
Limited Usefulness of Microsatellite Markers from the Malaria Vector Anopheles gambiae when Applied to the Closely Related Species Anopheles melas (2012a) Deitz KC, VP Reddy, M Reddy, N Satyanarayanah, M Lindsey, HJ Overgaard, J Musa, A Caccone and MA Slotman. Journal of Heredity 103(4): 585-593