Female limited Batesian mimicry: epistasis and the path to perfection

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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.