An analysis of population structure using microsatellite DNA in twelve Southern African populations of the Mozambique tilapia, Oreochromis mossambicus (Peters)

[摘要] ENGLISH ABSTRACT:DNA micro satellite loci express extensive allelic variation making them convenient markers forresearch in many fields employing population genetic tools, including aquaculture and conservationgenetics. Twelve Oreochromis mossambicus populations from wild, captive and introduced sourcesin Southern Africa were screened for genetic variation at ten CA repeat micro satellite loci. Three ofthe loci - UNHI04, UNHlll, and UNH123 - were sufficiently well resolved to screen extensivelyand were interpreted according to a model of Mendelian inheritance. Data was analyzed in terms ofgenetic structure and levels of genetic variation, the effect of management regime in captivitythrough successive generations on genetic diversity, and the nature of phylogenetic relationshipspresent between populations.Exact tests, carried out using Monte Carlo type multiple resampling techniques, and F-Statisticswere 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 ST of 0.17 all indicatedsignificant structuring among the populations. The evident genetic structuring endorsed the practiceof maintaining the populations as separate genetic stocks, in separate tanks, in order to preserveunique genetic material for aquaculture strain development.Populations also exhibited some significant deviations from Hardy Weinberg equilibriumcharacterised by an overall reduced heterozygosity across the loci. In microsatellite studies, nullalleles 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 examinedfor expected parental allelic ratios at the UNHI04, UNHlll and UNH123 loci. All three locipresented evidence of possible null alleles.Accelerated inbreeding and genetic drift through successive generations in captivity can reduceheterozygosity and gene diversity. To investigate loss of diversity a sample taken from theBushmans population in 1999 (N = 25) was compared with a Bushmans 2000 sample (N = 36). Thecomparison highlighted altered allele frequencies, a significant increase in average observedheterozygosity and a non-significant change in average expected heterozygosity using the UNHI04and UNH123 loci.Calculation of genetic distances and phylogenetic comparisons between the populations providedinsight into the degree of management required in conserving genetic diversity in naturalpopulations of Mozambique tilapia. UPGMA and Neighbour-Joining techniques were used toconstruct phylogenetic trees using Dm and ({)~)2 distance matrices. Clustering of populationsappeared to reflect geographic locality of the source populations, however certain populations werenot congruent with geography. Mantel tests were used to expose a possible association betweengenetic distance matrices generated from each individual locus. An association would support a geographic background to population genetic structure. The Mantel tests did not provide conclusiveevidence. Mantel tests for association between the combined locus Dm and (81l)2 genetic distancematrices and a geographic distance matrix were similarly non-significant.Multi-dimensional scaling (MDS) plots of Euclidean distance values for Dm and (81l)2 matricespresented a two-dimensional view of the genetic distance data. The degree of similarity with theUPGMA and Neighbour-Joining tree-clustering pattern was higher for the (81l)2 than for the DmMDS plots. Scatter plots indicated a reliable non-linear correlation between Euclidean distance andgenetic distance for the two-dimensional MDS.The micro satellite markers employed in this research provided molecular information needed forcomplimenting a co-study on quantitative genetic evaluation of the twelve populations. Thequantitative co-study provided measures of average length and weight gain indices for thepopulations 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 providedrationale for implementing strategies to conserve natural 0. mossambicus populations as geneticresources, and manage captive populations for long term maintenance of genetic diversity.

[发布机构] Stellenbosch University