Thesis (PhD)--Stellenbosch University, 2008. / ENGLISH ABSTRACT: Introduced species displace native species and alter ecological communities, affect agriculture as well as human health and are economically costly to eradicate. Long term monitoring of introduced species including the documentation of levels of genetic variation is therefore of the utmost importance. This study investigated the distribution of genetic variation in two introduced species distributed across South Africa the bud gall-forming wasp Trichilogaster acaciaelongifoliae and the Argentine ant Linepithema humile. The bud gall-forming wasp was introduced into South Africa as a biological control agent to curb the spread of the invasive long-leaved wattle Acacia longifolia. In addition to the intended (target) host, the bud gall-forming wasp has also colonised A. floribunda, a noninvasive ornamental plant. Limited genetic variation was found across South Africa based on the mitochondrial DNA cytochrome oxidase subunit I (COI) gene. Only 3 haplotypes characterized 53 individuals collected from 23 localities (nucleotide diversity £k = 0.002 ¡Ó 0.001, haplotype diversity h = 0.482 ¡Ó 0.045). No significant partitioning of genetic variation was found across South Africa including between host plants (target host = A. longifolia, non target host = A. floribunda) or between the core (sites of introduction) and edge (naturally dispersed) sites (ƒ¶ST = 0.094, P = 0.288). The limited genetic variation and the absence of significant genetic structure are congruent with patterns described for many other introduced species and may suggest that propagule pressure plays only a minor role in species establishment and spread of the gall-forming wasp across South Africa. Mitochondrial and nuclear markers were used to describe the distribution of genetic variation within Argentine ants across their introduced range in South Africa. For the mitochondrial DNA, low genetic diversity was found for the COI gene with only five haplotypes, separated by single mutational changes, characterizing 101 specimens from 35 localities (nucleotide diversity π = 0.001 ± 0.001, haplotype diversity h = 0.151 ± 0.048). Notwithstanding the low levels of genetic diversity, mitochondrial variation was significantly structured (ST = 0.54, P < 0.001) across the landscape. In contrast, microsatellite analyses of 230 ants from 23 localities, employing six polymorphic microsatellite markers, revealed a relatively high amount of genetic diversity (HE = 0.51 ± 0.22). Significant population structure was similarly evident (RST = 0.14, P < 0.001) with the localities of Elim2, Porterville2 and Bloemfontein2 clustering as a distinct population from the remainder of the localities. Importantly, individuals from these localities also had a unique mitochondrial haplotype and, when taken with the nuclear results, may indicate the occurrence of more than one introduction event (and possibly more than one colony) in South Africa. This is further underscored by the presence of unique microsatellite alleles in these three populations. In an attempt to establish the source populations for the introduction of Argentine ants into South Africa, mitochondrial cytochrome b sequences were generated for a subset of ants representing the two major genetic clades across South Africa. A comparison with the published data from across the world including the native range of the Argentine ant in South America grouped Argentine ants from South Africa with three potential source populations namely Ocampo and Rosario in Argentina and Passo do Lontra in Brazil. The results of this study underscore the role of human-mediated dispersal in shaping the levels of genetic variation in both species. Human-mediated dispersal can lead to genetic homogenization across populations. / AFRIKAANSE OPSOMMING: Indringer spesies verplaas of verander ekologiese gemeenskappe, beinvloed landbou asook menslike gesondheid en is ekonomies duur om te verwyder. Langtermyn monitering van indringer spesies asook die dokumentasie van genetiese variasie is dus baie belangrik. Hierdie studie bestudeer die verspreiding van genetiese variasie in twee indringer spesies wat regoor Suid-Afrika voorkom, naamlik die kroongal-vormende wespe Trichilogaster acaciaelongifoliae en die Argentynse mier Linepithema humile. Die kroongal-vormende wesp is na Suid Afrika gebring as biologiese beheeragent om die verspreiding van indringer lang-blarige wattle Acacia longifolia te beveg. Die kroongalvormende wespe het sowel die teiken spesies, asook A. floribunda, ‘n nie-indringer ornamentele plant gekoloniseer. Beperkte genetiese variasie is gevind regoor Suid Afrika gebasseer op die mitochondriale DNA sitokroom-oksidasie subeenheid I (COI) geen. Slegs 3 haplotipes karakteriseer 53 indiviue van 23 bevolkings (nukleotied diversiteit π = 0.002 ± 0.001, haplotiep diversiteit h = 0.482 ± 0.045). Geen beduinde groepering van genetiese variase is gevind regoor Suid Afrika nie (ST = 0.094, P = 0.288). Hierdie bevinding geld onafhanklik van die gasheer plant (teiken gasheer = A. longifolia, nie-teiken gasheer = A. floribunda). Ook is geen beduidende genetiese groepering gevind tussen die sentrale (plek van inisiele blootstelling) en perifêre (natuurlik verspreide) lokaliteite nie. Die kleinskaalse genetiese variase en die afwesigheid van beduidende genetiese struktuur wat hier gevind is, verskil van die patrone wat voorheen vir baie ander indringer-spesies beskryf is. Dit mag daarop dui dat ‘propagule’ druk slegs ’n klein rol speel in spesies-vestiging en verspreiding van die galvormende wespe regoor Suid-Afrika. Mitochondriale asook kern merkers is gebruik om die verspreiding van genetiese variasie in Argentynse miere in Suid Afrika te beskryf. Vir die mitochondriale DNA is lae genetiese variase gevind vir die COI geen, met slegs 5 haplotipes, gedifferensieer deur enkele mutasie veranderinge wat 101 monsters van 35 lokaliteite karakteriseer (nukleotied diversiteit π = 0.001 ± 0.001, haplotiep diversiteit h = 0.151 ± 0.048). Desondanks die lae genetiese variasie, is gevind dat mitochondriale variasie beduidend gestruktureerd is (ST = 0.54, P < 0.001) oor die landskap. Hierteenoor het mikrosatelliet analises van 230 miere van 23 lokaliteite, deur gebruik te maak van ses polimorfiese mikrosatelliet merkers, ’n relatiewe hoë hoeveelheid genetiese diversiteit aangedui. Beduidende bevolkingstruktuur was ook gevind (RST = 0.14, P < 0.001) in die areas Elim2, Porterville2 en Bloemfontein2 wat saam groepeer as eiesoortige bevolkings vergeleke met die res van die areas. Ook van belang is dat individue van die areas ’n unieke mitochondriale haplotipe besit, en in kombinasie met die kern resultate, mag dit die voorkoms van meer as een blootstellingsgeleentheid (en moontlik meer as een kolonie) in Suid Afrika aandui. Hierdie bevinding word verder beklemtoon deur die teenwoordigheid van unieke mikrosatelliet allele in die drie bevolkings. In ‘n poging om die oorsprong van die oorsprong-bevolking vir die blootstelling van Argentynse miere in Suid Afrika vas te stel, is mitochondriale sitokroom b volgordes gegenereer vir ’n substel miere wat die twee hoof genetiese klades in Suid Afrika voorstel. Vergelyking met gepubliseerde data van regoor die wêreld, insluitende die endemiese gebied van die Argentynse mier in Suid Amerika, het die Argentynse mier van Suid Afrika met drie potensiele oorsprong-bevolking verbind, naamlik Ocampo en Rosario in Argentina en Passo do Lontra in Brazil.