Living at the edge: biogeographic patterns of habitat segregation conform to speciation by niche expansion in Anopheles gambiae

Abstract

BackgroundOngoing lineage splitting within the African malaria mosquito Anopheles gambiae is compatible with ecological speciation, the evolution of reproductive isolation by divergent natural selection acting on two populations exploiting alternative resources. Divergence between two molecular forms (M and S) identified by fixed differences in rDNA, and characterized by marked, although incomplete, reproductive isolation is occurring in West and Central Africa. To elucidate the role that ecology and geography play in speciation, we carried out a countrywide analysis of An. gambiae M and S habitat requirements, and that of their chromosomal variants, across Burkina Faso.ResultsMaps of relative abundance by geostatistical interpolators produced a distinct pattern of distribution: the M-form dominated in the northernmost arid zones, the S-form in the more humid southern regions. Maps of habitat suitability, quantified by Ecological Niche Factor Analysis based on 15 eco-geographical variables revealed less contrast among forms. M was peculiar as it occurred proportionally more in habitat of marginal quality. Measures of ecological niche breadth and overlap confirmed the mismatch between the fundamental and realized patterns of habitat occupation: forms segregated more than expected from the extent of divergence of their environmental envelope – a signature of niche expansion. Classification of chromosomal arm 2R karyotypes by multilocus genetic clustering identified two clusters loosely corresponding to molecular forms, with 'mismatches' representing admixed individuals due to shared ancestral polymorphism and/or residual hybridization. In multivariate ordination space, these karyotypes plotted in habitat of more marginal quality compared to non-admixed, 'typical', karyotypes. The distribution of 'typical' karyotypes along the main eco-climatic gradient followed a consistent pattern within and between forms, indicating an adaptive role of inversions at this geographical scale.ConclusionEcological segregation between M and S is consistent with niche expansion into marginal habitats by chromosomal inversion variants during early lineage divergence; presumably, this process is promoted by inter-karyotype competition in the higher-quality core habitat. We propose that the appearance of favourable allelic combinations in other regions of suppressed recombination (e.g. pericentromeric portions defining speciation islands in An. gambiae) fosters development of reproductive isolation to protect linkage between separate chromosomal regions.

Spatiotemporal dynamics of gene flow and hybrid fitness between the M and S forms of the malaria mosquito, Anopheles gambiae

“…The M form is generally believed to be the more recently evolved (62,63), presumably via niche expansion into marginal habitats in parapatry with the S form (64). It is in these marginal habitats that they are believed to have diverged via adaptation to the peculiar ecological conditions that occur there and ultimately by the acquisition of premating mechanisms reducing the possibility of mating with the S form.…”

Adaptive divergence between incipient species of Anopheles gambiae increases resistance to Plasmodium

“…The M and S forms cannot be distinguished morphologically or by other specific phenotypic traits. However, general ecological differences are known (8)(9)(10)(11). The S form breeds in small ephemeral pools and puddles across sub-Saharan Africa, whereas the M form exploits larger, more stable breeding sites closely associated with agricultural or urban activity in West and Central Africa.…”

Humming in Tune: Sex and Species Recognition by Mosquitoes on the Wing

“…The complexity of malaria epidemiology and the extreme resilience with which malaria has established itself in human populations is due, in part at least, to the remarkable genetic plasticity of certain members of species complex, enabling them to adapt rapidly to an ever widening range of human-influenced habitats. This leads to rapid ecological speciation when reproductive isolation mechanisms develop (Coluzzi 1982;Powell et al 1999;della Torre et al 2001;Ayala and Coluzzi 2005;Costantini et al 2009). Although reproductive isolation is essential for speciation, little is known about how it occurs in sympatric populations of incipient species (Costantini et al 2009).…”

“…This leads to rapid ecological speciation when reproductive isolation mechanisms develop (Coluzzi 1982;Powell et al 1999;della Torre et al 2001;Ayala and Coluzzi 2005;Costantini et al 2009). Although reproductive isolation is essential for speciation, little is known about how it occurs in sympatric populations of incipient species (Costantini et al 2009). …”