Najas minor (Hydrocharitaceae) in North America: A reappraisal

Genetic studies of nonindigenous populations can help to determine their geographical origin, whether single or multiple introductions have occurred, and provide evidence of hybridization. We broadly surveyed Najas minor populations at several nuclear and chloroplast loci. Sequence data were obtained from nonindigenous N. minor populations in North America and portions of its native range in Europe (Italy) and Asia (China, Korea). North American populations were mapped to evaluate the observed patterns of genetic variation geographically. We detected multiple genotypes of N. minor in collections originating from within Eurasia and North America. In North America, the presence of two, distinct, widespread genotypes indicated at least two separate introductions. These genotypes shared partial genetic similarity with accessions from China. Two other genotypes (restricted to Michigan, Ohio, and West Virginia) represented either additional introductions of hybrid plants, and/or post-introduction hybrid events that occurred in North America. The existence of several genotypes among nonindigenous North American N. minor populations indicates a history of multiple introductions, and raises concerns over the potential proliferation of novel hybrid genotypes in this region. Because the two main genotypes essentially remain allopatric, management efforts should focus on preventing their contact, which could result in proliferation of hybrid plants with enhanced invasive attributes.