Phylogenetic trees derived from nuclear ribosomal ITS sequences reveal several well-supported lineages within Isoëtes e.g., an Australasian clade, a southern Africa clade, and a clade containing species from Europe, northern Africa, and western North America. Caribbean species I. cubana and I. jamaicensis and Central American I. panamensis are in a more ancient "Gondwanan" clade that includes I. coromandelina from India. Isoëtes lechleri, I. novo-granadensis, and I. savatieri from South America and I. storkii from Central America form a clade of recently derived and rapidly radiated taxa that also includes most of the species from North America. Morphology, chromosome counts, and DNA sequences indicate that there are undescribed species and interspecific hybrids between the South American members of this clade. Study of Isoëtes collections from the paramos of Ecuador and Venezuela reveal populations of multiple taxa growing together with chromosome counts of 2n=22, 33, 44, 55, 66, 88, and 132, indicating a pattern of reticulate evolution similar to that found in North America. Plants with 2n=33, 55, and 66 produce irregularly formed spores typical of interspecific hybrids. Morphology and chromosome counts were used to construct hypothetical pedigrees of the Andean Isoëtes that will be further tested with ITS and LFY sequence data. Morphology and chromosome counts suggest that Isoëtes killipii, which appears to be an octoploid (2n=88), has crossed with an undescribed tetraploid species (2n=44) to form a hexaploid hybrid (2n=66) that subsequently doubled its chromosome number to form the dodecaploid, I. novo-granadensis (2n=132). Similarly, I. karstenii, a basic diploid (2n=22), has crossed with a tetraploid, possibly I. palmeri, to produce a triploid hybrid (2n=33) and with I. andina, an octoploid, to produce a pentaploid hybrid (2n=55). Morphology, chromosome counts, and DNA sequences provide clues to resolving a phylogeny in Isoëtes.

Key words: allopolyploidy, interspecific hybrids, Isoëtes, nrITS