Species limits in Vachellia (Acacia) karroo (Mimosoideae: Leguminoseae): Evidence from automated ISSR DNA “fingerprinting”

Vachellia karroo (Hayne) Banfi & Galasso, previously known as Acacia karroo Hayne, is a very common woody species in South Africa, and displays a large amount of morphological and architectural variation. Previous morphological studies have provided evidence to support the recognition of a number of segregate species, but this has not been supported by early protein electrophoresis studies. Genetic variability of the species throughout South Africa was examined by means of the Inter-Simple Sequence Repeat (ISSR) DNA “fingerprinting” to determine whether there is any genetic structure that correlates to the morphological diversity. Based upon 33 samples from across the South African distribution of this species, we found no evidence for any genetic structuring, suggesting the species is one panmictic entity. The existence of morphologically discrete varieties in light of this genetic finding is difficult to explain, and perhaps the species ought to be considered as an ochlospecies. A scenario whereby the various morphotypes evolved in refugia during previous climates followed by subsequent expansion and introgression during the current interglacial may explain this conundrum. Alternatively, the evolution of the morphotypes has been recent and rapid, and the genetic variation observed here represents the ancestral gene pool that has not yet undergone lineage sorting as a consequence of isolation.