Molecular evidence of hybridization in Florida’s sheoak (Casuarina spp.) invasion



    1. USDA Agricultural Research Service, Northern Plains Agricultural Research Laboratory, 1500 North Central Avenue, Sidney, MT 59270, USA
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    1. USDA Agricultural Research Service, Invasive Plant Research Laboratory, 3225 College Avenue, Fort Lauderdale, FL 33314, USA
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    1. USDA Agricultural Research Service, Australian Biological Control Laboratory, CSIRO Entomology, 120 Meiers Road, Indooroopilly, Qld 4068, Australia
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    1. Australian Centre for Evolutionary Biology and Biodiversity, and School of Earth and Environmental Sciences, The University of Adelaide, North Terrace, Adelaide, SA 5005, Australia
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  • John Gaskin is a botanist whose research is focused on population genetics and systematics of invasive plant taxa. Gregory Wheeler and Matthew Purcell are both entomologists studying the biological control of invasive plants. Gary Taylor is an entomologist interested in coevolutionary patterns of Casuarinas and associated herbivores.

John F. Gaskin, Fax: 406 433 5038; E-mail:


The presence of hybrids in plant invasions can indicate a potential for rapid adaptation and an added level of complexity in management of the invasion. Three Casuarina tree species, Casuarina glauca, Casuarina cunninghamiana and Casuarina equisetifolia, native to Australia, are naturalized in Florida, USA. Many Florida Casuarina trees are considered unidentifiable, presumably due to interspecific hybridization. We collected tissue from over 500 trees from Australia and Florida and genotyped these using amplified fragment length polymorphisms. Our goal was to determine the exact identity of the Florida species, including any putative hybrid combinations. In Australia, we found high assignment values to the three parental species, and no evidence of hybridization. In Florida, we found many trees with strong assignment to any one of the three species, as well as 49 trees with assignment values intermediate to C. glauca and C. equisetifolia, suggesting hybridization between these species. One population of 10 trees had assignment values intermediate to C. cunninghamiana and C. glauca, suggesting additional hybridization. For 69 of these putative hybrid and parental types, we sequenced a low-copy intron of nuclear G3pdh, and these sequences indicated that some Florida trees contain heterozygotic combinations of C. glauca and C. equisetifolia haplotypes. The presence of novel hybrids in the Florida invasion may enhance evolution of invasive traits in these species. Novel Casuarina hybrids in Florida have no coevolutionary history with any insects or diseases, which may be problematic for biological control efforts.