Doctoral Degrees (Botany and Zoology)

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Browsing Doctoral Degrees (Botany and Zoology) by browse.metadata.advisor "Anderson, Bruce"

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    The convergence and divergence of floral traits are driven by the heterogeneity of pollinator and plant communities
    (Stellenbosch : Stellenbosch University, 2017-03) Newman, Ethan; Anderson, Bruce; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Substantial evidence suggests that pollinators are responsible for generating floral variation when they select upon floral traits involved in attraction as well the efficiency of pollen transfer. Another source of floral variation that is often neglected, is the role of community structure which drives floral divergence of traits involved in competition for the same pollinator resource. When these traits are selected upon across a heterogeneous landscape, it is thought to give rise to the formation of local floral forms which are often referred to as ecotypes. This thesis uses the long-proboscid fly (Prosoeca longipennis) study system as a platform for asking questions regarding the generation of floral variation. In my first data chapter (chapter two), I show how this variation can arise when tube length converges upon fly proboscis length across several localities, leading to a pattern of geographic trait matching. In addition, I also identify populations of species which appear to be morphologically divergent, because in populations where P. longipennis is absent, they are often pollinated by morphologically different pollinators. In chapter three, I investigate pollinator driven floral divergence in Nerine humilis, a species pollinated by long-proboscid flies in some populations and short proboscid insects in others. In this chapter, I demonstrate local adaptation of different floral forms associated with different pollinators. In addition, I take an extra step and demonstrate that the mechanical fit between flower and pollinator morphology is the mechanism behind local pollinator adaptation. In my fourth chapter, I show that floral adaptation may not always be as clearly illustrated as in chapter three, because most plants are visited by a multitude of functionally different pollinators. Here, I explore pollinator-mediated selection in Tritoniopsis revoluta and Nerine humilis with multiple functional pollinator types. Using single visitations, I demonstrate that flowers adapt to the optima and slopes of the additive fitness functions from all functional pollinator types. In my last data chapter, chapter five, I demonstrate how floral divergence may occur through the context of the floral guild community, where floral divergence does not occur through selection exerted by pollinators, but occurs as a result of competition for the placement of pollen on the bodies of long-proboscid fly’s across different localities. This resultant process of ecological character displacement gives rise to a pattern where mean style lengths of Pelargoniums are more different when they co-occur compared to when they occur on their own, where they may have style lengths that are similar or different to those in sympatry. This thesis contributes to the existing literature by providing much needed evidence on how selection exerted by pollinators as well as the structure of the floral guild community may drive adaptive divergence of floral morphology across a heterogeneous landscape.
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    Dispersal, dormancy, life history and breeding systems of southern African Asteraceae : risk-reducing strategies in unpredictable environments
    (Stellenbosch : Stellenbosch University, 2015-04) De Waal, Caroli; Anderson, Bruce; Ellis, Allan G.; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: How organisms respond to unpredictable environments is a fundamental question in evolutionary ecology. For example, plants may reduce the risk of reproductive failure by spreading their reproductive effort in space (dispersal) or in time (dormancy, iteroparity). Similarly, different plant breeding systems, (for example the ability to autonomously self-fertilise) may reduce the risk of reproductive failure in environments where pollination in particular is unreliable. Each of these strategies may be affected by selective pressures exerted by heterogeneous abiotic and biotic environments (e.g. unreliable rainfall patterns or range edge habitats). However, there is little theoretical or empirical consensus on how these strategies are related. In Chapter 2, I explore the association between dispersal and breeding system traits and range edge proximity. I show that annual daisies from Namaqualand, South Africa, are characterised by two discreet syndromes: high selfing ability associated with good dispersal and obligate outcrossing associated with lower dispersal, regardless of range position. This chapter illustrates that selection on both breeding system and dispersal traits may act consistently across distribution ranges. Because co-flowering plants often share pollinators, their fecundity is likely affected by changes in pollinator visitation rates or the transfer of conspecific relative to heterospecific pollen. In Chapter 3 I experimentally investigate the effects of con- and heterospecific density and spatial distribution pattern on pollination and fecundity in annual Namaqualand daisies. I show that increasing conspecific density and aggregation enhanced fecundity through increased mate availability and reduced heterospecific interference, independent of pollinator visitation rates. Moreover, I demonstrate the benefits of autonomous selfing when mates are limited and the potential for interspecific pollen transfer is high. In Chapter 4, I examine relative investment in dispersal vs. dormancy in seed heteromorphic Dimorphotheca (Asteraceae) species in relation to life history, rainfall unpredictability and range edge proximity. I show annuals and perennials differ significantly in the relative investment in different dispersal strategies. However, my findings provide little support for theoretical predictions of bet-hedging strategies in unpredictable or range edge habitats. This chapter emphasises the role of local environmental factors on fruit set that may obscure expected patterns across broad climatic gradients. Because of different costs and benefits of dispersal in space and time, we may expect negative patterns of covariation among dispersal and dormancy as alternative risk-reducing strategies. In Chapter 5, I provide evidence for a trade-off between these traits across 27 wind- dispersed daisy species from South Africa. This trade-off did not depend on life history effects, but was inconsistent at different levels of biological organisation. I also show that the effects of life history on spatial and temporal dispersal were inconsistent. Taken together, my research illustrates the importance of simultaneously investigating different risk-reducing strategies, because associations among them are clearly complex and often contradict theoretical expectations. Moreover I show that the effects of life history and phylogenetic relatedness cannot be disregarded. My findings underscore the importance of dispersal in space and time as well as autonomous selfing as risk-reducing responses to unreliable environments.
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    Evolutionary drivers of temporal and spatial host use patterns in restio leafhoppers Cephalelini (Cicadellidae)
    (Stellenbosch : Stellenbosch University, 2015-12) Augustyn, Willem Johannes; Ellis, Allan G.; Anderson, Bruce; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Understanding how divergent selection results in the evolution of reproductive isolation (i.e. speciation) is an important goal in evolutionary biology. Populations of herbivorous insects using different host plant species can experience divergent selection from multiple selective pressures which can rapidly lead to speciation. Restio leafhoppers are a group of herbivorous insect species occurring within the Cape Floristic Region (CFR) of South Africa. They are specialised on different plant species in the Restionaceae family. Throughout my thesis I investigated how bottom-up (i.e. plant chemistry/morphology of host plant species) and top-down (i.e. predation and competition) factors drive specialisation and divergence in restio leafhoppers. I also investigated interspecific competition as an important determinant of restio leafhopper community structure. In chapter 2 I quantified host specificity of restio leafhopper species within a local community for 24 months. I found that restio leafhopper species are highly host specific and potentially synchronised with the growth phases of their host plants. In chapter 3 I used a network metric, modularity, to determine whether host plant partitioning in a restio leafhopper community is non-random (i.e. driven by a deterministic process). This metric allows the identification of the components underlying host plant partitioning (modules). I then performed experiments to show that modules, and therefore host plant partitioning, can mostly be explained by preference and performance relationships (i.e. bottom-up factors). In chapter 4 I used null models to test whether niche partitioning in restio leafhopper communities is a general pattern across the landscape. I found non-random niche partitioning, which results from strong host specificity, in all investigated restio leafhopper communities. In addition, I performed binary host choice experiments in the presence and absence of interspecific competition, but found no evidence that interspecific competition narrows host preferences. These findings suggest that host specificity, the cause of niche partitioning, is likely shaped over evolutionary time. Sampling multiple interaction networks across the CFR, in chapter 5, I tested whether restio leafhopper populations are more host specific in species rich communities and regions in the CFR than in species poor communities and regions. I found no positive relationship between restio leafhopper species richness and host specificity at any scale. These findings suggest that specialisation is not driven by interspecific competition. In chapter 6 I investigated host shifts in Cephalelus uncinatus. C. uncinatus has a broader distribution than any single restio species that it can use; suggesting that host plant related divergence may result from geographic range expansion. I found that allopatric and parapatric populations, but not sympatric individuals, using different host plants have divergent host preferences. I also found evidence for morphological divergence in traits related to predator avoidance in population pairs that exhibit divergent host preferences. My findings emphasise the importance of both bottom-up and top-down factors, with the exception of interspecific competition, as determinants of specialisation and divergence in restio leafhoppers. I find no evidence that interspecific competition is an important force structuring restio leafhopper communities. Instead, strongly niche partitioned community structure appears to emerge from the speciation process.
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    Novel insights into pollen movement and floral evolution revealed by quantum dots
    (Stellenbosch : Stellenbosch University, 2018-12) Minnaar, Corneile; Anderson, Bruce; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: To understand the evolution of flowers and mating systems in animal-pollinated plants, we have to directly address the primary function for which flowers evolved—the movement of pollen from anthers to stigmas. Yet, despite a long and distinguished history of making significant advances in understanding of natural selection and evolution, the field of pollination biology has largely studied pollen movement indirectly (e.g., pollen analogues or paternity assignment to seeds) due to a lack of suitable pollen tracking methods. Consequently, understanding of pollen export mechanisms and male reproductive strategies has been limited. In Chapter 2, I describe and test a novel technique to label and track the movement of pollen grains using quantum dots. I show that quantum dots can be attached to pollen grains of several different species and that their attachment to pollen appears not to affect pollen dispersal. In Chapter 3 I employ quantum dot pollen-labelling to test the placement and transfer of pollen in a unique population of Lapeirousia anceps (Iridaceae) with a bimodal distribution in floral tube length. I find that floral-tube length acts as a strong reproductive isolation barrier between plants with short-tubed flowers and longtubed flowers. In Chapter 4 I use quantum dots to explore the function of floral handedness in Wachendorfia paniculata. Based on pollen transfer experiments, pollen moves predominantly between left- and right-handed flowers, rather than between flowers of the same type. These experiments allowed the creation of the first map of anther-level pollen grain placement on the bodies of bees. Pollen placement maps revealed pollen quality heterogeneity across pollinator bodies, and that stigmas of W. paniculata aligned with areas on bee bodies where the capture of outcrossed pollen is most likely. This led to greater than expected outcross pollen movement. These findings underline the importance of studying micro-scale pollen landscape composition on pollinator bodies and how stigmas interact with them. The thesis concludes with a review which assesses the history of studying male function in plants and identifies critical gaps in our understanding of the ecology and evolution of pollen transport. I explore male reproductive function along the male fitness pathway, from pollen production to ovule fertilization. At each step of the pathway to paternity, I discuss evolutionary options to overcome barriers to siring success. In particular, I highlight a newly emerging idea that bodies of pollinators function as a dynamic arena facilitating intense male–male competition, where pollen of rival males is constantly covered or displaced by competitors. This perspective extends the pollen-competitive arena beyond the confines of the stigma and style, and highlights the opportunity for important new breakthroughs in the study of male reproductive strategies and floral evolution.