Geographic distribution and genotypic composition of invasive hybrid watermilfoil (Myriophyllum spicatum x M-sibiricum) populations in North America

The recent recognition of invasive hybrid watermilfoil (Myriophyllum spicatum x M. sibiricum) in North America has necessitated a more thorough evaluation of its overall distribution and occurrence in natural populations. A comprehensive survey of watermilfoil populations was conducted in five Minnesota lakes, three of which were suspected a priori to contain hybrid watermilfoil. DNA sequence data verified that hybrid plants between the nonindigenous M. spicatum L. and indigenous M. sibiricum Kom. occurred in three of the five lakes sampled. Myriophyllum spicatum was not detected in lakes where hybrids were prevalent. Further sampling of lakes in Idaho, Michigan, Minnesota, Wisconsin and Washington identified 30 additional hybrid watermilfoil populations. In only three of these populations the hybrid watermilfoil was found to co-occur with M. spicatum. To facilitate the field identification of the two parental species and their hybrid, morphological data from watermilfoil specimens collected across the United States were evaluated. We determined that leaf segment/leaf length measurements can effectively distinguish M. spicatum and M. sibiricum; however, hybrids are intermediate for these characters and such measurements frequently overlap with respect to their parental taxa. By incorporating a combined molecular and morphological approach to identifying watermilfoils, the hybrids can be identified readily and their distributions elucidated both within and between lakes. Because hybrids may respond differently to local ecological conditions than their parents, information on their presence and distribution should be of particular importance to management and conservation programs.