Doctoral Degrees (Genetics)
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Browsing Doctoral Degrees (Genetics) by Subject "Abalones -- Population genomics"
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- ItemSignatures of selection in natural and cultured Abalone (Haliotis midae) : a population genomics study(Stellenbosch : Stellenbosch University, 2013-03) Rhode, Clint; Roodt-Wilding, R.; Bester-van der Merwe, Aletta Elizabeth; Stellenbosch University. Faculty of Science. Dept. of Genetics.ENGLISH ABSTRACT: The South African abalone, Haliotis midae, commonly known as perlemoen, is an economically important gastropod mollusc. Historically, this species maintained a lucrative fisheries sector; however with increasingly lower landings there has now been a shift to aquaculture. Efforts to conserve natural populations and to improve abalone aquaculture production are thus running in parallel. Previous studies reported significant disparities in parental contributions in aquaculture populations that could explain the rapid divergence of commercial stocks from wild populations. Furthermore, subtle, but significant, population differentiation has also been reported for wild populations on the west-, south-, and east coast of the South African coastline. This study therefore aimed to investigate the evolutionary forces, in particularly selection, facilitating population divergence in wild and cultured H. midae populations using a population genomics approach. By using both microsatellite- and single nucleotide polymorphism (SNP) markers it was found that approximately 10% to 27% of the H. midae genome may be influenced by selection. When incorporating these loci into analyses of population differentiation (e.g. AMOVA, factorial correspondence analysis and estimates of genetic distance) there was a marked increase in genetic divergence between wild and cultured populations (especially when using microsatellite loci) and amongst populations from different geographic regions (particularly supported by the SNP loci). The differences in population clustering as highlighted by microsatellite- and SNP markers can most likely be attributed to the genomic distribution of the respective loci: The SNP markers were developed from EST sequences and therefore mostly represents protein structural variation; whereas the microsatellite markers, found to be putatively under selection, were mainly located in regulatory motifs. The results of this study therefore confirmed previous observations of divergence amongst wild- and cultured populations, but more importantly demonstrated that selection is an important factor driving this divergence. In wild populations selection probably facilitates adaptation to local environmental conditions, whilst amongst aquaculture population adaptation to captivity, husbandry practices and artificial selection may be important determinants. There is evidence for population bottlenecks in wild- and cultured populations; nonetheless long-term effective population sizes seem to be large. Amongst the wild populations, however, short-term population sizes appear to be small most likely due to differential spawning rates amongst reproductively active animals leading to temporal fluctuation in genetic diversity. The results indicate that contact between wild and cultured abalone should be minimised to prevent any adverse effects due to outbreeding depression. With regards to conservation, an emphasis on maintaining adaptive diversity of the wild stocks might be warranted. Continued genetic monitoring is advisable for both wild and cultured abalone populations as to optimally manage the abalone resource for both conservation and commercial viability and sustainability.