Introgression in Betula Species of Different Ploidy Levelsand the Analysis of the Betula nana Genome
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Two of the most rapid drivers of evolution are hybridisation and polyploidisation. Hybridisation allows the rapid introduction of novel genetic material, potentially much faster than mutations, but this process is impeded by reproductive barriers between species. Differencesinploidylevelcanformsuchabarrier. Hybridisationaswellaspolyploidyareknown to occur frequently in the plant kingdom, including the genus Betula, which is investigated in this thesis. Three species of the Betula genus that exist in the United Kingdom are studied here: B. nana (dwarf birch), B. pendula (silver birch), and B. pubescens (downy birch). They differ in ploidy: B. nana and B. pendula are diploid and B. pubescens is a tetraploid. HybridisationandgeneﬂowbetweenthesethreespecieswasanalysedbyusingaRAD-seq dataset derived from 196 wild individuals. It was found that introgression acts unidirectionally from the diploid into the tetraploid species and that there is a cline of introgression between the north and south of the UK. This result suggests a range shift of the species from different distributions in the past. Gene ﬂow from B. nana to B. pubescens could be a neutral or even maladaptive consequence of their past species distributions. Alternatively, it could be an adaptive process: alleles from B. nana could be helping B. pubescens to adapt to harsher, more northerly populations. To gain a preliminary understanding of the possible effects of introgression, the loci in close linkage to RAD tags introgressed from B. nana into B. pubescens were investigated and their putative function inferred by comparing their homologs in related species. To enhance the analyses, a draft whole genome sequence assembly of a B. nana individual was improved with long read data generated by PacBio sequencing, as well as the addition of RNA-seq data. This produced a more contiguous and complete reference sequence, enabling a closer look at more genes in linkage to the RAD tags.
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