The pondweeds, Potamogeton (Potamogetonaceae), display
heterophylly, both within and among species. In addition, some species
display seasonal heterophylly. Consequently, the genus has long been
considered to be taxonomically difficult. Potamogeton, sensu
stricto, has conveniently been divided into two groups,
broad-leaved species and narrow-leaved species. Phylogenetic analysis
of 5S nuclear ribosomal non-transcribed spacer (5S-NTS) sequences
supports the monophyly of two major clades, one essentially those
previously considered to be broad-leaved species and the other those
previously considered to be narrow-leaved species. Within each main
clade, species groups are recognized, mostly along the lines of
previously accepted subsections. For example, Potamogeton
epihydrus and P. tennesseensis form a clade
sister to the P. spirillus, P.
bicupulatus, and P. diversifolius clade. These
two subclades are in turn sister to the species previously recognized
as the narrow-leaved pondweeds.
Key words: Heterophylly, pondweeds, Potamogeton, Potamogetonaceae