title: Elucidating the native sources of an invasive tree species, Acacia pycnantha, reveals unexpected native range diversity and structure.
abstract: OBJECTIVE
  Understanding the introduction history of invasive plant species is important for their management and identifying effective host-specific biological control agents. However, uncertain taxonomy, intra- and interspecific hybridization, and cryptic speciation may obscure introduction histories, making it difficult to identify native regions to explore for host-specific agents. The overall aim of this study was to identify the native source populations of Acacia pycnantha, a tree native to south-eastern Australia and invasive in South Africa, Western Australia and Portugal. Using a phylogeographical approach also allowed an exploration of the historical processes that have shaped the genetic structure of A. pycnantha in its native range.
  
  
  METHODS
  Nuclear (nDNA) and plastid DNA sequence data were used in network and tree-building analyses to reconstruct phylogeographical relationships between native and invasive A. pycnantha populations. In addition, mismatch distributions, relative rates and Bayesian analyses were used to infer recent demographic processes and timing of events in Australia that led to population structure and diversification.
  
  
  RESULTS
  The plastid network indicated that Australian populations of A. pycnantha are geographically structured into two informally recognized lineages, the wetland and dryland forms, whereas the nuclear phylogeny showed little geographical structure between these two forms. Moreover, the dryland form of A. pycnantha showed close genetic similarity to the wetland form based on nDNA sequence data. Hybrid zones may explain these findings, supported here by incongruent phylogenetic placement of some of these taxa between nuclear and plastid genealogies.
  
  
  CONCLUSIONS
  It is hypothesized that habitat fragmentation due to cycles of aridity inter-dispersed with periods of abundant rainfall during the Pleistocene (approx. 100 kya) probably gave rise to native dryland and wetland forms of A. pycnantha. Although the different lineages were confined to different ecological regions, we also found evidence for intraspecific hybridization in Victoria. The invasive populations in Portugal and South Africa represent wetland forms, whereas some South African populations resemble the Victorian dryland form. The success of the biological control programme for A. pycnantha in South Africa may therefore be attributed to the fact that the gall-forming wasp Trichilogaster signiventris was sourced from South Australian populations, which closely match most of the invasive populations in South Africa.
authors.affiliation: Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa.
authors.forename: Joice
authors.initials: J
authors.lastname: Ndlovu
authors.forename: David M
authors.initials: DM
authors.lastname: Richardson
authors.forename: John R U
authors.initials: JR
authors.lastname: Wilson
authors.forename: Martin
authors.initials: M
authors.lastname: O'Leary
authors.forename: Johannes J
authors.initials: JJ
authors.lastname: Le Roux
mesh_terms: D000045:Acacia / Q000235:genetics*; D001315:Australia; D001483:Base Sequence; D001499:Bayes Theorem; D002467:Cell Nucleus / Q000235:genetics; D060146:Ecotype; D014644:Genetic Variation; D054627:Genome, Plastid / Q000235:genetics; D005843:Geography; D006239:Haplotypes; D058865:Introduced Species; D014197:Trees / Q000235:genetics*