Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The genus Seriola includes several important commercial fish species, yet the phylogenetic relationships between species have not been fully investigated to date. This study reports the first molecular phylogeny for this genus based on two mitochondrial (Cytb and COI) and two nuclear gene (RAG1 and Rhod) fragments for all extant Seriola species (nine species, n = 27). The phylogenetic patterns resolved three main lineages: a ((S. fasciata and S. peruana), S. carpenteri) clade, a (S. dumerili and S. rivoliana) clade and a (S. lalandi and S. quinqueradiata) clade. The closure of the Tethys Sea (12 - 20 MYA) coincides with divergence of the ((S. fasciata and S. peruana), S. carpenteri) clade from the rest of the Seriola species; while the uplifting of the Isthmus of Panama (± 3 MYA) played an important role in speciation between S. fasciata and S. peruana. The climate and water temperature fluctuation in the Pliocene played important roles during the divergence of the remainder of the Seriola species. This study is also the first to describe the evolutionary history of the commercially important species Seriola lalandi across its distribution range. Global patterns of genetic variation within S. lalandi (n = 190) were examined using three genes fragments (mitochondrial DNA COI, Cytb and nuclear RAG1). Three distinct clades were identified, corresponding to three different geographic regions (North-western Pacific - Japan, North-eastern Pacific - USA, and the southern hemisphere clade). These groupings correspond with the previously identified subspecies of S. lalandi (North-western Pacific – S. lalandi aureovittata, North-eastern Pacific – S. lalandi dorsalis, and the southern hemisphere clade - S. lalandi lalandi). AMOVA results and pairwise FST values revealed significant population differentiation between these groups. The population subdivision between these clades in all probability is maintained by biogeographic or oceanographic barriers (such as the equator and East Pacific Barrier) that disrupt gene flow. The southern hemisphere clade comprised of samples from the southern Pacific (AUS, NZL and Chile) and the southern Atlantic (SA). No haplotypes were shared between these areas in the southern hemisphere. This southern hemisphere clade was further investigated with six microsatellite markers. The analyses revealed the South African populations as genetically distinct from populations of the South Pacific oceans (AMOVA, FCA and STRUCTURE results). In summary, the South African and southern Pacific grouping could be the result of recent vicariant events during the Pleistocene glacial / interglacial periods and / or contemporary oceanographic forces acting on these populations. Further population differentiation was found within the South African samples, but not in the South Pacific. In the southern Pacific clade this lack of population structure is the result of high gene flow (analysed with MIGRATE) between the sampling localities. This is the first report on the genetic structure of this commercial important species for South African populations. Five sampling localities from the west- to the east coast of South Africa were sampled (n = 201). The microsatellite analyses revealed two potentially genetically distinct groups. AMOVA, FST and FCA results suggest small but significant differentiation between populations from the west coast and from the south- and east coast, suggesting a potential genetic break in the Cape Point region (BARRIER). However, the program STRUCTURE showed a high level of admixture along the South African coast and the migration results (MIGRATE and BAYESASS) also suggest a high degree of gene flow between these regions. / AFRIKAANSE OPSOMMING: Die genus Seriola bevat verskeie kommersieel belangrike vis spesies, tog is filogenietiese verwantskap tussen species is nog nie ten volle bestudeer nie. Hierdie studie is die eeste molekulêre filogenietiese studie vir die genus gebaseer op twee mitochondriale DNA (Cytb en COI) en twee nukleêre DNA fragmente (RAG1 en Rhod) vir al die bestaande Seriola spesies (nege spesies, n = 27). Drie beranglike filogenitiese patrone is geïdentifiseer: ‘n ((S. fasciata en S. peruana), S. carpenteri) groep, ‘n (S. dumerili en S. rivoliana) groep en ‘n (S. lalandi en S. quinqueradiata) groep. Die diversifikasie van die ((S. fasciata en S. peruana), S. carpenteri) groep van die ander Seriola spesies stem oor een met die sluiting van die Tethys See (12 - 20 MYA). Terwyl die opligging van die Isthmus van Panama (± 3 MYA) ‘n belangrike rol gespeel het in die spesiasie tussen S. fasciata en S. Peruana. Die veranderlike klimaat toestande en water temperature tydens die Pliocene help met die diversifikasie van die res van die Seriola spesies. Hierdie studie is die eerste om verslag te doen op die evolusionêre geskiedenes van kommersiele belangrike vis spesies Seriola lalandi oor sy hele verspreidings gebied. Die globale patrone van genetiese variasie binne S. lalandi (n = 190) is ondersoek deur gebruik te maak van mitokondriale (Cytb en COI) en nukleêre (RAG1) DNA volgorde data. Drie groepe geassosieer met geografie is geïdentifiseer (Noord-westelike Stille Oseaan - Japan, Noord-oostelike Stille Oseaan - FSA en die suidelike hemisfeer groep). Hierdie groepeerings stem oor een met voorheen geïdentifiseerde subspesies van S. lalandi (Noord-westelike Stille Oseaan - S. lalandi aureovittata, Noord-oostelike Stille Oseaan - S. lalandi dorsalis en die suidelike hemisfeer groep - S. lalandi lalandi). AMOVA en FST resultate bevestig die hoë graad van genetiese struktuur tussen groepe. Hierdie populasie groepeerings is moonlik die oorsaak van - en onderhou deur giogeografiese and oseaanografiese grense (soos the ewenaar en EPB) wat geen vloei ontwrig. Die suidelike hemisfeer groep kan verder verdeel word in ‘n suidelike Stille Oseaan (AUS, NZL en Chile) groep en ‘n suidelike Atlantiese groep (SA). Geen haplotiepes word gedeel deur hierdie twee groepe. Hiedie suidelike hemisfeer groep was verder ondersoek met ses mikrosatelliet merkers. Die analises bevestig dat die Suid Afrikaanse populasies geneties verskillend is van die suidelike Stille Oseaan groep (AMOVA, FCA and STRUCTURE). Verdere genetiese struktuur kan verkry word binne die Suid Afrikaanse monsters, maar nie in die suidelike Stille Oseaan nie. In die suidelike Stille Oseaan hiere gebrek aan populasie struktuur is as gevolg van die hoë geen vloei (MIGRATE) tussen die lokaliteite. Hierdie is ook die eerste studie oor die genetiese structure van S. lalandi vir Suid Afrikaanse populasies. Monsters (n = 201) was geneem by vyf lokaliteite vanaf wes tot oos kus van Suid Afrika. Die mikrosatelliet analises dui op twee moontlike genetiese groepe. AMOVA, FST en FCA dui ‘n klein maar betekenisvolle differensiasie tussen populasies van die wes kus en die van die suid en oos kus. Dit dui aan op moontlike breek in die Kaap Punt omgewing (BARRIER). STRUCTURE analises toon egter ‘n bymengsel van twee groepe langs die Suid Afrikaanse kus en migrasie resultate (MIGRATE en BAYESASS) toon ook ‘n hoë graad van gene vloei tussen hierdie gebiede.