Lythrum salicaria is much more invasive in North America than its native Europe (Batra et al. 1986). One possibility for how it became more invasive is that it hybridized with a close relative in North America, such as L. alatum, and gained genes that made it better adapted to its new environment. This hypothesis was first tested by searching for morphological evidence of hybridization. By examining a variety of purple loosestrife populations across the northeastern United States, several traits were found that are not present in European populations of purple loosestrife but are found in North American winged loosestrife. These unique morphs found in North American purple loosestrife suggest hybridization between the two species. In support of this, we identified intermediate sized L. salicaria where the two species grow sympatrically and could have hybridized.; Amplified fragment length polymorphism (AFLP) markers were then used to further analyze the relationship between L. salicaria and L. alatum, and search for evidence of hybridization. Twenty-seven North American and eleven European populations of L. salicaria and L. alatum were screened with 5 primer pairs, and then eight Michigan populations (selected for allopatry or sympatry between the two species) were screened with an additional 18 primer pairs. When the resulting patterns of molecular diversity were examined, North American L. salicaria clustered more closely to European L. salicaria than to North American L. alatum, and no evidence of inter-species hybridization was found. However, a considerable amount of differentiation was observed among the North American L. salicaria. This differentiation may be the real reason why L. salicaria became so successful in North America. |