Costantini Carlo, Ayala Diego, Guelbeogo Wamdaogo M, Pombi Marco, Some Corentin Y, Bassole Imael Hn, Ose Kenji, Fotsing Jean-Marie, Sagnon N'Falé, Fontenille Didier, Besansky Nora J, Simard Frédéric
Institut de Recherche pour le Développement (IRD), UR016, and Institut de Recherche en Sciences de la Santé (IRSS), Bobo-Dioulasso, Burkina Faso.
BMC Ecol. 2009 May 21;9:16. doi: 10.1186/1472-6785-9-16.
Ongoing 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.
Maps 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.
Ecological 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.
非洲疟疾蚊子冈比亚按蚊持续的谱系分化与生态物种形成相符，生态物种形成是指通过对利用不同资源的两个种群进行趋异自然选择而实现生殖隔离的进化。在西非和中非，由核糖体DNA的固定差异所识别的两种分子形式（M和S）之间正在发生分化，其特征是存在明显但不完全的生殖隔离。为了阐明生态和地理在物种形成中所起的作用，我们在布基纳法索全国范围内对冈比亚按蚊M和S及其染色体变体的栖息地需求进行了分析。
通过地统计插值法生成的相对丰度图呈现出一种独特的分布模式：M型在最北部的干旱地区占主导，S型在更湿润的南部地区占主导。基于15个生态地理变量通过生态位因子分析量化的栖息地适宜性图显示，不同形式之间的差异较小。M型较为特殊，因为它在质量边际的栖息地中所占比例相对较高。生态位宽度和重叠度的测量结果证实了栖息地占用的基本模式和实际模式之间的不匹配：不同形式之间的隔离程度超过了根据其环境包络的差异程度所预期的水平——这是生态位扩张的一个特征。通过多位点遗传聚类对染色体臂2R核型进行分类，识别出两个大致对应分子形式的聚类，“不匹配”代表由于共享祖先多态性和/或残留杂交而产生的混合个体。在多变量排序空间中，与非混合的“典型”核型相比，这些核型分布在质量更边际的栖息地中。“典型”核型沿主要生态气候梯度的分布在不同形式内部和之间遵循一致的模式，表明在这个地理尺度上倒位具有适应性作用。
M和S之间的生态隔离与早期谱系分化过程中染色体倒位变体向边际栖息地的生态位扩张一致；据推测，这一过程是由优质核心栖息地中的核型间竞争所推动的。我们提出，在其他重组受抑制的区域（例如在冈比亚按蚊中定义物种形成岛的着丝粒周围部分）出现有利的等位基因组合会促进生殖隔离的发展，以保护不同染色体区域之间的连锁。
