Nucleotide sequences from the internal transcribed spacer (ITS) region of nuclear ribosomal DNA and the atpB-rbcL spacer region of the chloroplast genome were used to resolve species relationships of Isoetes from a worldwide perspective. Basic diploid species from North and Central America, Europe, Africa, Asia, and Australasia were included. A well-supported, putatively Gondwanian clade consisted of I. stellenbossiensis and I. capensis of South Africa, I. coromandelina of India, and I. panamensis of Costa Rica. Results demonstrated a close, well-supported relationship of the Asian taxa (I. kirkii of New Zealand, I. drummondii of Australia, and I. taiwanensis). North American species of Isoetes are not monophyletic. Two western North American species (I. nuttallii and I. orcuttii) were found in a well-supported clade consisting of European and African species. The remaining, weakly resolved North American clade of 15 species included I. mexicana and I. storkii of Costa Rica. The two species from Costa Rica (I. panamensis and I. storkii) and two species from Spain (I. setacea and I. velata) were not closely related, appearing in widely divergent clades. The resulting phylogeny and the number of substitutions and indels supporting the branches suggest that some clades are of relatively ancient origin, whereas others (the weakly resolved North American clade) are much more recent.

Key words: biogeography, evolutionary relationships, Isoetes, nucleotide sequences