Hybridisation and phylogenomics of Betula L. (Betulaceae).

Abstract

Hybridisation and polyploidy are important in the evolution of species. The genus Betula L. is an ideal model to study these processes as species of this genus hybridise frequently and polyploid species are common. In this thesis, I investigated the hybridisation of three Betula species in Britain; the phylogenetic relationships and genome size of all known Betula species, and conducted phylogenomic analysis of diploid Betula species. A cline of introgression of microsatellite marker alleles from B. nana was found extending into B. pubescens populations far to the south of the current B. nana range in Britain. We suggest that this genetic pattern is a footprint of a historical decline and/or northwards shift in the range of B. nana populations due to climate warming in the Holocene. The Atkinson Discriminant Function (ADF) is a leaf-morphology metric to distinguish between B. pubescens and B. pendula. We test it on 944 trees sampled across Britain against species‟ discriminations made using 12 microsatellite loci and found that the accuracy of the ADF can be raised to 97.5% by using an ADF of -2 rather than zero as the boundary line between the species. The taxonomy of the genus Betula is debated and a new classification has been proposed in a recent monograph. We evaluated it using ITS and restriction site associated DNA sequencing (RADSeq). The result based on ITS largely agrees with species classification in the recent monograph but with uncertainties. Phylogenomic analysis of 587 RAD loci for Betula diploid species using coalescence-based methods, a concatenation method and binary presence/absence of RAD loci produced wellresolved trees with similar topology. Based on these analyses, we propose a new classification of Betula into four subgenera and seven sections. Further research is needed to infer the parental origins of polyploid species within Betula.