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The evolutionary history of Cochlearia L. : Cytogenetics, phylogenomics and metabolomics of a cold relic in a warming world

Wolf, Eva Maria

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The cold-adapted genus Cochlearia L. (scurvy grass), a young polyploid complex within the Brassicaceae family (Cruciferae), displays a range of highly interesting cytogenetic and ecotypic characteristics and might serve as an excellent model system to study general evolutionary mechanisms such as polyploidization, hybridization, or cold and edaphic adaptation. The presented study provides, for the first time, comprehensive cytogenetic and highly-resolving phylogenomic analyses, and first metabolomic insights into the Cochlearia cold response. Thus, the findings presented herein might constitute a good starting point for further in-depth analyses of said evolutionary aspects e.g. based on population-genomic datasets. In chapter 1, the cytogenetic evolution within the genus Cochlearia is analyzed via both conventional chromosome counts and flow cytometry measurements. Based on a comprehensive literature review on published chromosome counts, the geographical distribution of cytogenetic variability is described, suggesting an early evolutionary separation of the two diploid karyotypes (2n=12 and 2n=14). The high frequency of aberrant chromosome numbers in polyploid taxa is interpreted as a result of frequent interploidal hybridization, given the near absence of interspecific fertility barriers, and thus reflecting the dynamics of polyploid evolution within the genus Cochlearia. Moreover, a correlation between genome size and chromosome number, as well as genome downsizing in polyploid taxa are revealed. Chapter 2 provides comprehensive phylogenomic analyses based on Illumina high-throughput sequencing data. Chloroplast and mitochondrial phylogenies are largely in congruence and indicate a glacial survival of the whole genus in arctic refuge areas as well as repeated adaptation to alpine habitats in Central Europe. Divergence time estimates, based on complete chloroplast genomes, imply a diversification of the whole genus over the course of several Pleistocene glaciations within the last ~700,000 years. Results from nuclear data analyses support both the clear evolutionary separation of the two diploid karyotypes as described in chapter 1, as well as the basal phylogenetic position of arctic taxa as shown in organellar phylogenies, and they reveal new insights into the evolutionary origins of the different polyploid taxa. In chapter 3, the metabolomic analysis of the Cochlearia cold response via metabolite profiling using gas chromatography-mass spectrometry (GC-MS) is described. Based on temperature-related bioclimatic variables (WorldClim), four bioclimatic ecotypes are defined and utilized as group priors for statistical analyses. All studied plants/ecotypes show strong metabolomic adjustments in reaction to 20 days of cold treatment under 5°C, especially with regard to increased levels of soluble carbohydrates and amino acids. Statistical analyses do not show a strong discrimination of the four bioclimatic ecotypes based on the analyzed metabolites, yet slight intrageneric variation among the bioclimatic clusters is described, implying similarities in the cold response between arctic and alpine taxa.

Item Type: Dissertation
Supervisor: Koch, Prof. Dr. Marcus A.
Date of thesis defense: 30 August 2017
Date Deposited: 23 Nov 2017 11:07
Date: 2017
Faculties / Institutes: The Faculty of Bio Sciences > Dean's Office of the Faculty of Bio Sciences
Subjects: 570 Life sciences
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