Determining the mechanisms driving the invasion success of the smallmouth bass (Micropterus dolomieu, Lacepède 1802)

Thesis (PhD)--Stellenbosch University, 2017.

ENGLISH Abstract: Biological invasions represent a significant threat to the Earth’s biota, including freshwater ecosystems. Whilst various studies have been conducted on invasive species in the hopes of establishing a set of general guidelines to facilitate our understanding of the invasion pathway and the features (organismal or environmental) that may assist in a species’ invasion success, predictions and generalisations remain challenging and limited. While general guidelines and unified frameworks are essential in furthering our understanding of biological invasions, exploring the genetic and phenotypic variation in a species and the influence of extrinsic factors during adaptation to novel environments could provide insight into a species’ invasion success. By utilising an array of methods, spanning different disciplines, this dissertation evaluates the potential mechanisms driving a species’ invasive success using the smallmouth bass, Micropterus dolomieu, as model organism. First, I attempt to unravel the role of genetic diversity in a species’ invasive success, by assessing the genetic differentiation and diversity within M. dolomieu populations in the invaded South African (SA) range, and examine how the genetic diversity may change over time in both native (USA) and invasive ranges (Chapter 2). By conducting a three-way comparison on two mitochondrial (mtDNA) and nine microsatellite loci for a total of 572 M. dolomieu specimens, representing the contemporary invasive SA range, contemporary native USA range and the historic native USA range (dating back to the period of introduction into SA), I reveal the presence of elevated levels of genetic diversity for the invasive SA range. The levels of genetic diversity for both the contemporary native and invasive ranges are, however, substantially lower than those of the historic native range, suggesting that both contemporary populations experienced a recent genetic bottleneck followed by a rapid population expansion. As admixture, and more specifically introgressive hybridisation, may contribute to these elevated levels of genetic diversity observed within the invaded SA range, I subsequently test the hypothesis that hybridisation and introgression can occur between two invasive species in a novel invaded range (Chapter 3). Using two notorious freshwater invaders (M. dolomieu and M. salmoides), I assess the level of introgressive hybridisation between the two species, using two mtDNA and nine microsatellite loci. Despite large uncorrected pairwise distances being observed between the two species, unidirectional mitochondrial introgression was detected, suggesting that introgressive hybridisation may play a pivotal role in the successful establishment and spread of alien invasive species upon introduction. The remaining chapters focus on the role of morphological variation in response to environmental variation in the novel invaded range. First, I test the hypothesis that environmental variation drives morphological changes in phenotype (Chapter 4). Analyses of linear and geometric morphometrics, as well as environmental variables, show a strong correlation between body depth and flow regime, with streamlined fish inhabiting high-flow environments. In addition, the presence of variation in gape size among localities suggests a link between cranial morphology and prey composition associated with substrate type. These results support the idea that similar environments have convergent phenotypes and highlight the importance of phenotypic plasticity in facilitating the successful colonisation, establishment and spread of invasive species. However, as gene flow may erase phenotypic variation associated with plasticity, I continue by exploring the genetic basis to local adaptation (Chapter 5). By combining linear morphometrics and genotypic data, I demonstrate slight population structuring among sampled localities, corresponding to three distinct sections of the river (i.e., tributary, impoundment and mainstem). However, the presence of high levels of gene flow observed among populations suggesting that both local adaptation and phenotypic plasticity may play a key role. In conclusion, M. dolomieu appears to be a successful invader that fully exploits genetic and phenotypic variability to invade, persist and establish in an array of non-native environments.

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Doctoral