Nucleotide sequences from the highly variable second intron of a LEAFY homolog (a meristem identity gene) were used to determine the parents of allotetraploid Isoetes species. By comparing the positions on the cladogram of the two cloned sequences from the allotetraploids with an extensive sampling of sequences from diploid species, parental species were identified. In some cases, we have been able to identify both parents with some certainty. For example, the allotetraploid I. maritima from the Pacific Northwest resulted from a hybridization event between I. echinospora (circumboreal) and I. bolanderi (Pacific Northwest and Rockies). Isoetes tuckermanii (northeastern North American) resulted from a cross between I. engelmannii (eastern North America) and a species with sequence similarities to I. prototypus (northeastern North America). In other cases, we have been able to identify only one of the parents; the other "dead-beat" parent is missing from our data set. For example, I. appalachiana (southeastern U.S.) is a cross between I. engelmannii and an unknown taxon with affinities to I. flaccida (Florida). I. azorica (Azores Islands) resulted from a hybridization event between I. engelmannii and a missing or undiscovered Isoetes species from North America or the Mediterranean region. A previous hypothesis had suggested that I. azorica was morphologically similar to I. tuckermanii . The fact that I. azorica and I. tuckermanii both have I. engelmannii as one of their parents may account for such an interpretation.

Key words: allotetraploids, hybrid origins, Isoetes, sequence data