DNA barcode assessment of Xanthium section Xanthium cockle burrs in Australia

Abstract Xanthium cockle burrs are invasive weeds in many countries, affecting local ecosystems and agriculture. All Xanthium in Australia are naturalised weeds from multiple overseas sources, but their taxonomic identities are contentious. In particular, species identities of four burr varieties or “complexes” under Xanthium section Xanthium in Australia are unclear. These complexes, commonly referred to as “Noogoora”, “Californian”, “Hunter”, and “South American” burr, are regarded as either varieties of a single species ( Xanthium strumarium ) or four separate species in the section. To genetically determine the species identities of these complexes in Australia, we compared their chloroplast DNA barcodes to reference sequences of vouchered Xanthium species reported by Tomasello (2018). We barcoded 173 plants from 26 sites consisting of single complex burrs and 175 plants from sites containing mixed burr complexes. Single complex sites contained either of two DNA barcode clades. One clade consisting of Noogoora and Californian burrs matched reference sequences of X. chinense . The second clade consisting of Hunter and South American burrs matched X. orientale . Genetic exchange between these two species in Australia was partially evident at some sites containing sympatric complexes or putative hybrids. Reference X. strumarium sequences were unmatched to the DNA barcodes of specimens sampled in Australia, and subsequently, we advise against future claims to a historical presence of this species in Australia. Our findings are significant for taxonomic treatment of Xanthium species in Australia and for consideration of source-specific biological controls of these weeds.