| Summary: | St Helena is an isolated volcanic island (lat. 15° 56'S, long. 5°42'W) in the South Atlantic Ocean. The endemic flora of St Helena, comprised of 49 plant species, is considered to be one of the most threatened in the world. This thesis investigates the evolution and conservation of two threatened groups of plants endemic to St Helena: i) trees in the genera Commidendrum and Melanodehdron (Asteraceae) and ii) epiphytic and terrestrial elaphoglossoid ferns in the genera Elaphoglossum and Microstaphyla (Lomariopsidaceae)Chapter two investigates species relationships of Commidendrum and Melanodendron using the ITS region of ribosomal DNA. Despite showing a range of morphological and ecological variation the four species of Commidendrum form a closely related monophyletic group. Melanodendron integrfolium is sister to Commidendrum indicating that the two genera evolved from a common ancestor which arrived to St Helena via a single dispersal event. The role of heterochrony in the evolution of Commidendrum is discussed. Chapter three investigates self-incompatibility and hybridisation in two of the most threatened Commidendrum species, C. rotundfolium and C. spurium. RAPD data indicated the presence of hybrids in the seed orchards of C. rotundfolium and C. spurium. Self-incompatibility in C. rotundfolium and C. spurium was investigated using a series of pollination experiments which examined pollen-stigma interaction at the stigma interface. Both C. rotundifolium and C. spurium possess a sporophytic self-incompatibility system, and poor seed viability in C. rotundfolium is due to a paucity of S-alleles. The conservations implications of this and interspecific hybridisation are discussed. Chapter four investigates the evolutionary relationships of the four elaphoglossid ferns, E. dimorphum, E. nervosum, E. conforme and M furcata from St Helena using sequences of the chloroplast trnL intron (partial) and trnL-F intergenic spacer. The investigation revealed the close relationship of E. nervosum, E. dimorphum and Mi furcata, whilst E. conforme was found to be distantly related. Microstaphyla furcata is shown to belong to Elaphoglossum confirming the previous transfer of this species to Elaphoglossum bfurcatum. Species relationships of the endemic Elaphoglossum, and the extent and distribution of population genetic diversity were investigated using allozyme analysis in chapter five. As well as supporting the relationships of the taxa in the molecular phylogeny, the allozyme data suggest a hybrid origin of E. dimorphum between E. nervosum and E. bfurcatum. In addition the allozyme data revealed significant genetic differentiation in populations of E. nervosum and E. bfurcatum which should be taken into consideration in any future conservation programme. To conclude, Chapter 5 is a general discussion on the evolution and conservation of island plants, highlighting my research findings from St Helena and comparing it to other studies.
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