Browsing by Author "Arendse, Brittany"

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    Can and do Ericas self pollinate?
    (2011) Arendse, Brittany; Van der Niet, Timotheus; Midgley, Jeremy J
    Within the Cape Floristic Region many lineages are characterised by large floral diversity. The genus Erica is one of these lineages, making up -7% of the CFR. Surprisingly, even though pollinators have been suggested to be a driving force of floral morphology, the role of pollinators in the floral diversification and speciation of this genus is not yet well understood. Therefore the aim of this paper was to establish if Erica species can and do self-pollinate. Two Erica species, E. plukenetii and E. urnaviridis, were obtained from Kirstenbosch nursery, where hand-pollinations were performed on 15-20 flowers of each of three treatments (self-pollination, crosspollination and autogamous self-pollination. Additionally pollen tube analyses were performed on seven Erica species, which were collected from the Constantia Mountain. In E. plukenetii, self-incompatibility seems to be the predominant breeding system. It appears that, like E. urna-viridis, most of the other species analysed, via pollen tube analyses, have the potential to self-pollinate. However, more experiments are required to establish if these species are truly self-compatible. Autogamy, on the other hand, does not appear to set seed in the species studied. Therefore it would be fair to say that some ericas can self-pollinate but none actually do self-pollinate. These results indicate that ericas have a strong dependence on pollinators for seed set and in the past speciation may have occurred due to adaptation to different pollinators, when opllinators were scarce.
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    Physico-chemical factors influencing the spatial and temporal distribution of the seagrass Zostera capensis in Langebaan lagoon
    (2011) Arendse, Brittany; Pillay, Deena; Lawrence, Cloverley
    Seagrass meadows have important ecological roles in coastal ecosystems and provide high-value ecosystems services compared to other marine and terrestrial habitats. Despite this, there is growing evidence that this key ecosystems is declining on a global scale. Much of these declines have been attributed to the effects of human alterations of marine habitats. Continued destructive activities, in both South Africa and elsewhere, if not managed properly will result in the demise of seagrass beds from coastal areas. With this in mind, the aim of the project is to determine the main physico-chemical factors influencing Zostera capensis biomass and distribution in Langebaan Lagoon. Five sites within the Langebaan Lagoon were selected and within these sites a minimum of three and maximum of ten Z. capensis beds were sampled, depending on the availability of the beds. Five 10 cm core samples were collected from each bed, from which seagrass biomass, density, leaf length, leaf width and epiphytic algal biomass were estimated. Physico-chemical data were collected with a CTD to test for the differences in: temperature, turbidity, pH, salinity, dissolved oxygen and dissolved chlorophyll levels. The results indicated that temperature and salinity are the main factors driving biomass and distribution along Langebaan Lagoon. Increasing temperature appears to have a negative effect on both leaf length and leaf width, while increasing salinity has a positive effect on seagrass biomass. Although, the ancestral seagrass populations thrived in conditions of higher temperatures, today the rates of change along the coast is so rapid that seagrass populations cannot keep up with this change. In addition, sites near the open ocean experienced a higher seagrass biomass than the sites further away, indicating that salinity is an important factor driving growth and distribution. However, temperature and salinity are not the sole factor responsible for the distribution of Z. capensis at Langebaan lagoon. Turbidity and dissolved chlorophyll also appear to impact certain sites more than others. It is thus clear that at least a few other factors must be considered: the interaction between nutrients, epiphytes and macrograzers and the plants tolerance to the sediment quality. These different aspects need to be studied in concert in order to perceive the full range of impacts on the Z. capensis beds and to better manage these ever-declining key ecosystems.
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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.