Browsing by Author "Johnson, Steven D"

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    Pollination and the evolution of floral traits : selected studies in the Cape flora
    (1994) Johnson, Steven D; Bond, William J; Steiner, Kim
    The study of plant pollination mechanisms has produced some of the most convincing examples of natural selection and adaptation. The aims of this thesis were to determine the role of pollinators in the evolution of floral traits in selected Cape plants, and to reach a better understanding of the relationship between floral adaptation and speciation. To establish a set of testable hypotheses, I asked specifically how adaptation to pollinators can explain three striking patterns in the Cape flora, namely (1) convergence in floral form between unrelated lineages (2) floral mimicry and (3) adaptive radiation within genera.
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    The reproductive biology of four geoflorous Protea species (Proteaceae)
    (2014) Zoeller, Kim Christie; Midgley, Jeremy J; Steenhuisen, Sandy-Lynn; Johnson, Steven D
    Pollination systems of previously unstudied plant species are often inferred from floral traits that are shared among unrelated plant taxa. However, these ‘pollination syndromes’ are often not reliable predictors of the primary pollinator of a plant species, and may in fact represent adaptations to multiple pollinator groups or even minor pollinators. As such, conducting comprehensive field-based research into the pollination and breeding system of unstudied plant species is important for furthering our understanding of evolutionary processes underlying shifts in pollination systems within plant lineages. Protea cordata, P. decurrens, P. scabra and P. subulifolia are cryptic, geoflorous shrubs that are predicted to be pollinated by non-flying mammals based on their floral traits, although there has been no experimental evidence in support of this. The aims of this study were to quantify key functional traits that mediate interactions between these four Protea species and their pollinators, as well as to experimentally determine their pollination and breeding systems. We further aim to establish the main vertebrate pollinators of the study species, their relative rates of visitation and their temporal patterns of activity using remote camera footage. Rodents were found to be the most frequent visitors to the flower heads of the study species. Floral morphology, nectar volume and sugar concentration of the study species conform to the general syndrome of non-flying mammal pollination system in this genus. In particular, the short (14-20 mm) distance between pollen and nectar ensures pollen deposition on the body of rodent foragers. Excluding small mammals from flower heads did not significantly reduced seed set, suggesting that insects can play an important role in cross-pollination in the study species. Breeding system experiments revealed that Protea cordata and P. scabra are self- incompatible, and are reliant on animal vectors for successful seed set. Supplemental hand pollination on P. cordata and P. scabra indicated that resource limitation is prevalent among these plant species. This study adds to the growing knowledge of the reproductive biology of Protea and will improve our ability to reconstruct the history of pollinator shifts in the genus.
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    Variation in breeding systems and consequences for reproductive traits in Erica
    (2015) Arendse, Brittany; Midgley, Jeremy J; Johnson, Steven D
    Erica makes up 7% of all species in the Cape flora. It is the most species-rich genus in the Cape Floristic Region (CFR), which has an area 0f 90 000km². Erica species have great inter- and intra-specific variation in floral form as well as in post-fire regeneration strategies. Previous studies of other plant groups (Barrett et al. 1996; Button et al. 2012) have illustrated changes in floral traits with a shift from outcrossing to selfing. The aim of this thesis was to determine whether similar changes occur in Erica. I thus analysed breeding systems in Erica in relation to floral traits, pollinators, and fire survivals strategies. Seed and fruit set data obtained from hand-pollination treatments suggested that self- fertilization occurs rarely. Autonomous selfing did not generally yield significant seed set. When comparing changes in floral traits with increased selfing ability and pollination syndromes weak relationships were observed. Average plant height showed a positive relationship with selfing ability but this correlation was not significant. Comparing traits within small and large flowered species separately yielded significant relationships between corolla size and selfing ability of small-flowered species.. Furthermore, general trends indicate that small-flowered species have increased ability to self-fertilise when corolla sizes are larger, corolla apertures larger and herkogamy reduced. Large-flowered species employ the same strategy but average corolla aperture is reduced. A trade-off between the size and the number of flowers was seen within the Erica genus. Small-flowered species had significantly more flowers compared to larger- flowered species that had markedly fewer flowers per unit height. This finding has implications for the selfing potential of small-flowered species as increased self- incompatibility may have evolved in order to reduce the effects of increased geitonogamy due to increased floral number. The prediction that self-fertilisation would be increased in seeders compared to resprouters, on the basis that seeders are more reliant on seeds for persistence than resprouters, was not realised when comparing the selfing ability of different fire- survival strategies. Erica mammosa, a species with morphs possessing both fire-survival strategies, shows no significant differences in selfing ability, this includes differences in pollen-ovule (P/O) ratios. However, indices suggest the resprouting form of E. mammosa to have an increased ability to self-fertilize while the seeder form has an increased ability for autonomous selfing. The 29 species analysed were divided into outcrossers and facultative outcrossers based on selfing indices but these did not fit neatly within Cruden’s proposed P/O ranges (facultative autogamy: P/Os= 32-397; facultative outcrossers: P/Os= 160.7 - 2258.6; outcrossers: P/Os= 1062 - 19525). This may be due to his classification of species into breeding systems being based on relatively few distantly related species per category with extremely variable P/O ratios per category. For example, outcrossers ranged an order of magnitude (from 1000+ to 20 000). It is also true that this ratio can be influenced by a variety of different factors, these include: habitat, evolutionary history and pollination syndrome. Consequently, P/O ratios in Erica do not seem to reflect pollination syndromes very well. Presumably, sex allocation theory may explain the relationship of breeding system with P/O ratios better. Histological studies of pollen tube growth for self- and cross-pollinated flowers of eight species suggested that Erica has late acting self-incompatibility (LSI). LSI is a barrier to selfing that occurs in the ovary. However, I could not determine if the rejection process occurs pre- or post-fertilization. Although, a large amount of knowledge is still lacking, this preliminary study provided insight into the reproductive biology of Erica.