The North American beachgrass, Ammophila breviligulata, is widespread along the Atlantic coast. Beachgrass populations also occur around the Great Lakes and Lake Champlain. Frank Seymour recognized the Champlain plants as a species distinct from Ammophila breviligulata, however the specific status of the Champlain beachgrass has remained in question. All of the inland populations are presumed to have been derived from coastal populations following the retreat of the Laurentide ice sheet. To reconstruct the biogeographical history of the North American beachgrasses and to resolve the relationship of the Champlain beachgrass to the rest of the group, we undertook a study of DNA sequence variation in the ITS region of nuclear ribosomal DNA. In our survey of 48 individuals representing 18 populations from across the range of North American Ammophila, we detected 17 different ITS variants. Although sequences from isolated populations were divergent, variation within populations was typically nil. Parsimony analyses of ITS sequence variation revealed four separate lineages within North American Ammophila: a northern coastal lineage, a southern coastal lineage, a Great Lakes lineage, and a Champlain/St. Lawrence lineage. Although these four clades were well supported, the relationship among them was not fully resolved. Topology of the ITS trees for Ammophila suggests that there was a single incursion of coastal beachgrasses into the continental interior. Further, the association of Champlain and St. Lawrence ITS variants supports the hypothesis that interior populations were derived from populations in the northern Atlantic via the St. Lawrence Seaway. Finally, nucleotide diversity is highest in the northern Atlantic lineage, which suggests that the Pleistocene refugium for beachgrass was located in that region. With respect to the status of the Champlain beachgrass, evidence from ITS sequence variation and a morphometric analysis of beachgrass structural characters suggests that the Champlain beachgrass is most appropriately interpreted as a subspecific variant of Ammophila breviligulata.

Key words: Ammophila breviligulata, Champlain beachgrass, DNA sequence, ITS