The importance of species identity in the biocontrol process: identifying the subspecies of Acacia nilotica (Leguminosae: Mimosoideae) by genetic distance and the implications for biological control

Abstract

Aims A molecular genetic distance study has been used in an initial survey to identify subspecies and genotypes of the weed Acacia nilotica in Australia, information needed to find suitable biocontrol agents. We use patterns of DNA sequence variation (in two DNA fragments) from each of the nine described subspecies of Acacia nilotica (L.) Delile (Leguminosae: Mimosoideae) that is to determine their genetic similarity, to verify if the Australian populations are A. nilotica ssp. indica (Benth.) Brenan, and to establish if any other subspecies are present in Australia.Location Australia and southern Africa through the Arabian peninsular to the Indo-Pakistan subcontinent.Methods Representative specimens from the global distribution of the nine A. nilotica subspecies were sourced primarily from herbaria sheet specimens where available, and secondarily from field collections. These specimens together with related outgroups from Mimosoideae were genetically analysed using the DNA fragments trnL and internal transcribed spacer one (ITS1). We calculated a similarity index as set out in paup* using upgma (Unweighted Pair-Group Method Arithmetic average) methods to cluster taxa to produce a genetic distance phenogram.Results Sequence results from ITS1 and trnL DNA fragments identified seven of the described subspecies of A. nilotica. Acacia nilotica ssp. cupressiformis (J. Stewart) Ali & Faruqi and A. nilotica ssp. adstringens (Schumach. & Thonn.) Roberty were not found to be genotypically distinct from A. nilotica ssp. indica and A. nilotica ssp. nilotica, respectively, based on the two DNA fragments. Subspecific ITS1 genotypes were geographically distributed similarly to previous reports that were based on morphology, with the exception that the hemispherica ITS1 genotype also occurred in Somalia. We confirmed that the Australian A. nilotica populations are mostly comprised of subspecies indica, but in addition, some individuals were found to be genetically identical to an unidentified Pakistan genotype not previously reported as occurring in Australia.Main conclusions Australian A. nilotica populations originated from India and Pakistan and we recommend further analysis to determine the complete genetic diversity profile and origins of the Australian populations. We highlight the importance of determining any hybridization between Australian populations of A. nilotica and native subgenus Acacia species. This study demonstrates the importance of genotyping weed species targeted for biocontrol and/or listed host specificity test species that may be easily misidentified.

A systematic approach to biological control agent exploration and prioritisation for prickly acacia (Acacia nilotica ssp. indica)

“…data 2005). This finding supports the need to search for coevolved biological control agents in India and Pakistan (Anonymous 1995;Wardill et al 2005).…”

“…Genetic studies have revealed that the invasive prickly acacia population in Australia is the subspecies A. nilotica indica, which is native to India and Pakistan (Wardill et al 2005). This is further supported by plant morphological (Brenan 1983) and biochemical (Hannan-Jones 1999) studies.…”

Plio-Pleistocene history and phylogeography of Acacia senegal in dry woodlands and savannahs of sub-Saharan tropical Africa: evidence of early colonisation and recent range expansion

“…In Africa, there have been three major phylogeographic studies of African tree species found in these biomes. A study of Acacia nilotica populations showed genetic differences that broadly matched subspecific designations and a phylogeographic separation of North and West Africa from East and Southern Africa (Wardill et al, 2005). The other two studies, of the baobab tree (Adansonia digitata L., Malvaceae; Pock Tsy et al, 2009) and the shea tree (Vitellaria paradoxa C. F. Gaertn, Sapotaceae; Allal et al, 2011) showed strong phylogeographic structure, distinguishing Eastern and Western populations within the Sudano-Sahelian region.…”

Maximising the contribution of native‐range studies towards the identification and prioritisation of weed biocontrol agents

“…Two DNA fragments, the trnL and the ITS-1, were used to identify eight of nine described subspecies of A. nilotica (L.) Delile and to report a previously unknown genotype from Pakistan (Wardill et al 2005). The study also indicated that the populations of this plant that are very serious weeds in Australia are mainly A. nilotica ssp.…”