Clinal variation and genetic divergence in Acacia tumida (Mimosaceae)

Abstract

Acacia tumida F.Muell ex. Benth. is a morphologically variable species from north-western Australia. Variants range from low, multi-stemmed shrubs to single-stemmed trees. Acacia tumida is cultivated for firewood, windbreaks and sand stabilisation. To examine patterns of genetic variation that could be utilised in domestication programs, 22 populations representing its natural distribution were surveyed by using allozymes. Estimates of genetic diversity (H_E = 0.149) were within the range of other tropical, widespread woody plants, but varied widely among populations (H_E = 0.064–0.203). There were strong clinal trends in diversity estimates; A, P, H_O, H_E were correlated with longitude and declined from west to east. H_E decreased as mean monthly maximum temperatures increased and allelic richness (A) increased with mean minimum monthly temperatures. There were also significant correlations between allele frequencies at certain loci and geographic and climatic parameters. Significant isolation-by-distance and relatively high levels of differentiation (θ = 0.176) were detected among populations. Populations of the narrow-phylloded form (Pilbara region) were the most genetically divergent, despite their geographic proximity to other populations. The low-shrub and tall-tree forms could not be distinguished by allozyme profiles. Slight genetic differences were detected between the non-pruinose and pruinose forms from within one population. Phylogenetic analysis supported morphological evidence of hybridisation between A. tumida and A. difficilis Maiden in their zone of overlap. The results are discussed in relation to the taxonomy and domestication of A. tumida.