Scottish Ocean Institute, School of Biology, University of St Andrews, St Andrews, Fife, UK.
Mol Ecol. 2011 Aug;20(15):3167-84. doi: 10.1111/j.1365-294X.2011.05172.x. Epub 2011 Jun 29.
Ecological factors have a major role in shaping natural variation in body size, although the underlying mechanisms are poorly understood. Icelandic Arctic charr (Salvelinus alpinus L.) populations represent an ideal model to understand body-size evolution, because adult dwarfism has arisen independently on multiple occasions in response to parallel environmental pressures. The mechanistic target of rapamycin (mTOR) pathway transmits signals from the environment to control cellular growth and is a primary candidate to be under selection for the dwarf phenotype. To test this hypothesis, we modified 'inputs' to this pathway in five dwarf and two generalist populations (with ancestral life history and body-size traits), using a standardized manipulation of food intake in a common environment. The skeletal muscle transcript levels of 21 mTOR-pathway genes were quantified in 274 individuals (∼6000 datapoints), and statistical modelling was used to elucidate sources of variation. Constitutive expression differences between populations were the main component of variation for around three-quarters of the studied genes, irrespective of nutritional-state and body-size phenotype. There was evidence for stabilizing selection acting among populations, conserving the nutritionally dependent regulation of pathway genes controlling muscle atrophy. There were three genes (mTOR, 4E-BP-1 and IGFBP4), where the expression variation between dwarf and generalist populations exceeded the between-population variation. Divergence in the expression of these candidate adaptive genes was most evident during a period of rapid growth following sustained fasting and was directionally consistent with their functions regulating growth and protein synthesis. We concluded that selection has operated efficiently to shape gene expression evolution in Icelandic charr populations and that the regulation of certain mTOR-pathway genes evolved adaptively in locations favouring dwarfism, resulting in reduced muscle protein accretion under growth-favouring conditions.
生态因素在塑造生物体大小的自然变异方面起着重要作用，尽管其潜在机制尚不清楚。冰岛北极红点鲑（Salvelinus alpinus L.）种群是理解体型进化的理想模型，因为在多次平行的环境压力下，成年矮化现象独立出现。雷帕霉素靶蛋白（mTOR）途径将环境信号传递到控制细胞生长的位置，是选择矮化表型的主要候选途径。为了检验这一假说，我们在五个矮化和两个普通型种群（具有祖先的生活史和体型特征）中，通过在共同环境中标准化的饮食摄入操作，改变了该途径的“输入”。我们在 274 个个体（约 6000 个数据点）中量化了 21 个 mTOR 途径基因的骨骼肌转录水平，并使用统计模型来阐明变异的来源。无论营养状态和体型表型如何，种群之间的组成型表达差异都是研究基因约四分之三变异的主要组成部分。有证据表明，在种群之间存在稳定选择，保守控制肌肉萎缩的途径基因的营养依赖性调控。有三个基因（mTOR、4E-BP-1 和 IGFBP4），矮化和普通型种群之间的表达差异超过了种群间的差异。这些候选适应性基因的表达差异在持续禁食后快速生长期间最为明显，并且与它们调节生长和蛋白质合成的功能方向一致。我们得出结论，选择在冰岛红点鲑种群的基因表达进化中发挥了有效的作用，某些 mTOR 途径基因的调控在有利于矮化的位置发生了适应性进化，导致在有利于生长的条件下肌肉蛋白质积累减少。
