Thesis (MScAgric)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: DNA micro satellite loci express extensive allelic variation making them convenient markers for
research in many fields employing population genetic tools, including aquaculture and conservation
genetics. Twelve Oreochromis mossambicus populations from wild, captive and introduced sources
in Southern Africa were screened for genetic variation at ten CA repeat micro satellite loci. Three of
the loci - UNHI04, UNHlll, and UNH123 - were sufficiently well resolved to screen extensively
and were interpreted according to a model of Mendelian inheritance. Data was analyzed in terms of
genetic structure and levels of genetic variation, the effect of management regime in captivity
through successive generations on genetic diversity, and the nature of phylogenetic relationships
present between populations.
Exact tests, carried out using Monte Carlo type multiple resampling techniques, and F-Statistics
were used to detect and quantify genetic structure among the twelve populations. The Exact test X2
(P < 0.001), a FST of 0.27 (P < 0.001), eST of 0.26, RsT of 0.28, and a <l>ST of 0.17 all indicated
significant structuring among the populations. The evident genetic structuring endorsed the practice
of maintaining the populations as separate genetic stocks, in separate tanks, in order to preserve
unique genetic material for aquaculture strain development.
Populations also exhibited some significant deviations from Hardy Weinberg equilibrium
characterised by an overall reduced heterozygosity across the loci. In microsatellite studies, null
alleles are often suggested as major contributors to heterozygote deficits. To test for null alleles,
two controlled crosses of 0. mossambicus were made. The progeny from each cross were examined
for expected parental allelic ratios at the UNHI04, UNHlll and UNH123 loci. All three loci
presented evidence of possible null alleles.
Accelerated inbreeding and genetic drift through successive generations in captivity can reduce
heterozygosity and gene diversity. To investigate loss of diversity a sample taken from the
Bushmans population in 1999 (N = 25) was compared with a Bushmans 2000 sample (N = 36). The
comparison highlighted altered allele frequencies, a significant increase in average observed
heterozygosity and a non-significant change in average expected heterozygosity using the UNHI04
and UNH123 loci.
Calculation of genetic distances and phylogenetic comparisons between the populations provided
insight into the degree of management required in conserving genetic diversity in natural
populations of Mozambique tilapia. UPGMA and Neighbour-Joining techniques were used to
construct phylogenetic trees using Dm and ({)~)2 distance matrices. Clustering of populations
appeared to reflect geographic locality of the source populations, however certain populations were
not congruent with geography. Mantel tests were used to expose a possible association between
genetic distance matrices generated from each individual locus. An association would support a geographic background to population genetic structure. The Mantel tests did not provide conclusive
evidence. Mantel tests for association between the combined locus Dm and (81l)2 genetic distance
matrices and a geographic distance matrix were similarly non-significant.
Multi-dimensional scaling (MDS) plots of Euclidean distance values for Dm and (81l)2 matrices
presented a two-dimensional view of the genetic distance data. The degree of similarity with the
UPGMA and Neighbour-Joining tree-clustering pattern was higher for the (81l)2 than for the Dm
MDS plots. Scatter plots indicated a reliable non-linear correlation between Euclidean distance and
genetic distance for the two-dimensional MDS.
The micro satellite markers employed in this research provided molecular information needed for
complimenting a co-study on quantitative genetic evaluation of the twelve populations. The
quantitative co-study provided measures of average length and weight gain indices for the
populations based on progeny growth trials. No significant correlation of average heterozygosity
(gene diversity) with either average weight or length gain was found.
The significant genetic diversity and structure present between the twelve populations provided
rationale for implementing strategies to conserve natural 0. mossambicus populations as genetic
resources, and manage captive populations for long term maintenance of genetic diversity. / AFRIKAANSE OPSOMMING: Die verstaffing van groot alleliese variasie deur DNA mikrosateliete maak van hulle gerieflike
merkers vir navorsing in 'n verskeidenheid van velde wat gebruik maak van populasie genetiese
gereedskap, ingesluit akwakultuur en bewarings genetika. Twaalf 0. mossambicus populasies wat
verkry was vanuit die natuur, in gevangeneskap en ingevoerdes, van Suidelike Afrika was getoets
vir genetiese variasie by tien verskillende CA-herhalende mikrosateliet loci. Drie van die loci -
UNHI04, UNHlll en UNH123 - is op grootskaal getoets en volgens In model van Mendeliese
oorerwing geinterpreteer. Die data was ontleed volgens genetiese struktuur en vlakke van genetiese
variasie, die effek wat bestuur strategie in gevangeneskap op genetiese diversiteit in opeenvolgende
generasies uitgeoefen het, so wel as die aard van die filogenetiese verhoudings wat teenwoordig is
tussen die populasies.
"Exact" toetse is uitgevoer deur gebruik te maak van Monte Carlo tipe veelvuldige
hermonsterinsamelings tegnieke en F-statistieke is gebruik vir die deteksie en kwantifisering van
die genetiese struktuur tussen die twaalfpopulasies. Die Exact toets X2 (P < 0.001), 'n FST van 0.27
(P < 0.001), SST van 0.26, RsT van 0.28, en 'n <DST van 0.17 gee almal 'n indikasie van
betekenisvolle strukturering tussen die populasies. Die genetiese struktuur bevestig die beleid dat
die populasies behou moet word as aparte genetiese voorraad, in aparte tenke, om te verseker dat
die unieke genetiese materiaal behoue bly om akwakultuur variante te ontwikkel.
Populasies het ook betekenisvolle verskuiwings van die Hardy Weinberg ekwilibrium getoon, wat
gekarakteriseer word deur 'n algemene verlaging van heterosigositeit oor die loci. Nul allele word
dikwels aanbeveel om in mikrosateliet studies groot bydraes te maak tot hetersigotiese defekte. Om
vir nul allele te toets was twee gekontroleerde kruisings van 0. mossambicus gemaak. Die nageslag
van elke kruising was getoets vir verwagte ouer alleliese verhoudings by die UNHI04, UNHlll en
UNH123 loci. Al drie loci het getoon dat dit moontlike nul allele kan wees.
Versnelde inteling en genetiese drywing deur opeenvolgende generasies in gevangeneskap kan die
heterosigositeit en diversiteit verminder. Om die vermindering van diversiteit te toets was 'n
monster van die Busmans 1999 (N=25) populasie vergelyk met 'n monster van die Bushmans 2000
(N=36) populasie. Die vergelyking het veranderde alleel frekwensies, 'n betekenisvolle vermeerding
in gemiddelde waargeneemde heterosigositeit en 'n onbetekenisvolle verandering in gemiddelde
verwagte heterosigositeit getoon deur gebruik te maak van die UNHI04 en UNH123 loci.
Berekening van genetiese afstande en filogenetiese vergelykings tussen die populasies het nuwe
insig gegee oor die graad van bestuur wat nodig is om genetiese diversiteit in die natuurlike
populasies van 0. mossambicus tilapia te behou.UPGMA en Neighbour-Joining tegnieke was
gebruik om filogenetiese bome op te stel deur gebruik te maak van Dm en (OIl)2 afstand matrikse.
Populasie bondeling het geblyk om geografiese lokaliteit van die bron populasies te toon, alhoewel van die populasies nie met die geografiese lokaliteit ooreengestem het nie. Mantel toetse is gebruik
om 'n moontlike assosiasie tussen genetiese afstand matrikse wat verkry is van elke loci bloot te
stel. 'n Assosiasie sou 'n geografiese agtergrond tot populasie genetiese struktuur steun. Oortuigende
bewys is nie deur die Mantel toetse verskaf nie. Mantel toetse vir assosiasie tussen die
gekombineerde loci Dm en (OJ..l)2 genetiese afstand matrikse en In geografiese afstand matriks was
ook onbetekenisvol.
'n Tweedimensionele beskouing van die genetiese afstand data is voorgestel deur multidimensionele
skaal (MDS) grafieke van Euclidean afstand waardes van die Dm en (OJ..l)2 matrikse te
teken. Die graad van ooreenstemming met die UPGMA en Neighbour-Joining boom samevoeging
patroon was hoër vir die (OJ..l)2 as vir die DmMDS grafieke. Verspreiding grafieke het 'n vertroubare
nie-liniêre korrelasie tussen Euclidean afstande en genetiese afstande vir die twee-dimensionele
MDS grafieke getoon.
Die mikrosateliet merkers wat in die studie gebruik was het molekulêre informasie verskaf wat
nodig is vir 'n komplimentêre studie oor die kwantitatiewe genetiese evalueering van dié twaalf
populasies. Die kwantitatiewe studie het afmetings van gemiddelde lengte en gewig vermeerdering
van die populasies verskaf gebaseer op nageslag proewe. Geen betekenisvolle korrelasie van
gemiddelde hetersigositeit (geen diversiteit) was getoon met óf gemiddelde gewig óf lengte
vermeerdering.
Die betekenisvolle genetiese diversiteit en struktuur teenwoordig tussen die twaalf populasies het
rede gegee om strategieë te implimenteer om natuurlike 0. mossambicus populasies te konserveer
as genetiese bronne en om populasies in gevangeneskap te bestuur vir langtermyn instandhouding
van genetiese diversiteit.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52093 |
| Date | 12 1900 |
| Creators | Hall, Edward G. |
| Contributors | Lewis, Rupert, Brink, Danie, Stellenbosch University. Faculty of Science. Dept. of Microbiology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | xvi, 102 p. : ill. |
| Rights | Stellenbosch University |
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