Evidence of Tamarix hybrids in South Africa, as inferred by nuclear ITS and plastid trnS–trnG DNA sequences

Abstract

Tamarix usneoides (Tamaricaceae) is a species native to southern Africa where it is currently being used in the mines for phytoremediation. Tamarix aphylla, Tamarix ramosissima, Tamarix chinensis, and Tamarix parviflora have been reported as exotic species in South Africa, with T. ramosissima declared invasive. The alien invasive T. ramosissima is hypothesized to be hybridizing with the indigenous T. usneoides. Accurate identification of Tamarix is of great importance in southern Africa because of the invasive potential of T. ramosissima and also the potential usefulness of T. usneoides. In this study, nuclear DNA sequence markers (ITS1 and ITS2 regions), together with the plastid marker trnS–trnG, are used to identify the genetic distinctiveness of Tamarix species and their putative hybrids. Phylogenies based on the ITS and trnS–trnG regions revealed that the indigenous T. usneoides is genetically distinct from the exotic species, which, however, could not clearly be separated from their closely related hybrids. The lack of congruence (p>0.0001) between the ITS and trnS–trnG phylogenies suggests that there is high incidence of hybridization in Tamarix populations in South Africa. Importantly, molecular diagnosis of Tamarix was able to identify hybrids using polymorphisms and phylogenetic signals. Close to 45% of Tamarix genotypes were hybrids with more than 50% of them occurring on the mines. Spread of Tamarix hybrids in South Africa through phytoremediation could enhance invasiveness. Therefore, the outcome of this study will ensure that only pure indigenous T. usneoides is propagated for planting on the mines in South Africa and that a proper control measure for the alien invasive Tamarix is used. Interestingly, the molecular diagnosis of Tamarix species supported the preliminary morphological identification of the species using eight key characters. However, the molecular markers used were not informative enough to separate hybrids from their closely related parent species. Hybrids were more reliably identified using polymorphisms than morphological features.