High Levels of Hybridization between Molecular Forms of Anopheles gambiae from Guinea Bissau

Abstract

In the malaria vector Anopheles gambiae Giles sensu stricto, two molecular forms denoted M and S are considered units of incipient speciation within this species. Very low hybrid frequencies and significant genetic differentiation have been found in sympatric M- and S-form populations. We studied the molecular form composition and the degree of genetic differentiation at 15 microsatellites in two samples of An. gambiae collected in two consecutive years from Bissau, Guinea Bissau. High frequencies of M/S hybrids (19-24%) were found in this area. Coincidently, very low levels of genetic differentiation were detected between forms when analysis involved microsatellites mapped at chromosome-3 (mean Fst, 0.000-0.002). The single exception was the X-linked AGXH678, for which high differentiation was measured (Fst, 0.158-0.301). This locus maps near the centromere of chromosome X, a low recombination region in which selection is likely to promote divergence between M and S forms. These results strongly suggest that the degree of isolation between M and S forms, considered the units of incipient speciation within An. gambiae, is not homogenous throughout the species distribution range.

Reproductive Isolation among Sympatric Molecular Forms of An. gambiae from Inland Areas of South-Eastern Senegal

“…gambiae s.s. is the prevailing species in coastal regions of The Gambia and surrounding areas in Senegal, whilst its sibling species, An. arabiensis , is more common in inland areas [10], [11]. Similar observations were also made by Onyabe and Conn [22] and Kristan et al [23], who found An.…”

“…However, studies in coastal areas at the western extreme of the geographical range found a higher than expected hybridization rate. In areas surrounding The Gambia, high levels of M/S hybrids were reported at sites near the west coast (3% at Dielmo, Senegal [9]; 7% at Njabakunda, The Gambia [10]; and 24% at Antula, Guinea-Bissau [11]). More recently, Nwakanma et al [12] reported high frequencies (5–42%) of M/S hybrid forms at 12 sites in the four contiguous countries of The Gambia, Senegal, Guinea-Bissau and Republic of Guinea.…”

Long Distance Linkage Disequilibrium and Limited Hybridization Suggest Cryptic Speciation in Atlantic Cod

“…Georges Bank) and declined to the south in the east and west, with only a single F1 individual being present in the sample from Ireland. Despite the high proportion of F1’s in some samples, the overall proportion among all samples was low (∼15%) suggesting that some samples or regions may have been biased towards hybridization or the collection of hybrids (e.g., [59]). Admittedly it remains unclear to what degree these linked genomic islands are a product of selection and speciation (“speciation island” hypothesis) or due to an absence of gene flow and the clustering of ancestral regions of divergence in regions of low recombination (“incidental island” hypothesis) [60].…”

Swarming behaviour in natural populations of Anopheles gambiae and An. coluzzii: Review of 4 years survey in rural areas of sympatry, Burkina Faso (West Africa)

“…Although these molecular forms are able to hybridise in the laboratory, M/S hybrids are very rare in nature (della Torre et al 2001Torre et al , 2005. The issue of reproductive isolation between these molecular forms has created much debate della Torre et al 2002;Caputo et al 2008Lawniczak et al 2010 as the assessment of reproductive isolation varies considerably across their geographical distribution (della Torre et al 2001;Tripet et al 2001;Oliveira et al 2008;Diabaté et al 2009;Caputo et al 2011), and we do not yet know how reproductive barriers between these molecular forms function. In fact a cryptic group of An.…”