Female limited Batesian mimicry: epistasis and the path to perfection

非洲燕尾雀Papilio dardanus是巴特西亚模仿的最早也是最具标志性的例子之一。该物种表现出十几种不同的雌性颜色形式，都由一个单一的遗传位点控制，这被称为H.耐人寻味的问题是这些不同的形态是如何在达达努斯的进化史上获得的，因为模仿者必须离开保护性表型提供的适应高峰，才能到达另一个提供同等甚至更好保护的位置。迄今为止，很大程度上没有得到证实的“超基因”假设可以解释H基因座的复杂影响。此外，表观基因假说提出了一个两步进化模型，通过该模型，“大效应”突变提供了与一个新模型的一般相似性，该模型随后被所谓的“修饰物”改进。这项拟议的研究试图测试这一模型，从而理解自然界中形态多样性是如何产生的。我最近对H基因座的分子特征揭示了关于基因组相应区域的真正令人兴奋的事实，并暗示了一种可能的机制。我发现，编码转录因子的两个非常相似的基因--嵌入的和被感染的--都位于H处，特定的等位基因与特定的晶型在遗传上完全相关。这将解决H在分子水平上的身份问题，但不能解决其功能。为了产生巨大的形态多样性，H基因产物必须与上游和/或下游成分的信号级联相互作用。这些组件是什么，它们在变形之间有什么不同？观察到的差异是否证实了假设的修饰物是一种渐进进化的手段(正如达尔文提出的，但在进化大步可能更可信的情况下没有得到证实)？这项研究将使用最新的DNA技术来解决这些问题。首先，使用DNA足迹技术，我将调查积聚/侵袭区域是如何调节的，即哪些转录因子与它结合，以及确切的位置？这些足迹在不同的变形中是否有所不同，如果是这样的话，对这种机制的影响是什么？其次，我将调查哪些基因受转录因子的控制，再一次寻找不同的变体。第三，遗传方法被用来通过映射潜在的修饰物来理解H控制的信令级联的组成部分。最后，由于这种多态仅限于雌性，而雄性有非常不同的外表，因此形态的指定也必须与性别决定联系在一起。这是一个特别耐人寻味的方面，将通过评估所描述的男性和女性的基因表达实验来研究(也包括与男性相似的“类似男性的女性”)。这项研究将解决以下问题：形态进化的主要步骤如何在基因组中显现，它们与形态(或其他特征)变异的关系如何，以及这些变化在多大程度上是由于网络相互作用的复杂性，而不是特定基因座的变化。

The African Mocker Swallowtail Papilio dardanus is one of the original, and most iconic, examples of Batesian mimicry. The species exhibits more than a dozen different female colour forms all controlled by a single genetic locus, which has been termed H. Intriguing is the problem of how these diverse morphs were acquired in the evolutionary history of P. dardanus, as mimics have to leave the adaptive peak provided by a protective phenotype to reach another position providing equal or even better protection. Hitherto largely unproven postulates of a 'supergene' could explain the complex effects of the H locus. In addition, the supergene hypothesis proposed a two-step model of evolution, by which 'large-effect' mutations provide general resemblance with a novel model, which is subsequently refined by so-called 'modifiers'. The proposed study is an attempt to test this model and hence to understand how morphological diversity is generated in nature. My recent molecular characterisation of the H locus has revealed truly exciting facts about the corresponding region of the genome and hints at a possible mechanism. I have found that two closely similar genes, engrailed and invected, coding for transcription factors, are located at H and specific alleles are genetically fully associated with particular morphs. This would solve the question about the identity of H on a molecular level, but not its function. In order to produce the great morphological diversity, the H gene products have to interact with a signalling cascade of upstream and/or downstream components. What are these components, and how do they differ among morphs? Do the observed differences corroborate the hypothesized modifiers as a means of gradual evolution (as proposed by Darwin but not proven where evolutionary large steps may be more plausible)? This study will use latest DNA technology to resolve these questions. First, using DNA footprinting techniques I will investigate how the engrailed/invected region is regulated, i.e. which transcription factors bind to it and where exactly? Do these footprints differ among morphs, and if so, what are the implications for the mechanism? Second, I will investigate what genes are controlled by the engrailed/invected transcription factors, again searching for differences among the morphs. Third, a genetic approach is used to understand the components of the H controlled signalling cascade, by mapping potential modifiers. Finally, as the polymorphism is limited to the females, while males have a very different appearance, the specification of the morphs also has to link to sex determination. This is a particularly intriguing aspect which will be studied by assessing the described gene expression experiments separately in males and females (and also including 'male-like females' which resemble the males). The study will resolve the question about how major steps in morphological evolution are manifest in the genome, how they relate to morphological (or other trait) variation, and to what degree these changes are due to complexity of network interactions, rather than changes in particular loci.