Final Report Summary - HYBRAD (The Impact of Hybridization on Functional Diversification - Experimental Evolution with Sexual Microbes) Project 1 - Hybrid transgressionEvidence is accumulating from across the tree of life that hybridization generates adaptive variation, but current knowledge mainly derives from post hoc analysis of existing hybrid species. Experimental tests are severely lacking. Especially transgressive segregation, the expression of extreme traits in hybrids, could promote hybrid speciation. In this first project (Stelkens et al. J Evol. Biol. 2014), we present experimental evidence for the ecological divergence of transgressive hybrids, which advances the hybrid speciation debate beyond mere description.Our results are in two parts.a) We crossed phenotypically and genetically divergent Saccharomyces yeast strains, and compared the ability of hybrids and parents to colonize seven types of increasingly stressful environmental clines, representing both natural and novel challenges (mimicking pollution events). We found that many swarms contained hybrids that could grow in more extreme habitats than their non-hybrid parents. Depending on the type of environment, more distant parental lineages produced either more, or less extreme hybrid phenotypes.b) Because hybrids can only be evolutionarily successful if they withstand competition from their parents, we tested the relative fitness of transgressive hybrids (identified and propagated from the above experiment) and their parents upon invasion of a common habitat. We found hybrids capable of directly outcompeting their parents under a large range of environmental conditions. Often hybrid fitness was 3-fold higher than parental fitness.In this paper, we thus demonstrate that hybridization readily gives rise to adaptive evolutionary novelty, particularly with respect to environmental stress. We also show that transgressive hybrids are able to outcompete their parents in a common habitat We believe that these findings significantly advance our understanding of the key processes underlying species diversification, especially in a world where anthropogenic impact leads to frequent relocation of organisms.Project 2 – Hybridization facilitates evolutionary rescueWe are entering a period of mass extinction driven by catastrophic man-made environmental change. Species that cannot survive future conditions will become extinct unless they can escape to refuges with less extreme conditions, or evolve rapidly enough to adapt to the deteriorating environment. This latter survival mechanism is called “evolutionary rescue”. Given the pace of man-made environmental change, there is an urgent need to understand the intrinsic and extrinsic conditions permitting evolutionary rescue of species.In this project (Stelkens et al. Evol. Applications 2014), we experimentally test a novel factor facilitating evolutionary rescue: interspecific hybridization. We show that by boosting the genetic variance and phenotypic novelty available to natural selection, hybridization increases the probability of evolutionary rescue compared both to parental lineages and intraspecific crosses. We conclude that hybridization can increase evolutionary responsiveness to environmental change and that taxa able to exchange genes with distant relatives such as plants will prove more resilient to rapid environmental change. Our findings have important implications for conservation of biodiversity and understanding the evolutionary origins of novelty. Project 3 – The genomic signature of transgression This project is investigating the genomic signature of transgressive segregation in phenotypically extreme Saccharomyces hybrids. I made an interspecific hybrid cross between genetically highly divergent parents, and selected the F2 hybrids in 10 different, increasingly stressful environments. After selection took place, I sampled genotypes from along the environmental clines for next-generation sequencing. I have promising preliminary results, using a modified RAD-tag protocol, which I developed in collaboration with Dr. Duncan Greig and Dr. Arne Nolte from the Max-Planck Institute in Plön, Germany). This method allows for the detection of both genomic recombination and aneuploidy in a highly multiplexed assay that permits screening large numbers of samples at once. Genomic libraries will answer questions such as: Is recombination beneficial in novel environments? Does the frequency of aneuploids increase with stress? Do genetic ‘solutions’ to adaptation differ between environments? This is ongoing work which I aim at publishing within the next 6 months. Work completed by Dr Rike Stelkens, Marie Curie fellow, with Advisory Team Prof G Hurst (Liverpool) Prof. M Brockhurst (York) and Dr D Greig (Max Planck Institute for Evolutionary biology)
