This thesis questions whether sympatric divergence, brought about by disruptive selection in a spatially heterogeneous environment, can occur under natural conditions and in the laboratory. I. An investigation, to detect micro-differentiation, was made on a Plantago lanceolata population, comprising a gradation of phenotypes and occurring in a small area (lx 5m) of dune pasture, heterogeneous by virtue of different vegetation heights. The prostrate phenotype with shorter leaves and inflorescences was correlated with low vegetation} the erect phenotype was correlated with taller vegetation. Vegetative propagules in standard conditions of garden and greenhouse showed persistent genotype differences of growth habit, leaf length and inflorescence length. Growth habit and leaf length also correlated with the original environment, indicating adaptive micro-differentiation. Some phenotypic plasticity was apparent. Investigation of the field population revealed flowering time differences between the phenotypes and rapid turnover of individuals less than six months old, particularly in low vegetation where the Plantago population was the most dense. Selection pressures appeared to be operating to maintain differentiation within this heterogeneous environment. Population control was evident, with mortality matching recruitment, but the chances of survival of an individual were independent of the season of establishment. II. In a second series of experiments, a population of randomly- mating Drosophila melanogaster was maintained for 20 generations in small 'population cages', heterogeneous because they contained two types of food medium, viz. normal food and normal food plus peppermint essence. The founder population yielded 40£ more progeny on the normal food. There were three control populations feeding on (l) normal food only, (2) peppermint food only, (3) homogeneous half-strength peppermint only. The experimental population initially responded to the heterogeneous environment (with its choice of food media), by yielding numbers of progeny and biomass in excess of expectation, which was calculated from the controls. This was thought to be an environmental response. The difference between the observed yield and expectation increased steadily for 10 generations, implying adaptation to the heterogeneity, but, after 17 generations of selection, the yield was significantly less than expectation. This persisted for a generation of lapsed selection on normal food, indicating a genetic response to some factor within the heterogeneous environment. Because females reared on peppermint showed a behavioural change and tended to choose this less palatable medium on which to lay their eggs, it is suggested that a genetic component of behavioural flexibility contributed to this result. There was also evidence of improved adaptation to normal food, possibly a genetic response to highly competitive conditions on this densely- populated medium. Although sympatric divergence was not conclusively demonstrated, a measure of habitat selection for egg-laying sites developed and the population became non-random. Peppermint retards the life-cycle of the flies living on it by approximately one day. Therefore, the heterogeneous population was experiencing conditions which might promote assortative mating. It was concluded from the two experiments, that a heterogeneous environment may act disruptively on a small, randomly-breeding population within a small area. The Plantago population, in an environment where selection pressures were probably high, showed evidence of micro-differentiation, indicating that sympatric divergence had occurred, although phenotypic plasticity was also evident in some morphological characters. The Drosophila population, in a heterogeneous environment where selection pressures may have been relatively low, also became non-random and evolved habitat-choice. In both investigations, forces enhancing assortative mating, helping to maintain genetic variation by reducing gene flow, were apparent. Therefore, it is concluded that sympatric divergence may be brought about by disruptive selection in a heterogeneous environment.

published_or_final_version / Dentistry / Doctoral / Doctor of Philosophy

This study of laboratory based instruction at higher secondary school level was an attempt to gain some insight into the effectiveness of three laboratory instruction methods: cooperative group instruction method, individualised instruction method and lecture demonstration method on biology achievement and retention. A Randomised subjects, Pre-test Post-test Comparative Methods Design was applied. Three groups of students from a year 11 class in Pakistan conducted experiments using the different laboratory instruction methods. Pre-tests, achievement tests after the experiments and retention tests one month later were administered. Results showed no significant difference between the groups on total achievement and retention, nor was there any significant difference on knowledge and comprehension test scores or skills performance. Future research investigating a similar problem is suggested / Master of Education (Hons)

This study of laboratory based instruction at higher secondary school level was an attempt to gain some insight into the effectiveness of three laboratory instruction methods: cooperative group instruction method, individualised instruction method and lecture demonstration method on biology achievement and retention. A Randomised subjects, Pre-test Post-test Comparative Methods Design was applied. Three groups of students from a year 11 class in Pakistan conducted experiments using the different laboratory instruction methods. Pre-tests, achievement tests after the experiments and retention tests one month later were administered. Results showed no significant difference between the groups on total achievement and retention, nor was there any significant difference on knowledge and comprehension test scores or skills performance. Future research investigating a similar problem is suggested / Master of Education (Hons)

A review of the literature established that there were areas of cytokinin transport which needed further investigation, in order to determine the function and/or mode of action of cytokinins during certain stages of plant development. Radioactive zeatin was applied to plant systems suitable for determining more about specific problems of cytokinin transport. The metabolism of the radioactive zeatin was monitored in relation to transport. The metabolism in, and possible export of, radioactive zeatin out of immature, mature and senescing Ginkgo biloba leaves was monitored using explants. The results showed that approximately the same percentage radioactivity was exported from the leaves at all three stages of their development. This indicates that these deciduous leaves could potentially export cytokinins, but the results were not regarded as being significant. Cytokinins would not have been expected to be transported out of expanding leaves, which rapidly utilize cytokinins, and in comparison greater cytokinin export would have been expected to occur from senescing leaves, but this did not occur. The resuIts could indicate that cytokinin glucosides in deciduous leaves are primarily inactivation products rather than storage compounds. The metabolism and transport of radioactive zeatin, applied to the leaves of Citrus sinesis trees, was monitored during the flush of new growth following a dormant period. Some of the radioactive zeatin applied to these leaves appeared to be utilized in the new shoot growth. This could imply that accumulated cytokinin glucosides in these evergreen leaves are exported out of the leaves and reutilized, and thus fulfil a storage function; although the extent to which export occurred as opposed to interconversion and/or catabolic metabolism in the leaves could not be determined. The distribution of radioactive zeatin applied to the xylem and phloem of Phaseolus vulgaris plants at three stages of development was also monitored. There appeared to be preferential transport of cytokinins in the transpiration stream, that is, applied to the xylem. The vegetative apices, buds and flowers appeared to be the major sinks for radioactive compounds in the vegetative and flowering plants, and the leaves were the most important sink in the fruiting plants. Radioactive zeatin in the phloem appeared to move passively with the assimilate stream. The transport of radioactive zeatin applied to the primary leaves appeared to confirm that cytokinins in the phloem were transported along with the assimilate stream. The results also indicated that the accumulated cytokinin glucosides in these annual leaves were primarily inactivation products, involved in regulating leaf metabolism, although some re-utilization of these cytokinins could potentially take place. could have reflected normal cytokinin transport occurring in the tissues. The main metabolic pathway, oxidation, did not, however, appear to be the main metabolic pathway in the plant tissues and may have been the result of the exogenous application of zeatin. The fact that metabolism did not appear to be the normal metabolism of the tissues, could potentially have affected transport. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1982.

Calcium is a ubiquitous intracellular signal responsible for controlling numerous cellular responses including development and proliferation. Calcium (Ca²⁺) is stored in both neutral and acidic stores and its release through gated channels has been implicated in regulating development in Dictyostelium discoideum. This thesis aims to understand the roles of the calcium channel proteins, in particular the two-pore channel proteins (TPCs), found on acidic stores in Ca²⁺ signalling during Dictyostelium development. Bioinformatic analysis indicates conservation of a gene encoding an orthologue of TPC2 in Dictyostelia and, similar to plant TPCs, a Ca²⁺ sensing domain is predicted along with a novel potential calmodulin binding site. To investigate the role of intracellular Ca²⁺ channels, a series of strains was generated, disrupted in one or more of genes encoding the channels TPC2 and mucolipin (TRP-ML), predicted to be located on acidic stores, and IplA, located on neutral stores. All disrupted strains, including one lacking all three channels, are able to complete development. However, strains lacking TPC2 show a pronounced delay in early development, correlating with reduced expression of some early developmental genes. Vesicles derived from tpc2-null cells show normal Ca²⁺ release compared to parental cells but an increased rate of Ca²⁺ uptake. During early development, the pH of acidic vesicles is increased in the absence of TPC2. However development of tpc2-null cells showed increased sensitivity to weak bases in producing fewer aggregates but resistance to sodium chloride and weak bases in later development suggesting a complex role for TPC during development. In vivo cytosolic Ca²⁺ responses were analysed in strains expressing an ultra-sensitive Ca²⁺ indicator YC-Nano 15. Growing tpc2^- and iplA^- cells have lower basal cytosolic Ca²⁺ than parental Ax2 cells. Intercellular Ca²⁺ waves were observed in aggregates from Ax2, mcln^- and tpc2^- cells but were greatly reduced in iplA- aggregates, as was the increase in cytosolic Ca²⁺ in response to extracellular cAMP. In tpc2- aggregates, wave frequencies were reduced and the response to cAMP addition abolished after treatment with caffeine, a known adenylyl cyclase inhibitor in Dictyostelium. This work demonstrates that TPC2 plays a role in the early stages of Dictyostelium development. TPC2 is important for pH regulation in acidic vesicles and cytosolic Ca²⁺ levels, either or both of which could influence development either directly or via changes in early developmental gene expression.