Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina.
Department of Biology, Duke University, Durham, North Carolina.
Mol Ecol. 2018 Nov;27(21):4270-4288. doi: 10.1111/mec.14784. Epub 2018 Jul 27.
The process of sympatric speciation in nature remains a fundamental unsolved problem. Cameroon crater lake cichlid radiations were long regarded as one of the most compelling examples; however, recent work showed that their origins were more complex than a single colonization event followed by isolation. Here, we performed a detailed investigation of the speciation history of a radiation of Coptodon cichlids from Lake Ejagham, Cameroon, using whole-genome sequencing data. The existence of the Lake Ejagham Coptodon radiation is remarkable as this 0.5 km lake offers limited scope for divergence across a shallow depth gradient, disruptive selection is currently weak, and the species are sexually monochromatic. We infer that Lake Ejagham was colonized by Coptodon cichlids soon after its formation 9,000 years ago, yet speciation occurred only in the last 1,000-2,000 years. We show that secondary gene flow from a nearby riverine species has been ongoing, into ancestral as well as extant lineages, and we identify and date river-to-lake admixture blocks. One block contains a cluster of olfactory receptor genes that introgressed near the time of the first speciation event and coincides with a higher overall rate of admixture. Olfactory signalling is a key component of mate choice and species recognition in cichlids. A functional role for this introgression event is consistent with previous findings that sexual isolation appears much stronger than ecological isolation in Ejagham Coptodon. We conclude that speciation in this radiation took place in sympatry, yet may have benefited from ongoing riverine gene flow.
在自然界中，同域物种形成的过程仍然是一个基本未解决的问题。喀麦隆火山口湖慈鲷辐射曾长期被视为最引人注目的例子之一；然而，最近的研究表明，它们的起源比单一的殖民事件后隔离更为复杂。在这里，我们使用全基因组测序数据，对喀麦隆 Ejagham 湖的 Coptoodon 慈鲷辐射的物种形成历史进行了详细调查。 Ejagham 湖 Coptoodon 辐射的存在是值得注意的，因为这个 0.5 公里的湖泊在浅深度梯度上的分歧范围有限，破坏性选择目前较弱，而且这些物种是性单态的。我们推断，Coptoodon 慈鲷在 Ejagham 湖形成后不久就在这里殖民，然而物种形成只发生在最近的 1000-2000 年。我们表明，来自附近河流物种的二次基因流一直在进行，进入了祖先和现存的谱系，我们确定并标记了河流到湖泊的混合块。一个块包含一个嗅觉受体基因簇，这些基因在第一次物种形成事件附近发生了基因渗入，并且与更高的总体混合率相一致。嗅觉信号是慈鲷择偶和物种识别的关键组成部分。这种基因渗入事件的功能作用与 Ejagham Coptoodon 中性隔离似乎比生态隔离强得多的先前发现一致。我们的结论是，这个辐射中的物种形成是同域的，但可能受益于持续的河流基因流。
