Evolutionary history and chloroplast DNA variation in three plant genera: Betula, Corylus and Salix. : The impact of post-glacial colonisation and hybridisation.

The great difference in the level of chloroplast variation and its geographic structure among the three main species studied here demonstrates that forest species do not form a homogeneous group. Hazel shows a genetic structure similar to many other thermophilous species and this structure, in combination with fossil evidence, indicates that the post-glacial colonisation of most of Europe originated in a refugium in western France while the Balkan and Italy were colonised from a south-eastern refugium. In sallow and silver birch the chloroplast DNA variation and its structure does not fit with a scenario of glacial restriction to southern refugia and survival at intermediate latitudes is suggested for both species. The chloroplast DNA variation in silver birch suggests the presence of one western and one eastern European post-glacial colonisation route and limited contribution of southern populations in the colonisation of the rest of Europe. Unique haplotypes by the Ural Mountains indicates the possibility of a separate glacial origin of these populations. The study of chloroplast DNA in species closely related to sallow and silver birch indicate that extensive hybridisation and cytoplasmic gene flow occurs within both the Salix and Betula genera in Europe. The nuclear and chloroplast phylogenies of 14 Betula species were not in complete agreement with each other or with the classical division of the Betula genus into subgenera or sections. The phylogenetic structure implies that hybridisation has played a role in the evolution of the Betula genus. This thesis focuses on the chloroplast DNA variation in three forest tree genera: Corylus, Betula and Salix. Chloroplast PCR-RFLP is used to evaluate the post-glacial history of hazel, Corylus avellana, silver birch, Betula pendula and sallow, Salix caprea and to explore the possibility of introgression in the Salix and Betula genera. In addition, the chloroplast matK gene, its flanking regions and the nuclear ADH gene were used to study the phylogenetic relationships within the Betula genus