| Summary: | The work presented here investigates two basic properties of mutation rates in the unicellular chlorophyte Chlamydomonas reinhardtii. The first chapter is devoted to an investigation of the mutational heritability $ rm (V sb{M})$ of fitness in asexually propagated populations. This is the rate at which novel variation for fitness accumulates in a population. In two trials, values of $ rm V sb{M}$ = 4.5 and $4.7 times 10 sp{-3}$ of the environmental variance $ rm (V sb{E})$ were obtained. These values were at least an order of magnitude greater than estimates from other organisms of $ rm V sb{M}/V sb{E}$ for fitness or for quasineutral variation. The possibility that this was due to disruptive selection for types specialized for different parts of the culturing environment was investigated, and rejected. Other possible explanations, and future avenues for research, are discussed. === The second chapter extends the investigation from normal culturing conditions into stressful ones. Specifically, it considers the hypothesis that C. reinhardtii might increase its mutation rate as a general response to environmental stress. Stressed lines were found to display reduced mean fitness and an increased variance of fitness after being returned to normal culturing conditions. This was interpreted as evidence for increased mutation rates in treated lines relative to controls. Possible mechanisms underlying this phenomenon are discussed, along with suggestions for further research.
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