Abstract
Biological invasions are drastically altering natural habitats and threatening biodiversity on both local and global levels. In one of the United States' worst invasions, Eurasian Tamarix plant species have spread rapidly to dominate over 600,000 riparian and wetland hectares. The largest Tamarix invasion consists of Tamarix chinensis and Tamarix ramosissima, two morphologically similar species. To clarify the identity, origins, and population structuring of this invasion, we analyzed DNA sequence data from an intron of a nuclear gene, phosphoenolpyruvate carboxylase (PepC). This intron proved to be highly variable at the population level, and the 269 native and invasive specimens yielded 58 haplotypes, from which we constructed a gene genealogy. Only four of these haplotypes were common to both the U.S. and Eurasia. Surprisingly, we found that the most common plant in this U.S. invasion is a hybrid combination of two species-specific genotypes that were geographically isolated in their native Eurasian range. Less extensive hybrids exist in the invasion, involving combinations of T. ramosissima and T. chinensis with Tamarix parviflora and Tamarix gallica. The presence of potentially novel hybrids in the U.S. illustrates how importation of exotics can alter population structures of species and contribute to invasions.