Palkovacs Eric P, Dion Kirstin B, Post David M, Caccone Adalgisa
Department of Ecology and Evolutionary Biology, Yale University, 165 Prospect Street, New Haven, CT 06520-8106, USA.
Mol Ecol. 2008 Jan;17(2):582-97. doi: 10.1111/j.1365-294X.2007.03593.x. Epub 2007 Dec 20.
Alewife, Alosa pseudoharengus, populations occur in two discrete life-history variants, an anadromous form and a landlocked (freshwater resident) form. Landlocked populations display a consistent pattern of life-history divergence from anadromous populations, including earlier age at maturity, smaller adult body size, and reduced fecundity. In Connecticut (USA), dams constructed on coastal streams separate anadromous spawning runs from lake-resident landlocked populations. Here, we used sequence data from the mtDNA control region and allele frequency data from five microsatellite loci to ask whether coastal Connecticut landlocked alewife populations are independently evolved from anadromous populations or whether they share a common freshwater ancestor. We then used microsatellite data to estimate the timing of the divergence between anadromous and landlocked populations. Finally, we examined anadromous and landlocked populations for divergence in foraging morphology and used divergence time estimates to calculate the rate of evolution for foraging traits. Our results indicate that landlocked populations have evolved multiple times independently. Tests of population divergence and estimates of gene flow show that landlocked populations are genetically isolated, whereas anadromous populations exchange genes. These results support a 'phylogenetic raceme' model of landlocked alewife divergence, with anadromous populations forming an ancestral core from which landlocked populations independently diverged. Divergence time estimates suggest that landlocked populations diverged from a common anadromous ancestor no longer than 5000 years ago and perhaps as recently as 300 years ago, depending on the microsatellite mutation rate assumed. Examination of foraging traits reveals landlocked populations to have significantly narrower gapes and smaller gill raker spacings than anadromous populations, suggesting that they are adapted to foraging on smaller prey items. Estimates of evolutionary rates (in haldanes) indicate rapid evolution of foraging traits, possibly in response to changes in available resources.
美洲西鲱（Alosa pseudoharengus）种群呈现出两种不同的生活史变体，一种是溯河洄游型，另一种是陆封型（淡水定居型）。陆封型种群与溯河洄游型种群在生活史上呈现出一致的分化模式，包括成熟年龄更早、成年个体体型更小以及繁殖力降低。在美国康涅狄格州，沿海溪流上修建的水坝将溯河洄游的产卵群体与湖泊定居的陆封型种群分隔开来。在此，我们利用线粒体DNA控制区的序列数据和五个微卫星位点的等位基因频率数据，来探究康涅狄格州沿海陆封型美洲西鲱种群是从溯河洄游型种群独立进化而来，还是它们拥有共同的淡水祖先。然后，我们利用微卫星数据来估计溯河洄游型和陆封型种群之间分化的时间。最后，我们检查溯河洄游型和陆封型种群在觅食形态上的差异，并利用分化时间估计值来计算觅食特征进化的速率。我们的结果表明，陆封型种群已经多次独立进化。种群分化测试和基因流估计表明，陆封型种群在基因上是隔离的，而溯河洄游型种群则进行基因交换。这些结果支持了陆封型美洲西鲱分化的“系统发育总状花序”模型，溯河洄游型种群形成一个祖先核心，陆封型种群从该核心独立分化出来。分化时间估计表明，陆封型种群从共同的溯河洄游型祖先分化出来的时间不超过5000年前，甚至可能近至300年前，这取决于所假设的微卫星突变率。对觅食特征的检查发现，陆封型种群的口裂明显比溯河洄游型种群窄，鳃耙间距也更小，这表明它们适应于捕食体型较小的猎物。进化速率估计（以霍尔丹为单位）表明觅食特征进化迅速，可能是对可用资源变化的响应。
