The molecular basis of Mullerian mimicry

蝴蝶物种间的模仿是进化和适应的范例，但其分子遗传基础从未被研究过。在这里，我们将使用现代基因组技术，以确定基因控制模仿模式的属Heliconius，一个众所周知的进化模式系统。尤其令人吃惊的是我们最近的发现，三种海葵使用相同的遗传区域来控制颜色图案的变化。在墨尔波墨涅人和埃拉托人这两个物种中，这些是通过相互模仿进化而来的趋同模式。在第三个物种，H. numata中，其模式非常不同，并模仿来自不同亚科的物种，Ithomiinae。此外，在numata中，该位点是一个“超基因”，即单个位点控制翅膀图案的所有方面，并且在种群中是多态的（在同一种群中发现许多不同外观的形式一起飞行）。因此，基因组的同一区域以某种方式进化来控制趋同和发散模式。在这里，我们将使用已经在H. melpomene中开发的标记来识别其他两个物种的感兴趣区域，并获得一个足够大的区域的DNA序列，我们确信这三个物种中都包含感兴趣的基因。这将允许比较这三个物种在这一地区的基因组组织，并测试可能构成翅膀图案进化基础的基因组重组。特别是有一个长期存在的假设，一个“超级基因”，如在人肉人身上看到的，可能是通过逐渐将几个基因移到一起而产生的（这种进化的优势是在野生种群中避免了不合适的中间模式）。这项研究将首次在分子水平上对这一假设进行明确的检验。然后，我们将确定该区域的哪个位点控制着H. melpomene的黄色带模式。我们将首先确定该区域所有可能的基因作为模式位点的候选基因，并确定哪些基因在发育翅膀中表达，以及它们是否显示出与翅膀模式相关的任何模式。因此，我们的目标是确定控制H. melpomene后翅上一个条带的Yb基因，并比较三个物种之间该位点区域的基因组组织。这项工作提供了一个不寻常的机会，将在自然种群中重要性得到充分研究的特征与DNA序列水平的变化联系起来。这将是第一次在分子水平上描述一个控制如此充分研究特征的自然变异的基因，并将刺激未来对Heliconius拟态的发育和种群遗传研究。特别是，这些结果将有助于未来研究基因的强选择如何影响周围基因组的变异。

Mimicry among butterfly species is a paradigm example of evolution and adaptation, but its molecular genetic basis has never been studied. Here we will use modern genomic techniques to identify genes controlling mimicry patterns in the genus Heliconius, a well-known evolutionary model system. What is particularly striking is our recent discovery that three Heliconius species use the same genetic region to control variation in colour patterns. In two of the species, H. melpomene and H. erato, these are convergent patterns that evolved through mutual mimicry. In the third species, H. numata, the patterns are very divergent and mimic species from a different sub-family, the Ithomiinae. Furthermore in H. numata the locus is a 'supergene' i.e. a single locus that controls all aspects of wing pattern and is polymorphic within populations (many different-looking forms are found flying together in the same population). Thus, the same region of the genome has somehow evolved to control both convergent and divergent patterns. Here we will use markers already developed in H. melpomene to identify the region of interest in the other two species and obtain DNA sequence for a sufficiently large region that we are sure contains the gene of interest in all three species. This will allow a comparison of genome organisation in this region between the three species and a test for reorganisation of the genome that may underlie the evolution of wing pattern. In particular there is a long-standing hypothesis that a 'supergene' such as that seen in H. numata might arise by gradually moving several genes together (the evolutionary advantage of this is that unfit intermediate patterns are avoided in wild populations). This study will allow the first explicit test of this hypothesis at a molecular level. We will then determine which locus in this region is controlling the yellow band pattern in H. melpomene. We will first identify all the possible genes in this region as candidates for being the patterning locus, and determine which ones are expressed in developing wings and whether they show any patterns that correlate with the wing patterns. We therefore aim to identify the Yb gene that controls a band on the hindwing of H. melpomene, and compare the organisation of the genome in the region of this locus between the three species. This work offers an unusual opportunity to link a trait whose importance in natural populations is well studied with a change at the level of DNA sequences. It will represent the first time that a gene controlling natural variation in such a well studied trait has been characterised at a molecular level, and will stimulate future developmental and population genetic studies of mimicry in Heliconius. In particular, the results will facilitate future studies of how strong selection on a gene affects variation in the surrounding genome.