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United States Department of Agriculture

Agricultural Research Service

Research Project: BIOLOGICAL CONTROL OF INVASIVE PLANTS OF THE NORTHERN GREAT PLAINS

Location: Pest Management Research Unit

Title: Molecular evidence of hybridization in Florida's sheoak (Casuarina spp.) invasion

Authors
item Gaskin, John
item Wheeler, Gregory
item Purcell, Matt - ARS-CSIRO
item Taylor, Gary - UNIVERSITY OF ADELAIDE

Submitted to: Molecular Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 15, 2009
Publication Date: May 27, 2009
Repository URL: http://hdl.handle.net/10113/44921
Citation: Gaskin, J.F., Wheeler, G.S., Purcell, M., Taylor, G. 2009. Molecular evidence of hybridization in Florida's sheoak (Casuarina spp.) invasion. Molecular Ecology. 18(15):3216-3226.

Interpretive Summary: Three Casuarina tree species, C. glauca, C. cunninghamiana, and C. equisetifolia, native to Australia, are naturalized in Florida, USA. Many Florida specimens are considered unidentifiable and may be hybrids. We collected tissue from over 500 trees from Australia and Florida and genotyped these using DNA methods. Our goal was to determine the exact identity of the Florida species, including any new hybrids. In Australia, we found the three parental species, and no evidence of any hybridization. In Florida, we found many trees with strong assignment to one of the three species, and also novel hybrids. The presence of novel hybrid genotypes in the Florida invasion may be problematic, as continued cross-pollination can increase levels of genetic diversity of the invasion, providing further opportunities for the evolution of invasive traits in these species and hybrids. As no hybrids were detected in the native range, the novel hybrids in Florida have essentially no evolutionary history with any insects or diseases, which may be problematic for biological control efforts.

Technical Abstract: Three Casuarina tree species, C. glauca, C. cunninghamiana, and C. equisetifolia, native to Australia, are naturalized in Florida, USA. Many Florida specimens are considered unidentifiable, presumably due to interspecific hybridization. We collected tissue from over 500 trees from Australia and Florida and genotyped these using AFLPs. Our goal was to determine the exact identity of the Florida species, including any putative hybrid combinations. We used principle coordinates analysis and Bayesian clustering and assignment tests on the AFLP data to determine number of genotype clusters in trees collected from the native range, then used that information to assign trees from Florida to species. In Australia, we found high assignment values to the three parental species, and no evidence of any hybridization. In Florida, we found many trees with strong assignment to one of the three species, and an additional cluster of closely related genotypes with assignment values intermediate to C. glauca and C. equisetifolia, suggesting hybridization. For 67 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 these two parental species’ haplotypes. We found only weak support for hybridization between C. glauca and C. cunninghamiana, and no support for hybridization between C. cunninghamiana and C. equisetifolia. For C. glauca and C. equisetifolia, genetic diversity in Florida was higher than in the native range collections. The presence of novel hybrid genotypes in the Florida invasion may be problematic, as continued interspecific gene flow can increase levels of genetic diversity of the invasion, providing further opportunities for the evolution of invasive traits in these species and hybrids. As no hybrids were detected in the native range, the novel hybrids in Florida have essentially no coevolutionary history with any insects or diseases, which may be problematic for biological control efforts.

Last Modified: 8/22/2014