Browsing by Author "Shuttleworth, Adam."

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Development of Beauveria brongniartii as a bio-insecticide to control white grub (Coleoptera: Scarabaeidae) species attacking sugarcane in South Africa.
(2016) Kheswa, Nozipho.; Conlong, Desmond Edward.; Laing, Mark Delmege.; Shuttleworth, Adam.
Abstract available in PDF file.
Ecology and evolution of the specialized hemipepsis-wasp (Hymenoptera : Pompilidae) pollination guild in South Africa.
(2011) Shuttleworth, Adam.; Johnson, Steven Dene.; Brothers, Denis John.
Pollinators are believed to have played a key role in the radiation of flowering plants. The Grant-Stebbins model of pollinator-mediated speciation, in which evolutionary shifts between pollinators result in phenotypic diversification and enforce reproductive isolation, is one of the most compelling hypotheses for the rapid diversification of angiosperms. A key principle in this model is that plant pollination systems tend towards specialization, resulting in convergent suites of floral traits (syndromes) associated with particular types of pollinators. However, the expectation of pollination system specialization is not always supported by ecological data and has also been questioned on theoretical grounds. In this thesis, I examine pollination by Hemipepsis spider-hunting wasps (Hymenoptera, Pompilidae, Pepsinae) and use this system to address questions about levels and proximal mechanisms of floral specialization, floral shifts and convergent evolution of floral traits. Specialized pollination by Hemipepsis wasps is a newly described pollination system within the angiosperms. I document pollination by these wasps for the first time in 15 South African grassland plant species, including two species of Eucomis (Hyacinthaceae) and 13 asclepiads (Apocynaceae: Asclepiadoideae). In one of the asclepiads, Xysmalobium undulatum, I describe a bimodal pollination system involving both Hemipepsis wasps and a cetoniine beetle. I also describe an unusual and potentially antagonistic pollination mechanism whereby wasps are systematically dismembered during the insertion of pollinia in the two asclepiads Pachycarpus asperifolius and P. appendiculatus. I have used these and previous case studies to establish the existence of a new pollination guild, consisting of at least 21 plant species (across 10 genera and three families), that are reliant on four functionally similar species of Hemipepsis wasp for pollination. Plants in the guild are distributed throughout the moist grasslands of eastern South Africa and flower from September through until early May, peaking in December/January. The Hemipepsis-wasp pollination guild is characterized by high levels of functional specialization (17 of the 21 known guild members are pollinated exclusively by Hemipepsis wasps), despite the absence of morphological adaptations to prevent non-pollinating insects from accessing nectar. I used field and laboratory based experiments to explore the function of floral traits in enforcing specialization. These showed that Hemipepsis wasps primarily use scent, rather than visual cues, to locate flowers, but I was unable to firmly identify specific compounds responsible for the attraction of these wasps (compounds that elicited antennal responses in preliminary GC-EAD experiments did not attract wasps in bioassays). The chemical composition of the floral scents of guild members was examined for 71 individuals representing 14 species in addition to previous studies, and found to comprise complex blends of volatiles (usually containing between 30 and 50 compounds), typically dominated by aliphatics and monoterpenes with small amounts of aromatics. I also showed that the floral colours of guild members are similar to background vegetation, suggesting that floral colours are adapted for crypsis to avoid detection by non-pollinating insects. Palatability choice experiments with honeybees showed that non-pollinating insects find the nectars of at least three of the asclepiad guild members distasteful. Plants in this guild thus appear to achieve specialization through biochemical filters (scent as an attractant and differentially palatable nectar) and cryptic coloration. Pollinator-mediated convergence in floral traits is the fundamental basis for pollination syndromes, but has seldom been rigorously analyzed. Flowers in the Hemipepsis-wasp pollination guild share several qualitative traits, including dull greenish- or brownish-white colour, often with purple blotches, exposed sucrose dominant nectar with a relatively high sugar concentration (typically over 50% sugar by weight) and a sweet/spicy fragrance to the human nose. To test for convergent evolution in guild members, I compared scent, nectar and colour traits of guild members to those of congeners with different pollinators. Although traits often differed between guild members and their congeners, I found little evidence for overall convergence in floral scent profiles and nectar properties, but floral colours in the guild were significantly closer to the colour of background vegetation than those of congeners. At this stage, the lack of knowledge about specific floral volatiles that influence Hemipepsis-wasp behaviour and secondary nectar constituents that limit non-pollinator visits makes it difficult to identify the extent of biochemical convergent evolution within the guild. The directions and functional traits involved in evolutionary transitions between pollination by Hemipepsis wasps and other vectors are currently difficult to ascertain as there is limited phylogenetic data for the plant families concerned. In the genus Eucomis, fly and Hemipepsis-wasp pollinated species are very similar in floral morphology and colour, but differ strongly in floral scent. Using manipulative field experiments in conjunction with detailed analyses of colour, scent and morphology, I was able to show that a shift between wasp and fly pollination could be induced simply by manipulating oligosulphides in the scent emission from inflorescences. When considered in combination with other experiments highlighting the importance of scent as a pollinator attractant for all guild members, this suggests that scent properties may have played a key role in the evolutionary transitions between pollination by Hemipepsis wasps and other vectors. This research has established that pollination by Hemipepsis spider-hunting wasps is more geographically and phylogenetically widespread than was previously known, and has confirmed that these wasps are important and consistent pollinators in southern African grassland ecosystems. I have shown that a distinct guild of plants is specialized for pollination by these wasps. The high levels of specialization within this guild highlight the effectiveness of biochemical filters and cryptic coloration in limiting the spectrum of flower visitors. The major challenge ahead will be to identify the floral volatiles that attract Hemipepsis wasps and the non-sugar constituents that make the nectars of some guild members differentially palatable. These would both contribute greatly to our understanding of floral specialization and the mechanisms involved in the radiation of the angiosperms.
Friendly neighbours? Investigating ecological facilitation between Thunbergia atriplicifolia (Acanthaceae) and Exochaenium grande (Gentianaceae).
(2018) Msweli, Simangele Mbali.; Johnson, Steven Dene.; Shuttleworth, Adam.
Wildflowers that co-exist, share similar floral signals and flower simultaneously, may interact with the same pool of pollinators, therefore sharing pollinator species and individuals. Floral similarity could be a result of a common ancestor, convergent floral syndrome or mimicry. Pollinator sharing may have consequences for the pollination success and fecundity of one or more of the interacting wildflowers. This study was conducted in grassland vegetation of southern KwaZulu-Natal to investigate an observed floral similarity between Thunbergia atriplicifolia and Exochaenium grande, document pollinator sharing between the two species and investigate the consequences of pollinator sharing on (1) interspecific pollen transfer between the two species and (2) seed set of individuals of E. grande that grow within sparse and dense populations of T. atriplicifolia. It was hypothesized that E. grande is a mimic of T. atriplicifolia. Investigated aspects of floral similarity include: flowering phenology, flower colour, flower size and nectar production. Weekly surveys indicated that both species flower simultaneously but E. grande occurs at much lower density and peaks in flowering later than does T. atrplicifolia. Flowers of the two species have similar spectral reflectance and in terms of a bee vision model can be considered indistinguishable to bees. There is also overlap in flower size of the two species. They provide similar amounts of nectar but nectar of E. grande has a higher sugar concentration. Both species are dependent on pollinators for seed production and are pollinated by generalist solitary bees belonging to the Apideae and Hactilidae family, with some species shared by both plant species. Stigmas of T. atriplicifolia and E. grande carried heterospecific pollen in varying degrees. Estimates of pollen production indicated that E. grande produces more pollen than T. atriplicifolia. Although interspecific pollen transfer was confirmed, it is not yet clear whether this improper pollen transfer affects seedset. Plant density of E. grande and T. atriplicifolia was not a significant predictor of seed set in E. grande. However, there were very high levels of seed predation in E. grande, and that hinders our ability to rule out the any effects of pollinator sharing in seed set. These findings reveal a striking similarity and pollinator sharing between the two species, but despite this, there appears to be no negative or positive effects of this on seed set. Seed set only accounts for half of fitness, however, and the effects of pollinator sharing may therefore be evident on other measures of fitness. The role of trait similarity for pollinator attraction still requires further experimental investigation before the hypothesis of ecological facilitation (or mimicry) can be accepted.
Pollination ecology and the functional significance of unusual floral traits in two South African stapeliads.
(2017) du Plessis, Marc Alexander.; Shuttleworth, Adam.; Johnson, Steven Dene.; Nicolson, Susan W.
Carrion and dung mimicking plants often exhibit unusual floral traits which are believed to attract necro- and coprophagous insects as pollinators. Our understanding of these unusual traits and their functions is very limited. Stapeliads (Apocynaceae: Asclepiadoideae: Stapeliinae) are a monophyletic group of some 400 species of stem-succulent plants, many of which emit foul odours and exhibit unusual morphological traits that have anecdotally been assumed to represent adaptations to enhance the flowers’ resemblance to carrion or dung. This study looked at the pollination biology of two stapeliads, Orbea variegata and Stapelia hirsuta var. hirsuta, and explored the functional significance of some of the floral traits commonly associated with carrion or dung mimicking flowers. Further, odours emitted by both species were compared to the odours of putative models to explore the chemical basis for the assumed mimicry. Orbea variegata attracted flies from the families Muscidae, Calliphoridae and Sarcophagidae (at sites near Scarborough and Clifton, Western Cape) and individuals from each of these families were found carrying pollinia. The scent of O. variegata flowers was found to be dominated by dimethyl disulphide, dimethyl trisulphide as well as phenol. The presence of both these compounds suggests mimicry of both carrion and dung, although an ANOSIM analysis indicated that the odour of O. variegata shared more similarities with dung. This suggests that O. variegata is fairly generalist and explains the attraction of various flies that are associated with carrion or faeces by the flowers of this species. In experiments testing the importance of black versus yellow colouring and the importance of patterning, flies were found to prefer black coloured models in the presence of O. variegata odour, whereas the presence and size of blotching on the corolla lobes had no significant effect on fly visits. The colours of the black blotching and yellow of the corolla lobes showed minimal chromatic contrast when interpreted using the Troje (1993) fly vision model, although background rocks showed chromatic contrast, suggesting flies can distinguish between the flowers and the background. In an experiment testing the importance of odour for attracting flies, significantly fewer flies were able to locate concealed flowers compared to visible flowers, suggesting an important role for visual cues for flies to locate the odour source. Stapelia hirsuta var. hirsuta was found to exhibit two floral colour morphs at Swellendam (Western Cape). The yellow morph was rarer than the maroon morph. These flowers attracted flies belonging to the Muscidae, Calliphoridae and Sarcophagidae families, although only Calliphoridae and Sarcophagidae were found to carry pollinia. The odour composition of these two morphs differed slightly, where the odour of the maroon morph was dominated by dimethyl disulphide, dimethyl trisulphide and p-cresol and the yellow morph was dominated by dimethyl disulphide, dimethyl trisulphide and limonene. The ANOSIM analysis of odours emitted by S. hirsuta var. hirsuta in relation to that of various fly oviposition substrates suggested that these flowers are dung mimics rather than carrion mimics, although the presence of sulphides suggests possible mimicry of both. The yellow morph had higher fly visitation rates than the maroon morph. In experiments testing the role of floral trichomes, the removal of floral trichomes significantly decreased the visitation rates to the flowers, as well as the amount of time visitors spend on the flowers. Again, visual cues were shown to be of importance, as visible flowers received more visits than concealed flowers. Analysis of colours of different floral morphs, using the Troje (1993) fly vision model, suggests that flies cannot perceive chromatic colour differences between morphs. In these studies, I have shown that O. variegata and S. hirsuta var. hirsuta are visited and pollinated by carrion associated flies, and the flowers emit odours associated with both carrion and dung. This work sheds light on some of the floral features that are often associated with carrion and dung mimicry by flowers and the roles they play in the attraction of flies.