Isozymes were examined in the six aquatic (including amphibious) diploid species of Isoetes of eastern North America in order to quantify their levels and patterns of genetic variation, to determine their genetic distinctness, and to evaluate the utility of allozymes in addressing ancestry in the I. riparia complex. Twenty-six populations representing diploid species, I. echinospora, I. engelmannii, I. flaccida, I. mattaponi, I. prototypus, and I. valida were compared for eleven enzymes coded by 16 interpretable loci. Levels of genetic variation in populations tended to be low ranging from none (means: P = 0.0, A = 1.0, H = 0.0) in I. echinospora to moderate (means: P = 22.9, A = 1.4, H = 0.093) in I. flaccida. Genetic variation in polymorphic species tended to be distributed more among than within populations, as indicated by high F_st values (F_st > 0.607 in all polymorphic species except I.mattaponi, for which F_st = 0.272). The genetic identities of conspecific populations were high, with mean values ranging from I = 0.779/S = 0.733 in I. flaccida to I = 1.000/S = 1.000 in I. echinospora. Interspecific genetic identities were lower, with means ranging from I = 0.438/S = 0.429 between I. mattaponi and I. prototypus to I = 0.736 between I. valida and I. flaccida, and S = 0.699 between I. valida and I. prototypus. Thus the six diploids were found to be genetically distinct, corroborating their recognition based on morphology. Evidence from this study indicates that isozymes may be informative in diagnosing ancestry of allopolyploids in the Isoetes riparia complex.

Communicating Editor: Anita Cholewa