Genomic patterns of introgression between hybridising birch species due to range shifts caused by climate change in the Scottish Highlands

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Scientists are certain the global warming is occurring at an alarming rate, but we do not yet fully understand the effects this will have on the living world. There is good evidence that many natural species are gradually moving the ranges in which they live, to stay in environmental conditions to which they are adapted. For many species in the UK, this means moving northwards, or up mountains. One consequence of this is that some species are forced into close contact with their relatives. This arrangement can work well if they are unable to have sex together, and the two species can often coexist in the same area. But where sex is possible, things get complicated. Sexual reproduction between two species is called hybridisation, and can result in the two species merging into a single new species, or one species eliminating the other. Which of these outcomes occurs depends on many factors, including how fit the two species are relative to each other and how successful the hybrid offspring are. This project plans to investigate this in two birch tree species growing in the UK. Downy birch is common and currently expanding its range up mountains where it meets its relative, dwarf birch, which is nationally scarce. The two species can hybridise together even when they are far apart, because they are wind pollinated. They produce hybrids that grow up and hybridise with their parents. As downy birches advance up the mountains, the dwarf birches have no where left to go. We need to know how long this has been happening in the past and how quickly it is happening in the present. We can get a window on this from historical records, but these can often be patchy. Another way is to look at the DNA of the two species and look for signs of past hybridisation. Because the species hybridise so much, bits of the dwarf birch genome will be found inside the downy birch genome wherever the two species have met, even if it was a long time ago. This means that we can discover where the two species first met, and how far dwarf birch has been pushed back by downy birch. For this to work, we have to look at very many sections of DNA, all over the genomes of these two wild tree species. Until recently, this would have been impossible, but new technology lets us do this relatively easily and cheaply. We can read many parts of the DNA sequences of the two species and discover where they differ, then go back and look for these differences in areas where they have hybridised. For every tree we look at, we can find which sections of the genome are from downy birch and which are from dwarf birch. Looking more closely at that data, we can use mathematics to map which sections of the genome are moving around and which are not. In another approach to this, we can use our DNA sequence data to design sticky tags that are targeted to one species or the other. When we attach these tags to dyes, we can 'paint' the genomes of individual plants and use microscopes to look at the mixtures of the two species that they contain. Then we can look even closer, and investigate the function of genes in the sections from different species. We expect different sections of the genomes to behave in different ways, due to the genes they contain. Some sections will be so integral to a species' identity that they cannot part with them. Others will be selected by the environment and move to wherever they are needed, regardless of the species. Still others will be neutral, and drift around at random; these are the best markers of historical hybridisation and species range movements. Scientists have only just started to be able to work on the question of which parts of genomes can move freely between species, even though we have been interested in it for a long time. So this work will make an important contribution to our knowledge of how species evolve and remain separate, as well as helping us to understand how global warming will affect rare species.