Elucidating the native sources of an invasive tree species, Acacia pycnantha, reveals unexpected native range diversity and structure

Abstract

Background and aims: 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.

Key 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.

Fig. 1.

Herbarium specimens of the extreme ecotypes of Acacia pycnantha in Australia for the dryland form (A) characterized by thin phyllodes (C) and the wetland form (B) characterized by broad phyllodes (D).

Fig. 2.

Haplotype network of Acacia pycnantha (based on the rpl32-trnL^(UAG) region) and geographical distribution of haplotypes in the native south-eastern Australia and invasive South Africa and western Australian ranges. Each unique haplotype is represented by a coloured circle and the size of the circle is proportional to the number of individuals possessing that haplotype. Pie charts represent the proportion and distribution of haplotypes across native (South Australia and New South Wales) and invasive (Western Australia and South Africa) ranges.

Fig. 3.

Bayesian tree for Acacia pycnantha using combined ITS and ETS regions. Numbers above the branches represent posterior probabilities derived from the analysis. Invasive taxa are indicated by asterisks for Australian, Portuguese and South African collections, as indicated in the key; also indicated are wetland, Victoria dryland and Flinders Range dryland forms of A. pycnantha.

Fig. 4.

Mismatch distributions for DNA sequence data of the plastid (A) and nuclear (B) genomes for Acacia pycnantha in Australia (native range). The solid line represents the expected mismatch distribution of a constant-size population and the dotted line represents the observed mismatch distribution based on A. pycnantha DNA sequence data.

Fig. 5.

Refuge areas and geographical barriers in Australia during the Pleistocene. Shaded areas represent refugia in Australia and those with black dots represent refugia for Acacia pycnantha (Eyre Peninsula and Mt Lofty). The arrows show the Eyrean and Mallee geographical barriers (Ford et al., 1987).