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一万年前的最后一个冰河时代结束时。北大西洋为北极鲑（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。新的栖息地和竞争物种的缺乏导致了不同形式的北极生物的出现，这些生物被称为变种。特别是，27个离散的矮鲑种群已经被确定为具有特殊的觅食形态，使它们能够利用溪流和湖泊底部的小幼虫裂缝。我们的冰岛和加拿大合作伙伴收集了大量关于每个侏儒种群的数据，包括栖息地特征（温度和底部类型）、饮食、最大体型、性成熟时的体型和年龄以及颅骨形态。其他正在进行的关于快速进化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的进化可重复性。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）激素控制的信号通路而产生侏儒症，这表明了一种普遍而保守的生物学机制。有趣的是，在经常用于发育研究的斑马鱼身上，所谓的igf结合蛋白基因的“敲除”也导致了头部形状的改变，这让人想起了在侏儒鲑身上发现的那些变化。因此，我们将通过实验验证这一假设，即在发育过程中环境和igf激素系统之间的相互作用可以产生侏儒表型的特殊颌骨和颅骨形态特征。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，以确定是母亲的环境，胚胎还是两者都对产生具有侏儒特征的鱼很重要。Thingvallavatn是冰岛最大、最古老的湖泊，湖中有四种北极char变体，其中一种是侏儒，它们专门利用不同的栖息地。室内育种实验表明，在体型、形态和性成熟大小等生活史上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。之前我们发现，Thingvallavatn基因的侏儒症导致躯干肌肉纤维数量减少，这被认为比祖先基因的维护成本更低。通过对大量北极鲑种群（15个侏儒鲑和5个通才鲑）的研究，我们将检验这一假设的普遍性，即发育可塑性与选择对设定肌肉纤维数量的相对重要性与栖息地的年龄和稳定性有关，并且取决于是否与其他形态竞争。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。同一物种的不同形态的进化是相对普遍的，例如，在棘鱼和非洲慈鲷中可以发现。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.