Genomic Patterns of Wing Pattern Introgression in Heliconius

Despite the fact the already Charles Darwin had recognised the role of hybridisation in the process of speciation in his famous work On the origin of species, it has been assumed to hinder speciation or to be rare and of no evolutionary significance among zoologists. Influenced by Ernst Mayr and his biological species concept, this view has been upheld for a long time. Within the last decade, however, evidence from across the animal kingdom suggesting that hybridisation represents an important evolutionary force in the speciation has been accumulating, e.g. for the evolution of Darwins finches and humans. Hybridisation can either directly lead to the emergence of new species via hybrid speciation or indirectly by transferring genes, that are advantageous under certain environmental conditions (i.e. adaptive introgression), and thereby provide the raw material for speciation through divergence in adaptations. One of the best studied systems with this respect is the tropical radiation of Heliconius butterflies, for which both cases of hybrid speciation and adaptive introgression are described. Recently, evidence has been presented suggesting that protective colour patterns on the wings are promiscuously shared among closely related Heliconius species. This allows now for testing the scenarios of adaptive introgression and hybrid speciation, for ruling out possible alternative explanations, and for a better understanding of the selective forces and genomic signatures of adaptive introgression. Furthermore, the frequency of these processes and therefore their relative importance for speciation needs to be evaluated. In this project, I first aim to reconstruct the number of independent colour pattern introgression events in order to assess their relative importance. Moreover, I investigate signatures of selection at genomic regions coding for colour pattern loci. In addition, I will rigorously test the hypotheses of adaptive introgression and hybrid speciation with a set of methods to exclude alternative scenarios. In order to achieve these goals I will sequence and analyse the genomic regions coding for warning patterns for numerous colour pattern morphs and species. By comparing the relationships between colour pattern regions and species I can draw conclusions about hybridisation and introgression. These regions will also be tested for signals of selection to better understand the evolutionary forces at work in these regions. The diversity of colour patterns and species in Heliconius butterflies and the well-established knowledge on colour patterns offers unique opportunities to assess the relevance of hybridisation for the process of speciation and consequently for our own evolution. Finally, the acquired skills and insights from the work on the Heliconius system will be transferred back to Austria to study hybridising species complexes in the Alps and the Greater Alpine region and to compare them with other systems, e.g. Heliconius butterflies.

The main outcome of this project is the generation of an extensive genomic dataset, comprising several hundred individuals across several populations and species of butterflies in the Heliconius melpomene clade, which enables me, together with the Heliconius community, to study patterns of selection and introgression at genomic region controlling the brightly coloured warning patterns these butterflies display on their wings.Heliconius butterflies are toxic, which they signal to predators through their warning patterns. Distantly related species occupying the same habitats have converged onto the same colour patterns to share the costs of training predators, and thereby enhance protection, an evolutionary process known as Muellerian mimicry. Closely related species, however, have taken a shortcut and exchanged the genomic regions controlling the colour patterns via hybridization. Even our own, human evolutionary history has been shaped by this process of gene flow, e.g. via interbreeding between humans and Neanderthals. This process of exchange of genetic material between species is increasingly recognized as a significant evolutionary process also in animals, however, its relative importance for the process of adaptation and the function and fate of introgressed genetic material in the new genomic background are often unknown.Within the H. melpomene clade, gene flow between populations of H. melpomene, H. timareta and H. cydno, and the silvaniform species H. elevatus and H. besckei is known. Using a priori knowledge on four genomic regions harbouring the main genetic variation underlying the colour patterns, Ive used different genomic sequencing techniques to gather sequence information for hundreds of individuals across this clade. With this comprehensive dataset we can i) assess how often introgression has occurred within in the clade and how many independent introgression events are detectable, i.e. is adaptive introgression rare or frequent, ii) investigate how selection has shaped the genetic variation in these regions and which elements and regions are subject to strong selection, and iii) narrow down the genomic regions coding for colour pattern variation, information that will be highly useful for functional validation studies using e.g. CRISPR/CAS9.In conclusion, weve found evidence for repeated introgression of colour pattern alleles across the Helicionius melpomene clade and discovered a previously unknown subspecies of H. timareta in Colombia, which is, due to adaptive introgression of colour pattern alleles, virtually indistinguishable from its co-mimic H. melpomene. Moreover, clear and narrow signals of selection were detected in regulatory regions of known colour pattern genes, allowing to identify functional entities. This dataset is a rich, new resource for the Heliconius community and further in-depth analyses are ongoing. Altogether this project has revealed the importance of gene flow between species in an adaptive radiation.