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Navarro-Quezada, Aura R. (2007): Molecular evolution of tropinone-reductase-like and tau GST genes duplicated in tandem in Brassicaceae. Dissertation, LMU München: Faculty of Biology



Gene duplication is an opportunity for evolving new functions from the newer gene, but also has a disadvantage due to local gene-rearrangement effects and, if duplications are numerous, through alterations of genome size. Therefore, selection is playing a central role in determining the fate of a duplicate gene. Plants are known to harbor numerous gene families, and are thus an ideal system to test the fate of gene duplicates. This thesis tackles the tropinone-reductase like enzymes (further TRL) and the tau GSTs located upstream from this gene family. TRL enzymes are short-chain dehydrogenases that are involved in a reduction step downstream in the synthesis of tropane alkaloids in Solanaceae, important defense compounds of plants. The function of TRLs in Brassicaceae is not clear, since most of the plants in this family do not produce tropane alkaloids, but some have been associated with the oxidative-stress response. This gene family contains 80\% of its members duplicated in tandem in Arabidopsis thaliana. We profited from this fact to isolate 12 TRL (+ pseudogenes) from this species, further six species of Brassicaceae (A. thaliana, A. lyrata, A. cebennensis, Capsella rubella, Boechera divaricarpa and Brassica rapa), and one species from a closely related plant family, Cleome spinosa. We tested the role that selection plays in maintaining large numbers of this gene family. We used phylogenetic methods to analyze non-coding sequence evolution and identified regulatory motifs. We analyzed non-coding sequence evolution. Microarray expression data from A. thaliana and qPCR for A. thaliana and A. lyrata were analyzed to detect divergence in the expression patterns of orthologs and paralogs. TRL genes follow a gene birth and death dynamics. More probable, they originated from non-equal recombination of tandem duplicated genes. Positive selection at the origin of the duplicated genes allowed these to acquire differential expression patterns, leading to the preservation of numerous TRLs. The analysis of coding and non-coding sequences shows them to display correlated evolution, particularly in species recently separated by speciation. We further tested for selection on the tau glutathione-S-transferases (GST) enzymes, adjacent 3' in the genome to TRLs. Tau GSTs are unique to plants and are involved in detoxification. Multiple copies of these enzymes will allow flexibility in substrate specificity, which is important for the detoxification function. We detected positive selection among paralogs of tau GSTs supporting their potential of functional diversity, but we also detected negative selection among paralogs and groups of orthologs, indicating that more often their functions are conserved.