Cuckoos versus hosts: next-generation sequencing of genes for cuckoo counter-adaptations

进化过程中的适应方式是目前生物学中的一个热门话题。特别是，人们对作为适应基础的基因变化非常感兴趣。这一点很重要，因为它使我们能够了解是否仅仅因为潜在的遗传因素，某些适应比其他适应更有可能出现，它还可以帮助我们了解生物体如何适应环境变化。杜鹃和它们的宿主提供了一个全新的、易于管理的系统，在这个系统中可以确定自然界适应的遗传基础。研究杜鹃及其宿主的一大优势是，我们可以非常准确地衡量自然界适应的选择优势，因为每一方都选择了直接影响生存或繁殖成功的另一方的特征。例如，野外实验表明，宿主对奇形怪状的蛋的排斥推动了杜鹃蛋的进化，这种蛋完美地模仿了宿主产下的蛋。对于寄主来说，排卵是适应的，因为这意味着它们的后代不会在稍后被巢中的幼小杜鹃杀死。蛋的模仿适用于杜鹃，因为这意味着当它们拒绝杜鹃蛋时，它们的后代不会被宿主杀死。在这个项目中，我们将研究闪亮的青铜杜鹃(Chaldes Lucidus)的一个关键反适应的遗传学基础，这是一种南半球的杜鹃，在雏鸟的颜色上表现出惊人的模仿。这一物种的不同种群拥有不同颜色的雏鸟，它们准确地模仿了当地宿主雀形目动物的雏鸟--因此，杜鹃雏鸟在一些种群中完全是黑色的，在另一些种群中是苍白的。我们最新的实验(仍未发表)表明，通过模仿宿主幼崽，杜鹃雏鸟不太可能被宿主识别为外来物种，因此不太可能被拒绝死亡。因此，小鸡的模仿是适应性的。此外，这是一个非常有利的遗传分析系统，因为肤色差异的遗传基础可能很简单，而且脊椎动物对黑色素着色的遗传途径也很了解。在这项初步研究中，我们建议使用下一代测序技术来根据基因表达和序列变异的差异来识别颜色变异的候选基因座。显色差异的强候选基因座的鉴定将在未来的研究中得到证实。除了颜色位点鉴定外，这项工作还将在杜鹃物种中产生第一个大规模的基因组资源，因此将对研究杜鹃的进化生物学家的相当大的社区有很大的帮助。我们还将能够确定杜鹃种群间羽毛差异的潜在候选基因座。该项目代表了研究杜鹃-宿主共同进化的一种新方法，并为研究自然界的遗传适应提供了一种新的研究体系。因此，它有可能开辟一个全新的研究领域。

The way that adaptations arise during evolution is currently a hot topic in biology. In particular, there is great interest in the genetic changes that underlie adaptations. This is important because it enables us to understand whether some adaptations are more likely to arise than others simply because of the underlying genetics and it can help us understand how organisms adapt to environmental change. Cuckoos and their hosts offer a completely novel, tractable system in which to determine the genetic basis of adaptation in nature. One great advantage of working on cuckoos and their hosts is that we can measure the selective advantages of adaptations in nature very precisely because each party selects characteristics in the other that directly affect survival or reproductive success. For example, field experiments have shown that rejection of odd-looking eggs by hosts has driven the evolution of cuckoo eggs that perfectly mimic those laid by the host. Egg rejection is adaptive for hosts, because it means their offspring are not killed later by a hatchling cuckoo in the nest. Egg mimicry is adaptive for cuckoos, because it means their offspring are not killed by hosts when they reject the cuckoo egg. In this project we will investigate the genetics underlying a key counter-adaptation in the shining-bronze cuckoo (Chalcides lucidus), a southern hemisphere cuckoo which shows remarkable mimicry in chick coloration. Different populations of this species have differently coloured chicks which accurately mimic the chicks of the local host passerine - consequently cuckoo chicks are completely black in some populations, and pale in others. Our most recent experiments (still unpublished) suggest that by mimicking host young, cuckoo nestlings are less likely to be recognised as foreign by their hosts and consequently less likely to be rejected to die. Chick mimicry is therefore adaptive. Furthermore, this is a very favourable system for genetic analysis since the genetic basis of the skin colour difference is likely to be simple, and the genetic pathways underlying the black melanin coloration are well understood in vertebrates. In this pilot study we propose to use next-generation sequencing technology to identify candidate loci for colour variation based on differences in gene expression and sequence variation. The identification of strong candidate loci for the coloration difference which will be confirmed in future research. In addition to colour locus identification, the work will produce the first large scale genomic resources in a cuckoo species and will thus be of great use to the considerable community of evolutionary biologists working on cuckoos. We will also be able to identify potential candidate loci for the feather down differences between the cuckoo populations. The project represents a novel approach to the study of cuckoo-host coevolution, and offers a novel study system to the study of genetic adaptations in nature. It therefore has the potential to open up an entire new field of research.