Rapid evolution of phenotypic divergence in fish populations

冰岛是研究鱼类在淡水栖息地定居的天然实验室，因为河流和湖泊都可以追溯到不到10 000年前的最后一个冰河时代结束时。北大西洋为北极红点鲑（Salvelinus alpinus）等物种提供了避难所，一旦冰层消退，它们就会入侵淡水。新的栖息地和缺乏竞争物种导致了不同形式的北极红点鲑的出现，称为变种。特别是，27个离散种群的矮红点鲑已被确定与专门的喂养形态，使他们能够利用小的幼虫裂缝底部的溪流和湖泊。我们的冰岛和加拿大合作伙伴已经收集了关于每个侏儒种群的大量数据，包括栖息地特征（温度和底部类型），饮食，最大体型，性成熟时的大小和年龄以及颅骨形态。其他正在进行的关于快速进化的DNA序列的研究将使我们能够确定每个种群之间的关系，并估计哪些种群是独立出现的，从而使我们能够研究生活在相似栖息地的种群进化的可重复性。涉及蠕虫、苍蝇和脊椎动物等多种生物的研究表明，营养不良本身就足以通过影响胰岛素样生长因子-I（IGF-I）控制的信号通路产生侏儒症：这表明了一种普遍和保守的生物学机制。有趣的是，在通常用于发育研究的斑马鱼中，所谓的IGF结合蛋白基因的“敲除”也会导致头部形状的改变，这让人想起在侏儒红点鲑中发现的那些。因此，我们将实验测试的假设，环境和IGF-激素系统之间的相互作用，在发展过程中可以产生专门的下巴和颅骨形态特征的侏儒表型。由于鱼类的早期发育完全依赖于通过蛋黄传递的遗传信息，我们将进行实验，以确定是否是母亲，胚胎或两者的环境对生产具有侏儒特征的鱼类很重要。Thingvallavatn是冰岛最大和最古老的湖泊，包含四种北极charr变体，包括一种侏儒形式，它们专门利用不同的栖息地。实验室育种实验表明，个体大小、形态和生活史（如性成熟时的大小）的巨大差异是可遗传的。这表明变种之间的激烈竞争和生殖隔离导致了自然选择和特征的专业化，帮助每个变种在他们选择的环境中生存。以前，我们表明，侏儒症在Thingvallavatn charr已导致在躯干中的肌肉纤维的数量减少，这被认为是降低成本的维护相对于祖先charr。通过研究大量的北极红点鲑种群（15个侏儒和5个通才），我们将测试的假设，即发育可塑性与选择的相对重要性设置肌肉纤维数量的年龄和稳定性的栖息地，是不同的，这取决于是否有竞争与其他变种的一般性。这项研究很重要，因为它解决了自然选择和可塑性如何在生理系统水平上产生相同物种的不同形式的基本问题。同一物种的不同变体的进化相对常见，例如在棘鱼和非洲慈鲷中发现。这项研究的实际应用是了解鱼类种群的生物多样性是如何产生的，以及如何为后代保护它。

Iceland represents a natural laboratory for studying the colonization of freshwater habitats by fish since rivers and lakes all date from the end of the last Ice-Age less than 10,000 years ago. The North Atlantic provided a refuge for species such as arctic charr (Salvelinus alpinus) which invaded freshwater once the ice retreated. New habitats and the lack of competing species led to the appearance of different forms of Artic charr, called morphs. In particular, 27 discrete populations of dwarf charr have been identified with specialised feeding morphology that enables them to exploit the small larval fissures on the bottom of streams and lakes. Our Icelandic and Canadian partners have collected an enormous amount of data on each of the dwarf populations including, habitat characteristics (temperature and bottom type), diet, maximum body size, size and age at sexual maturity and cranial morphology. Other studies in progress on rapidly evolving DNA sequences we will enable us to determine the relationships between each population and estimate which ones arose independently allowing us to study the repeatability of evolution for populations living in similar habitats. Studies involving such diverse organisms as worms, flies and vertebrates suggest that poor nutrition alone is sufficient to produce dwarfism via effects on the signaling pathways controlled by the hormone Insulin-like growth factor-I (IGF-I): indicating a universal and conserved biological mechanism. Intriguingly, in the zebrafish, which is often used for studies of development, so-called 'knock-outs' of an IGF-binding-protein gene also caused alterations to the shape of the head which are reminiscent of those found in dwarf charr. We will therefore experimentally test the hypothesis that interactions between the environment and the IGF-hormone system during development can produce the specialised jaw and cranial morphology characteristic of the dwarf phenotype. Since early development in fish is entirely dependent on genetic messages passed through the egg yolk we will conduct experiments to determine whether it is the environment of the mother, the embryo or both that are important for producing fish with dwarf characteristics. Thingvallavatn, the largest and oldest lake in Iceland, contains four Arctic charr morphs, including a dwarf form, which are specialised to exploit different habitats. Laboratory breeding experiments have shown that the large differences in body size, morphology and life history such as the size at sexual maturity are heritable. This suggests that intense competition between morphs and reproductive isolation has resulted in natural selection and specialization for characters helping each morph to survive in their chosen environment. Previously we showed that dwarfism in the Thingvallavatn charr has resulted in a reduction in the number of muscle fibres in the trunk, which is thought to lower costs of maintenance relative to the ancestral charr. By studying a large number of Arctic charr populations (15 dwarfs and 5 generalists) we will test the generality of the hypothesis that the relative importance of developmental plasticity versus selection for setting muscle fibre number is related to the age and stability of the habitat and is different depending on whether there is competition with other morphs. The research is important because it addresses the fundamental question of how natural selection and plasticity operate to produce different forms of the same species at the level of physiological systems. The evolution of different morphs of the same species is relatively common and is found, for example, in sticklebacks and African cichclids. The practical application of this research is in understanding how the biodiversity of fish populations arises and how it may be conserved for future generations.