Oceanic archipelagos are famous for their rich endemic biota and island lineages have played a central role in biogeography. Macaronesia is a region of archipelagos in the Atlantic Ocean and 22% of native flowering plants are endemic to the region. Diversity in Macaronesia is unevenly distributed. Darwin first recognized the paucity of richness and Single Island Endemics (SIBs) in the Azores compared to other archipelagos and this pattern is now known as the' Azores Diversity Enigma'. This PhD explores hypotheses to explain the 'Enigma' related to taxonomy and diversity patterns. Analyses of herbarium collections of endemic plants suggest that our current understanding of the Azores endemic flora may be insufficient to establish a robust taxonomic framework. The taxonomically complex genus Pericallis (Asteraceae) exhibits a diversity pattern that reflects the' Enigma', it therefore provides an ideal focus for testing hypotheses to explain the 'Enigma'. A dated phylogenetic analysis of Pericallis is established. Comparisons of diversity and evolutionary processes of Pericallis in both the Azores and Canaries enable two key hypotheses for the 'Enigma' to be tested: (i) taxonomic artefact and (ii) lack of ecological diversity in the Azores. Taxon delimitation issues are also resolved. The study umavels a complex evolutionary history involving both habitat and geographic shifts. Furthermore, in both the Canaries and Azores much ofthe diversity is recent (ca. 1.7 Myr). Analyses of Pericallis suggest that taxonomic artefact does not satisfactorily explain the 'Enigma'. Ecological diversification has played a significant role in both archipelagos; therefore hypothesis (ii) can be rejected. Novel molecular and morphological diversity patterns are revealed. One new species is described in Madeira and four new subspecies are recognized in the Canaries. The results of this PhD reveal novel patterns of diversity and evolution that are significant for exploring and challenging the assumptions of island biogeography studies.

The sensing and integration of environmental cues is of crucial significance to plants. Particularly important for these photoautotrophic organisms is the use of light as an informational signal. The presence of potentially-shading neighbours can be perceived by plants using specialised photoreceptors. This activates a switch to a developmental programme characterised by stem elongation growth and early flowering. Collectively, these responses are known as the 'shade-avoidance response'. One of the key triggers of shade-avoidance is a reduction in ratio of red: far-red light (R: FR) perceived by the plant. At a molecular level, low R: FR light promotes the stabilisation and activation of a class of transcription factors termed PHYTOCHROME INTERACTING FACTORS (PIFs). These promote the expression of auxin biosynthesis genes which then leads to stem elongation growth. In this thesis I show that UV-B, sensed by the newly characterised UV-B photoreceptor UV-RESISTANCE LOCUS 8 (UVRB), provides a strong brake on low R: FR-induced shade-avoidance across the entire Arabidopsis life cycle. This is achieved by a dual mechanism which results in both the inactivation of PIFs, potentially through competitive binding by DELLA proteins, and a direct destabilisation of PIF proteins. UV-B thus inhibits the upregulation of auxin activity usually induced by low R: FR conditions, thereby limiting stem elongation.

No description available.

Compositae plants of the genus Erigeron have ca. 390 species, widely distributed in most temperate regions around the world. Erigeron anl7UUS (L.) Pers. (fleabane) is one of the most valuable plants in this genus, used in Chinese foll< medicine to treat indigestion, malaria, enteritis, hepatitis and hematuria increasingly since the 1970s. However, it is not an indigenous species in China and has not been officially recorded in the Chinese Pharmacopeia. Very little research has been published on its biological activity and no activity against MRSA has been reported . In this research, whole plant material was collected in Shanghai, China and its chemical composition , antibacterial activity, DNA gyrase inhibitory activity and mutagenicity assessment were evaluated on isolated compounds and extracts.

High levels of inbreeding are expected to cause a strong reduction in levels of genetic variability, effective recombination rates and in adaptation compared with related outcrossing populations. The evolution of mating systems can thus have profound effects on the evolution of genome structure and diversity. In this thesis I test these predictions, using the plant genus Arabidopsis as a model system. I examine patterns of genome organisation, DNA sequence polymorphism and divergence in the highly self-fertilizing Arabidopsis thaliana, and compare them to those of its self-incompatible, outcrossing relative Arabidopsis lyrata. From comparisons of rates of substitution, there is no evidence for a higher rate of amino acid substitution in A. thaliana, suggesting that slightly deleterious amino acid mutations may not be the primary source of protein evolution in these species. In contrast with results from published data, analysis including polymorphism data also reveals no difference in the ratio of nonsynonymous to synonymous polymorphism between species, suggesting that there may not be a general elevation of amino acid polymorphism in A. thaliana. Comparisons of intron length reveal evidence for consistently smaller introns in A. thaliana, perhaps reflecting the action of directional selection on noncoding DNA length in an annual plant. Analysis of codon usage bias at orthologous loci shows evidence for consistently higher GC content at third codon positions in A. lyrata. Comparisons of base composition in introns and polymorphism patterns for preferred and unpreferred synonymous mutations show no evidence for a shift in mutation pattern or rates of biased gene conversion between species, suggesting that the difference in codon bias might reflect a relaxation of natural selection in A. thaliana. However, comparisons of codon usage between species using a measure of codon bias based on relative abundance of tRNA genes reveals no significant difference between species, and there is no evidence for a difference in the relative rates of preferred and unpreferred substitutions. As there is a good correlation between the frequency of preferred codons defined by tRNA abundance and levels of gene expression, these results suggest a neutral explanation for the difference in GC content. Comparisons of multilocus neutral variability between A. thaliana and A. lyrata show the expected decrease in average within-population diversity in A. thaliana, but this is complicated by strong geographic structuring of variability in A. lyrata, probably associated with recent demographic history. Consistent with an influence of demographic history, analysis of intralocus linkage disequilibrium suggests a strong deficiency of the effective rate of recombination in A. lyrata. In contrast, A. thaliana shows approximately the amount of linkage disequilibrium expected in a highly self-fertilising species. These results suggest a possible role for population admixture in northern, postglacial populations of A. lyrata. A whole-genome analysis of transposable elements in A. thaliana indicates that recombination rate heterogeneity does not play an important role in driving their distribution in this species, and that gene density is the primary determinant of TE abundance in the genome. Combined with evidence from other complete eukaryotic genomes, this pattern is consistent with a role of inbreeding in reducing effective rates of recombination genome-wide, and thereby reducing the effect of recombination rate heterogeneity on genome structure. Polymorphism analysis of 18 foci located in regions of contrasting recombination rate in an Icelandic population of Arabidopsis lyrata reveals no evidence for the expected positive correlation between recombination rate and nucleotide diversity. A maximum likelihood analysis of polymorphism and divergence for these data shows evidence for significantly elevated diversity compared with divergence at a centromeric locus, suggesting the possibility of balancing selection in this region. Alternatively, recent demographic history may have contributed to an uncoupling of the expected relationship between recombination and variability, and an inflation of the variance in diversity across loci. The results of this thesis provide some evidence for the evolution of genome structure between related Arabidopsis species, but no strong evidence for differences in the efficacy of natural selection. These results emphasize the importance of understanding the influence of population history on the action of natural selection at the molecular level.

No description available.

A major focus of research regarding non-native invasive plants is to understand what ecological variables (abiotic and biotic) increase or decrease the likelihood that a non-native plant species will become invasive, and to determine whether invasion has a negative impact on native plant species. Crassula helmsii is a plant indigenous to Australasia, which has invaded wetland habitats across much of Britain. It is a low growing, clonally spreading plant, which occupies the margins and shallows of freshwater and brackish waterbodies. Crassula helmsii is considered invasive due to its ability to produce vigorous aboveground growth, which in some locations spreads to form dense monospecific mats of vegetation. There are concerns that C. helmsii can exclude native plant species, and therefore that its invasion may have a negative impact on biodiversity. The research presented in this thesis was conducted with the aim of better understanding the mechanisms which allow C. helmsii to develop dense, dominating vegetative mats. This aim was addressed by investigating how C. helmsii abundance was related to variation in the abiotic environment, how C. helmsii abundance was affected by generalist herbivores, and whether C. helmsii abundance was related to the presence or absence of co-occurring plant competitors. To investigate how C. helmsii abundance is related to the abiotic environment, firstly field data were collected of pH, nutrient levels, water depth, and shade levels along C. helmsii invaded margins of waterbodies. Multiple linear regression was used to address the question: How well does abiotic variation predict variation in C. helmsii abundance in the field? Finding that C. helmsii cover was significantly greater at a higher pH and at lower shade levels. Secondly C. helmsii was grown in mesocosms at different strengths of salinity (0, 2, 4, 8 ppt) to address the question: What is the estimated minimum salinity level for the control of C. helmsii in brackish water habitats? Relative growth rate decreased significantly with increasing salinity and C. helmsii died in the highest salinity treatments (8 ppt). These two experiments demonstrated that C. helmsii abundance can be affected by the abiotic environment, and specifically by pH, shade levels, and salinity. Dense, dominating mats of C. helmsii are more likely to occur in freshwater conditions, which have a neutral to alkaline pH, and with minimal overhead shading. More effective control against C. helmsii dominance may be achieved by preventing C. helmsii from colonising sites with such abiotic conditions, or by focusing more effort on removal of C. helmsii vegetation which has colonised such sites. To investigate the effect of disturbance from generalist herbivores, a field-based experiment was set up, in which 4 m2 fenced plots which excluded large vertebrates were erected along a C. helmsii invaded drawdown zone. The abundances of all plants species within the fenced plots and within adjacent unfenced plots were recorded repeatedly over a year, and compared, in order to explore the question: What is the effect of grazing on the abundance of C. helmsii within a wetland plant community? Crassula helmsii declined in abundance in the fenced plots compared to the unfenced plots, and the abundance of co-occurring vegetation was consistently higher in the fenced plots than in the unfenced plots. Crassula helmsii appeared to benefit from grazing disturbance, attaining a higher abundance in the unfenced plots, where competitor plant species had been grazed out. Crassula helmsii could have a negative impact on native plant species diversity, if it is able to fill a niche usually occupied by native plant species of open, grazed drawdown zones (e.g. Teucrium scordium). The relationship between C. helmsii abundance and the presence or absence of co-occurring plants was investigated in order to ascertain the effect of competition on the abundance of C. helmsii, and also to ascertain the effect of competition from C. helmsii on the abundance and diversity of co-occurring plants, therefore giving an indication of whether the spread of C. helmsii can have a negative impact on native species. Firstly a common garden experiment and a field based experiment were separately conducted, both assessing how well C. helmsii could spread into available bare ground compared to native competitors, and addressing the question: Is available bare ground an important pre-requisite to high C. helmsii abundance? These experiments found that C. helmsii can rapidly respond to available bare ground, but that simultaneous spread of native competitors might limit C. helmsii community dominance. The effect of C. helmsii on native plants was species specific; the common garden experiment showed that C. helmsii limited the spread of Anagallis tenella, but not Hypericum elodes or Hydrocotyle vulgaris. Secondly field survey data of plant species abundance and diversity were collected to explore the question: Can the invasion of C. helmsii negatively affect the plant community in a Phragmites australis dominated fen habitat? Survey data were analysed to determine whether C. helmsii was negatively correlated with variables of the plant community, a pattern indicating that C. helmsii could increase in abundance by displacing native plants. The variables ‘total community abundance’ and ‘species diversity’ were not significantly correlated with C. helmsii. At the single species level, C. helmsii and the dominant plant species P. australis showed a high capacity to coexist, although C. helmsii was most abundant where P. australis was absent. Based on these three experiments, evidence was not found to suggest that C. helmsii has a negative impact through direct displacement of native plants. However, because dominating C. helmsii growth may be more likely to occur in an area with ample bare ground into which this species can spread, and where tall competitive plant species (e.g. P. australis) are in low abundance, management activities designed to increase species diversity by creating patches of open vegetation structure, might instead just encourage a higher abundance and dominance of C. helmsii.

No description available.

No description available.

No description available.