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University of Stellenbosch (2010)

Population genetics of bush-encroaching acacia mellifera at Pniel, Northern Cape Province, South Africa

Nxele, Beka Jeremia

Titre : Population genetics of bush-encroaching acacia mellifera at Pniel, Northern Cape Province, South Africa

Auteur : Nxele, Beka Jeremia

Université de soutenance : University of Stellenbosch

Grade : MScConEcol (Conservation Ecology and Entomology) 2010

Résumé
Two populations of Acacia mellifera were noted in Pniel, which is a semi-arid savanna, near Kimberley in the Northern Cape province of South Africa. One population appeared on a rocky, andesitic laval ridges (soil pHKCL 6.5-7.0) along the Vaal river. The other appeared in a sandveld area (soil pHKCL 4). Bush encroachment by A. mellifera was found to be more extensive on the rocky areas than in the sandveld and the two habitats differed extensively on soil pH, clay and silt contents and also on water holding capacities. The rocky habitat was thus deduced to have a higher CEC. Seeds were sampled on a logarithmic scale for allozyme analysis and also randomly in each of the two habitats for local adaptation tests, in which case lime (CaCO3) and organic matter (cow-dung) were used in a completely-crossed design. Detected interaction effects (between population source and pH ; population source and organic matter and between pH and organic matter) and significant differences could not separate the two populations as the differences occurred across populations. Random genetic differences leading to phenotypic plasticity in the two observed populations, might be responsible for the observed phenotypic differences. Allozymic data showed no significant differences between the two populations and the genetic distance between and within the populations also confirmed that the two populations had not genetically differentiated. The Mantel Test on the two populations, showed nonsignificant results. Nei‟s UPGMA dendrogram revealed that the game farm subpopulations were more primitive and genetically related to each other. Despite differences in allozyme frequencies, between the sampled sites, genetic differentiation was found to be low (FST = 0.337). Nei‟s (1972) original measures of genetic distance ranged between 0.871 and 1.000 with a mean of 0.949 ± 0.053. The study concluded that the two observed populations had not genetically differentiated and no local adaptation could be established rather phenotypic plasticity was evident and resulted in the observed divergent growth forms. Nonetheless, the overall direction of spread of encroachment appeared to be the eastward.

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