1,721,003 research outputs found
Adaptive and maladaptive sex ratios in a pollinating fig wasp
Full text linkDissertation (MSc (Genetics))--University of Pretoria, 2020.When a trait enhances fitness and arose as a result of natural selection, it is termed an adaptation. The optimization strategy employs selection thinking which makes the explicit assumption that the globally best trait will be fixed. Organisms, do not always respond optimally across all environments encountered as the strength of selection is finite. An excellent example of optimality is skewed sex ratios predicted by Hamilton's local mate competition (LMC) theory. It occurs when brothers compete in isolated groups for matings. Hamilton showed mathematically that in such groups, the unbeatable sex ratio is female-biased and this bias increase as the number of mothers that contribute to isolated patches decrease. One taxon that has been used to test LMC is the fig wasps.
Fig wasps are haplodiploid, with unfertilized haploid eggs developing into males and fertilized diploid eggs into females. One or a few mothers crawl into a fig and lay all their eggs within, resulting in extreme local mate competition and sib mating. Sibmating increases relatedness of the daughter to the mother as genes from the father are identical by descent to the mother. Relatedness to sons is unaffected. This relatedness asymmetry selects for even more female-biased ratios. Thus, an extra daughter instead of a son will decrease local mate competition, and an extra daughter provides a higher fitness to the mother due to relatedness. There are two mechanisms that can explain how fig wasps sex ratios are adjusted to the number of mothers in a patch. First, a byproduct of a constraint on clutch size, where mothers lay all or most of their sons first, followed by daughters. Mothers do not change their behaviour, but limited oviposition space prevents them from laying all their eggs destined to be daughters. Second, a facultative strategy where mothers sense other ovipositing females and adjust the number of sons they lay.
We studied sex ratio adjustments in Ceratosolen galili, the cuckoo wasp that oviposits in Ficus sycomorus, which is pollinated by C. arabicus. Different numbers of C. galili foundresses and a mix of these species were entered into figs to study the mechanism of sex ratio adjustment. We found that mothers adjust their sex ratios facultatively when another foundress is present, and there is no constraint on clutch size. However, if the fig becomes saturated with eggs from three foundresses, the mothers’ clutches are constrained, and sex ratios are also adjusted as a result of a byproduct. Since these foundress numbers are frequently observed in nature, selection effectively solved this problem. However, C. galili mothers often share figs with C. arabicus and C. galili should not adjust their sex ratios in their presence of C. arabicus. However, C. galili erroneously adjust their sex ratio facultatively in the presence of C. arabicus. Erroneous adjustment can be considered a maladaptation. Interestingly, the lack of pollination behaviour of C. galili is not sanctioned by reduced offspring survival.GeneticsMSc (Genetics)UnrestrictedFaculty of Natural and Agricultural Science
Analysis of the heritage and genetic diversity of influential Afrikaners
Full text linkDissertation (MSc)--University of Pretoria, 2012.Previous studies have claimed that the Afrikaner population of South Africa contains
approximately 7% non-European DNA. It is thus ironic that Afrikaners practiced racial
separation and discrimination against non-Europeans, which was formalized in the
Apartheid policy years. As racism is not dead in South Africa, conservative individuals of
both sides of the racial spectrum often have trouble believing in the chequered heritage of
Afrikaners. Since pedigree data of early Afrikaner individuals are available, this study
attempted to construct pedigrees of several prominent Afrikaner individuals including
former Apartheid leaders to establish the truth about Afrikaner genetic composition.
Analysis of these pedigrees suggests that the Afrikaner population is heterogeneous with
twenty two of the individuals studied having non-European ancestry but with eight
having none. The latter tend to have high proportions of French DNA and larger
inbreeding coefficients which suggests that the French tended not to admix with others.
Finally, this study illustrates that most inbreeding is the result of distant common
ancestors.GeneticsMScUnrestricte
Resolving the phylogeny and population genetic structure of South African pollinating fig wasps
No full textDissertation (MSc (Genetics))--University of Pretoria, 2011.A distinct pattern of obligate mutualism exists between fig tree hosts and their pollinating fig wasps. Normally one section or subsection of fig tree hosts is associated with one fig wasp genus. In general, each species is pollinated by a specific fig wasp species. This led to the hypothesis that the fig wasp and fig tree lineages diverged simultaneously. African fig wasps pollinating hosts of the Galoglychia section frequently break the normal one fig wasp species-to-one host species ratio. The phylogeny for these species was reconstructed using three DNA segments and compared to the morphological classification of their Ficus hosts. Pollinator genera were monophyletic for all analyses, however, the relative positioning of genera was inconsistent. Analyses suggest frequent host jumps between fig trees and fig wasps. Fig wasps of the genus Alfonsiella that pollinate Ficus craterostoma, Ficus stuhlmannii and Ficus petersii are morphologically similar in South Africa. Based on host association, genetic differentiation for this group was investigated. Molecular data indicated that the pollinator of F. craterostoma is a good species, while the F. stuhlmannii and F. petersii pollinators were genetically indistinguishable. Based on molecular data and morphological re-evaluation, a new Alfonsiella species is described, Alfonsiella pipithiensis sp. n. A key to all described species of Alfonsiella is provided. In order to resolve the population genetic differentiation of pollinating fig wasp species in South Africa, Platyscapa awekei was used as a model species. A few studies indicate that pollinating fig wasps can disperse between 30 and 55 kilometers. However, a recent study on two P. awekei populations in South Africa reported an FST value of 0.011, indicating that pollinators disperse approximately ten times further. This study aims to confirm these results with more detailed sampling of populations. In addition, possible temporal differentiation was tested for the South African population. Six microsatellite loci were used to detect spatial and temporal genetic differentiation in seven populations (collected from 2004 to 2006) over a 340 kilometer range. Genetic differentiation between sampled populations was low (FST = 0.0055), however, the data suggest stronger temporal genetic isolation than spatial genetic isolation.Geneticsunrestricte
Identification of the factors that determined fedundity in early Afrikaners
Full text linkDissertation (MSc(Agric))--University of Pretoria, 2010.Fecundity determines the magnitude and direction of evolutionary change. By
analyzing traits correlating to human fecundity we may understand our own evolutionary
roots, and the behaviour and choices that define modern reproduction. For this reason,
studies that look at historical populations prior to the demographic inversion and the use
of contraceptives have become popular. Very good genealogical records exist for the
early Afrikaner population (1652-1820), but only a few studies have utilized this wealth of
information, primarily because it is not available electronically.
My aim was to determine which factors affected fecundity in this historical
population. The Afrikaner population had a very high growth rate, presumably because
of ameliorated environmental conditions. Under high growth rates we can expect that the
determinants of fecundity may differ from those found to be important in European
populations.
I recorded fecundity and a number of predictor variables for 1138 first generation
offspring (referred to here as B generation) of 517 founder individuals (A generation), as
well as the B generation spouses. Data was compiled on nationality of origin; number of
marriages (when sequential remarriage occurred); dates of birth and dates of various
reproductive events (date of first and last child’s birth/christening, wedding dates).
I analysed the data with a variety of approaches to estimate the effects of a
number of predictor variables known to affect fecundity as well as a couple of new ones
we propose. A few of the key findings are: a gender specific role for the effect of multiple
marriages; a discrepancy in fecundity for the various nationality groups suggesting
cultural inheritance of fecundity; a new fertility measure, the time lapse between
marriage and the birth of first child, explained most of the variation in fecundity; and we
found very limited evidence to support the idea that fecundity is heritable.GeneticsMSc (Agric)Unrestricte
Causes of fighting in male pollinating fig wasps
Full text linkDissertation (MSc)--University of Pretoria, 2008.A striking variation in the behaviour of pollinating fig wasps (Agonidae) is the occurrence of male fighting in some species while in others it is completely absent. Fighting behaviour was investigated at two levels. Firstly, the variation in fighting behaviour between the species was used to examine factors that might cause the evolution thereof. Comparisons across species were done using phylogenetic regression. This method takes similarity due to phylogenetic constraints into account when data are compared. Kin selection theory implies that fighting is barred by the high degree of relatedness in competing males. We however find that the relatedness of the males do not have an influence on the evolution of fighting and this finding supports models suggesting that high LMC cancels benefits due to relatedness. Rather, that the only factor having a significant correlation with fighting is the release sex ratio. The release sex ratio and dispersal is also associated. Fighting and dispersal are not expected to have direct influence on each other and the association of both with the release sex ratio imply that this may be an indirect link between these two behaviours. A syndrome where fighting and dispersal is found together is in part explained by the release sex ratio. We conclude that the release sex ratio is the most likely cause of the evolution of fighting behaviour in pollinating fig wasps. The second part of this study deals with the proximal determinants causing fighting, in the males of the species Platyscapa awekei. We show that the sex ratio which, is less female biased than non-fighting pollinator species, rapidly becomes even less female biased as soon as both sexes becomes active. Numerous fights are fought by the males in the female limited environment. The activity of the wasps is shown to be regulated by the gaseous environment, which change from a high to a low CO2 concentration with the construction of an exit hole from the fig. The males of the species P. awekei are inactive, and do not engage in mating or fighting activities, in high CO2, contrasted to males of other species, which are active in this environment. P. awekei females rapidly release once the CO2 level is lowered and mating behaviour is only observed in this environment. The number of female to male encounters of every male decrease as the operational sex ratio becomes less female biased. Male fighting in this species is therefore expected due to the high sex ratio, which is enforced by the increase thereof. We conclude that the physical environment, in this species, affects the mating environment. The resultant reduction in the number of potential mating opportunities therefore escalates fighting between the males.Geneticsunrestricte
A study on the sex allocation behaviour of the pollinating fig wasp, Platyscapa awekei
Full text linkDissertation (MSc)--University of Pretoria, 2011.The behaviour of sex allocation has been extensively studied in hymenopterans (ants, bees and wasps) as an adaptive trait with respect to intra-specific competition within the framework of kin selection theory. Mating in these organisms often takes place in patchy populations established by the offspring of a few foundresses. Typically, there is a bias in favour of female dispersal from these patches. Theory predicts that foundresses that oviposit alone will do best to produce just enough sons to mate all of their daughters so as to maximize the number of dispersing daughters, under conditions of what is referred to as Local Mate Competition (LMC) between brothers to mate their sisters. If foundresses co-found a patch with other foundresses, they are expected to invest more resources in sons insofar as opportunity to sire offspring with the daughters of the other foundresses presents itself. Among organisms with such a life histories are fig wasps, the insects that pollinate and lay their eggs in the flowers that grow inside young figs. There is thought to be strong selective pressure for foundresses to use information about clutch size differences in species where clutch sizes are small and low foundress numbers are frequently encountered. However, less rigorous modes of sex allocation are thought to suffice in species encountering intermediate foundress numbers. Theory thus predicts a positive relationship between the degree of structure within mating populations and the information utilized by foundresses with respect to intra-specific competition for resources and mating opportunities. This is being extensively tested across the diverse species range of fig wasps and their hosts with the larger objective in mind of contributing to a better understanding of the role of natural selection in accounting for variation observed of intra-specific behaviour. This dissertation reports on a study of the sex allocation behaviour of the pollinating fig wasp Platyscapa awekei, a species characterized by low foundress numbers and clutch size differences brought about by foundress competition over oviposition sites. Offspring collected from experimentally controlled twofoundress broods were fingerprinted using microsatellite genetic markers to assign maternity and work out clutch size differences. These data are used to test what information foundresses use when allocating sex. It is reported that foundresses appear to use information of clutch size differences in two foundress broods. This observation provides evidence of advanced information utilization in fig wasps. More generally, the findings add support to the hypothesis that natural selection can bring about subtle adaptive behaviour at the individual level, but simultaneously highlights the importance of accounting for the selective regime of the organism being studied when attempting to understand the role of natural selection in the evolution of fine scale adaptive traits.Geneticsunrestricte
Identification of the factors that lead to dispersal and inbreeding
Full text linkThesis (PhD)--University of Pretoria, 2009.Individual-based simulation modelling is an excellent method for testing hypotheses, while including realistic and stochastic population parameters. This thesis considers the evolution of dispersal or inbreeding through individual-based simulation modelling. The occurrence of exclusive inbreeding and exclusive outbreeding is found in a number of organisms and are referred to as mixed mating. Mixed mating is suggested to be in response to low levels of inbreeding depression as well as simultaneous inbreeding- and outbreeding depression while intermediately related mating partners are not available. The results of this thesis show that stable mixed mating strategies evolve in the presence of both inbreeding and outbreeding depression, as well as, under conditions where low levels of inbreeding depression are present. Also, inclusive fitness allows higher levels of inbreeding in genetic systems where the mating partners are more related to each other. Dispersal evidently evolves in response to inbreeding depression. A number of other factors, such as local mate competition and the cost of dispersal also influence the rate of dispersal. In addition to these factors, it is shown in this thesis that male dispersal evolves when there is variation in patch sex ratios. Simulation data also supports parent offspring conflict models, as males have reduced dispersal rates when they, rather than their parents, determine the dispersal rate. Population structure is affected by dispersal rates. Using individual-based simulation modelling and various sampling strategies, reveals that few molecular markers, for a few individuals, are sufficient to accurately detect population subdivision, especially when the sub-populations are large. It is, however, indicated that planning prior to sampling are important for proper assessment of population structure. Lastly, molecular data from the pollinating fig wasp Platyscapa awekei reveals that this species suffers from low levels of inbreeding depression. However, when this data are simulated, stable mixed mating did not evolve although it is observed in P. awekei. Sex ratio variation, high local mate competitions and male only broods are therefore suggested to drive male dispersal. It is consequently advantageous to use various techniques to unravel the evolution of a trait and gain insight into the system.Geneticsunrestricte
Phenotypic plasticity of phages with diverse genome sizes
Full text linkDissertation (MSc (Genetics))--University of Pretoria, 2006.A key factor in studying evolutionary biology is an understanding of the mechanisms organisms utilise in the ongoing process of adaptation. When faced with a heterogeneous and unpredictable environment, we expect organisms to evolve either as specialists or generalists, yet a unifying theory as to which will evolve is still lacking due to conflicting hypotheses based on limited empirical evidence. Phenotypic plasticity allows a single genotype to express different phenotypes, and has been found as an adaptive response to changing environments in all major taxa. With the advent of genomics it has become possible to study the underlying genetics of this phenomenon. It is however becoming clear that there is no single principle governing the plastic response, but rather a complex set of interactions between what appears to be regulatory and structural genes. With empirical data only recently becoming more readily available, the modelling of plastic responses are often still founded on the theoretical predictions and assumptions for which there is little proof. To bridge the gap between theory and nature, the challenge facing scientists today is the construction of experimental systems where theoretical predictions can be scrutinised. Given that phenotypic plasticity is a widespread phenomenon, understanding the magnitude and constraints of this response is an important issue in the study of evolution. Models have predicted a correlation between genome size and phenotypic plasticity, with increased genome size (complexity) linked to higher levels of phenotypic plasticity. Experimental findings, however, increasingly point to plasticity being governed by complicated sets of interactions between various parts of the genome, the adaptive landscape, and environmental cues. In the work presented here, a study was designed to test for a correlation between genome size and the level of plasticity by, looking at the fitness response of phages exposed to varying temperature. Seven phages differing in genome size and genome composition were used. Genome sizes ranged from 5386 bp to 170 000 bp. Taking advantage of the short generation times of phages, fitness could be measured as the growth rate per hour, which was compared among the different phage groups. The growth of large populations within a constant, controlled environment minimized the complications of environmental heterogeneity, and allowed for quantitative measure of the response to different temperatures. This was used to gain insight into how genome size relates to the level of phenotypic plasticity. Limited generation numbers were allowed for, to ensure population growth could be directly related to the plasticity of the genome, since numerous generations would be required for the effects of selection to become apparent. Adsorption rates are influenced by temperature, and were therefore measured to determine if it had a significant effect on the resulting population density. Results showed a marginal interaction between genome size and phenotypic plasticity, with adsorption rate having no significant effect. More experimental work would be required to verify this finding.Geneticsunrestricte
Quantification of the genetic variation in Spirocerca lupi from the Pretoria area
No full textDissertation (MSc)--University of Pretoria, 2010.Geneticsunrestricte
Approximate Bayesian Computation on Ficus craterostoma genetic variation to understand the history of forests in South Africa
No full textDissertation (MSc (Genetics))--University of Pretoria, 2023.Over the past 2.58 million years, recurring glaciation cycles have influenced the distribution and demography of forest in South Africa. These cycles may have led to fluctuations in forest population number and size due to climatic factors. This is because one hypothesis for forest distribution in South Africa is that forests can only grow under specific climatic conditions that limit their distribution. In addition, it has been proposed that forests can only persist in areas protected from natural fires. Ficus craterostoma is a fig tree species confined to forests that should have experienced climatic and fire driven range limitations and potentially recent anthropogenic changes to its distribution. Anthropogenic events like the Bantu-speaking migration (approximately 800-500 YA) and the arrival of European settlers (around 400-300 YA) could have caused forests to contract due to exploitation. The recent history of this species and forests in general remains largely unexplored. Approximate Bayesian Computation analysis revealed that 1) severe bottlenecks occurred towards the end of last glacial maximum (LGM), 2) significant forest expansion was unlikely following the LGM and that 3) there was no human-induced effect on forests. Rather, after the historical bottleneck, forest populations recovered slightly but were unable to return to their once large populations. Therefore, I infer that fire and climate have limited forests in South Africa.NRF Postgraduate BursaryGeneticsMSc (Genetics)UnrestrictedFaculty of Natural and Agricultural SciencesSDG-13: Climate actionSDG-15: Life on lan
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