Genetic variation with Acacia karroo hayne

Abstract

The assessment and sustainable management of biodiversity have recently become the subject of increasing global concern. Recent advances in biochemical and molecular genetics have provided improved techniques for rapid assessment of the level and the distribution of genetic diversity; knowledge of both is a prerequisite for genetic conservation and improvement. This study involved the use of cytological and isozyme techniques to characterize the genetic diversity and the mating system of Acacia karroo Hayne. A. karroo is the most widespread acacia in southern Africa, and is a source of many products, including fuelwood, timber, forage, fencing material, cordage, medicine, tannin, dye, sweet gum and honey. As a species adapted to harsh environments, A. karroo has an important role. in soil enrichment and protection, and in microclimatic regulation. This study was based on populations sampled across the natural range of A. karroo. Based on cytological evidence from 10 populations surveyed, A. karroo was found to be a tetraploid, consisting of only one cytotype, with a chromosome number of 2n = 52. The tetraploid status was further supported by evidence from the isozyme inheritance patterns, which also indicated that the species is a segmental tetraploid. All 12 populations surveyed for allozyme variation expressed a high level of genetic diversity. Ninety eight percent of the isozyme loci were polymorphic. The total gene diversity (HT) was 88%, higher than that reported for other plant species; the mean number of alleles per locus was 3.7. The mean genetic identity was 90% within the expectations of conspecificity, and the coefficient of gene differentiation (Gst) was estimated at 5%, indicating low divergence between populations. Unweighted pairgroup and rooted dendrogram analyses clustered the populations into three phylogenetic groups, which may be characterized as the northern, eastern and southcentral- eastern groups. The distribution of some common alleles at the shikimate dehydrogenase and alcohol dehydrogenase loci were significantly correlated with some geographical factors, viz. latitude, longitude and rainfall. Estimates of mating system parameters in one population indicated that the species has a mixed mating system. Multilocus outcrossing rate was estimated at 0.88; singlelocus outcrossing estimates were heterogeneous, ranging from 0.53 to ~1.00, confirming the variable characteristics of the loci. The mean outcrossing rate, estimated using fixation indices for all 12 populations, was 0.72; the gene flow rate (Nm)was estimated at 4.47, consistent with the entomophilous pollination mechanism reported for other tree species. Gene distribution within the population studied showed no specific spatial pattern. The results of this study suggest that the gene pool of A. karroo can be sampled efficiently from relatively few trees within each of the three cluster groups, in conjunction with a systematic sampling for localized alleles. The high level of within population variability indicates a high potential for genetic improvement, while the mixed mating system ensures exchange of genes between selected trees. However, due to lack of evidence of correlations between genetic distribution and phenotypic or ecological variants, additional sampling based on morphological and adaptive traits is recommended.