Molecular studies of the evolutionary relationships of Brachypodium (Poaceae).

The evolutionary relationships of the isolated genus Brachypodium (Poaceae) with genera in the Triticeae and with Festuca and Bromus have been investigated at different DNA levels, i.e. repetitive DNA, both nuclear and chloroplast RFLPs, 5S ribosomal DNA spacer, and ribosomal DNA repeat units. Brachypodium was found to have one of the smallest genomes known, with a very small amount of repetitive DNA and a largely undermethylated genome. A group of cloned repeated DNA probes was found to be genus-specific. The low/single copy DNA probes isolated from Brachypodium hybridised to a wide range of grass genera, and detected RFLPs among them. The RFLPs appeared mostly to be derived from deletion/insertion events. A chloroplast DNA probe was highly polymorphic both between species in Brachypodium and between different genera. The presence of a single copy of rpsl5 gene in Brachypodium was observed. Two 5S gene families were amplified via PCR. The smallest 5S spacer was only 150bp, which is the smallest one identified so far in the Poaceae. The ribosomal DNA in the genus was analysed and four restriction maps of rDNA were constructed. The degree of length variation of rDNA in Brachypodium was found to be low. The genome organisation and the structure of 5S DNA and rDNA are different from those of Triticeae. The evolutionary position of Brachypodium in relation to Triticeae, Festuca and Bromus was analysed by parsimony method using RFLP data and 5S spacer sequences. The most parsimonious trees showed that Brachypodium is well separated from Triticeae, Festuca and Bromus. The evidence from the work reported in this thesis is in favour of placing Brachypodium in a separate tribe instead of combining it within Triticeae or any other tribe. In addition, the structure and the distribution of 5S gene families and of rDNA has a clear taxonomic value within the genus Brachypodium. Finally, the relative taxonomic value of various components of DNA at different taxonomic levels using a range of molecular techniques is discussed.