Garrick Ryan C, Benavides Edgar, Russello Michael A, Hyseni Chaz, Edwards Danielle L, Gibbs James P, Tapia Washington, Ciofi Claudio, Caccone Adalgisa
Department of Biology, University of Mississippi, Oxford, MS, 38677, USA.
Mol Ecol. 2014 Nov;23(21):5276-90. doi: 10.1111/mec.12919. Epub 2014 Oct 4.
Although many classic radiations on islands are thought to be the result of repeated lineage splitting, the role of past fusion is rarely known because during these events, purebreds are rapidly replaced by a swarm of admixed individuals. Here, we capture lineage fusion in action in a Galápagos giant tortoise species, Chelonoidis becki, from Wolf Volcano (Isabela Island). The long generation time of Galápagos tortoises and dense sampling (841 individuals) of genetic and demographic data were integral in detecting and characterizing this phenomenon. In C. becki, we identified two genetically distinct, morphologically cryptic lineages. Historical reconstructions show that they colonized Wolf Volcano from Santiago Island in two temporally separated events, the first estimated to have occurred ~199 000 years ago. Following arrival of the second wave of colonists, both lineages coexisted for approximately ~53 000 years. Within that time, they began fusing back together, as microsatellite data reveal widespread introgressive hybridization. Interestingly, greater mate selectivity seems to be exhibited by purebred females of one of the lineages. Forward-in-time simulations predict rapid extinction of the early arriving lineage. This study provides a rare example of reticulate evolution in action and underscores the power of population genetics for understanding the past, present and future consequences of evolutionary phenomena associated with lineage fusion.
尽管许多岛屿上的经典辐射被认为是反复的谱系分裂的结果，但过去融合的作用却鲜为人知，因为在这些事件中，纯种个体很快就会被一群混合个体所取代。在这里，我们在来自沃尔夫火山（伊莎贝拉岛）的加拉帕戈斯巨型陆龟物种——贝克氏陆龟（Chelonoidis becki）中捕捉到了正在发生的谱系融合。加拉帕戈斯陆龟的长世代时间以及对遗传和人口数据的密集采样（841个个体）对于检测和描述这一现象至关重要。在贝克氏陆龟中，我们识别出了两个基因上不同但形态上难以区分的谱系。历史重建表明，它们在两个时间上分开的事件中从圣地亚哥岛殖民到沃尔夫火山，第一次估计发生在约199,000年前。在第二批殖民者到来之后，两个谱系共存了大约53,000年。在那段时间里，它们开始重新融合在一起，正如微卫星数据所揭示的广泛的渐渗杂交。有趣的是，其中一个谱系的纯种雌性似乎表现出更强的配偶选择性。时间向前模拟预测早期到达的谱系将迅速灭绝。这项研究提供了一个正在发生的网状进化的罕见例子，并强调了群体遗传学在理解与谱系融合相关的进化现象的过去、现在和未来后果方面的力量。
