Population genetic structure of Vallisneria americana, a dioecious clonal macrophyte

The population genetic structure and spatial genetic structure of the dioecious aquatic macrophyte Vallisneria americana was examined near Turkey Island in the Detroit River in late summer of 1991. The density and distribution of male, female, and vegetative ramets was determined for three transects parallel, and three transects perpendicular to the island, at mean water depths of 38–306 cm. A strong male‐biased sex ratio was observed in shallow water, while sex ratios became increasingly female‐biased, and plant density increased, at greater water depths. Cellulose acetate gel electrophoresis was used to characterize the allozyme phenotype of each ramet that had been screened for seven enzyme systems. Overall, 91 allozyme phenotypes were identified. A single allozyme phenotype accounted for 33%–55% of all the ramets (depending on the transect), indicating extensive regional vegetative growth. However, the population as a whole displayed high genetic diversity (H T = 0.3403), with most of the diversity occurring within transects (H s = 0.3297) rather than between transects (D ST = 0.0106). The possible factors accounting for the high small‐scale genetic diversity of this clonal aquatic plant species are discussed.