Browsing by Author "Swart, Belinda Louisa"

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    The evolutionary history of the genus Seriola and the phylogeography and genetic diversity of S. lalandi (yellowtail) across its distribution range
    (Stellenbosch : Stellenbosch University, 2014-04) Swart, Belinda Louisa; Roodt-Wilding, Rouvay; Bester-van der Merwe, Aletta Elizabeth; Von der Heyden, Sophie; Stellenbosch University. Faculty of AgriSciences. Dept. of Genetics.
    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.