Using molecular methods to determine the origin of weed populations of Pereskia aculeata in South Africa and its relevance to biological control

New biological control agents are required in order to reach and sustain an adequate level of control of the declared environmental weed Pereskia aculeata Miller (Cactaceae) in South Africa. Identifying the origin of weed genotypes has been important in a number of biological control programmes and is likely to be of importance for the control of P. aculeata due to its disjunct native distribution and morphological polymorphisms between plants from different regions of the native and introduced distribution. DNA sequencing of the trnL chloroplastic intron and the phyC nuclear gene indicate that the South African weed population’s origin was in the southern region of native distribution. Inter-Simple Sequence Repeats (ISSRs) confirmed this result and added resolution to the analysis indicating that the native plants with the closest genetic distance to the South African weed population were found in Rio de Janeiro Province, Brazil. The relationship between the South African weed population and garden variety plants as well as the large genetic distance between the South African plants and the native plants suggests that the South African population may be the progeny of escaped garden variety plants that have been cultivated and possibly hybridized. The low levels of genetic variation within the South African population and the monophyly of the South African plants indicates that these plants are the progeny of a single introduction or multiple introductions from the same source. Rio de Janeiro Province in Brazil is the most appropriate region in which to survey for new biological control agents.