Invasive water milfoil (Myriophyllum) populations in North America are believed to represent nonindigenous species that have become ecologically aggressive outside of their native range. It has often been difficult to characterize invasive populations of M. heterophyllum and M. spicatum morphologically relative to closely related native North American milfoil species. Molecular studies of these presumed M. "heterophyllum" and M. "spicatum" populations have revealed widespread polymorphisms in biparentally inherited nrDNA ITS sequences. Subclones of these polymorphic regions revealed the occurrence of distinct sequences matching those acquired from both native and nonindigenous North American species. These data clearly demonstrate that invasive water milfoil populations in North America have resulted from hybridization. Data from uniparentally inherited cpDNA markers show hybridization to occur both uni- and bidirectionally. Invasive populations of M. "heterophyllum" in New England typically represent hybrids between M. heterophyllum (native to eastern USA) and M. pinnatum (native to southeastern USA). Morphologically aberrant invasive populations of M. "spicatum" in North America have been found to represent hybrids between M. spicatum (Eurasian) and M. sibiricum (native North American). We suggest that invasivity in these aggressive aquatic weeds may be linked to heterosis maintained by vegetative propagation.

Key words: hybridization, invasive aquatic plants, ITS, M. heterophyllum, M. spicatum, Myriophyllum