Sequences from the second intron of a homolog of LEAFY, a meristem identity gene, are highly variable in IsoŽtes and other plants (including flowering plants). The LEAFY data is approximately four times more variable than ITS sequences in IsoŽtes and is useful at both the species and population level. For species delimitation, we have identified numerous substitutions and indels that characterize the North American species of IsoŽtes. For example, I. echinospora has virtually identical sequences for populations from such distant locations as Iceland, Maine, Wisconsin, and Montana. In contrast, while two populations of I. melanopoda from Mississippi and Louisiana are very similar, a third population from Arkansas differs at numerous sites, indicating a cryptic species. For determining parental origins of hybrids and allopolyploids, we cloned the intron region to separate the sequences originating from the parental species. Using this method, we were able to enter the two hybrid sequences into an extensive data set of North American species to identify the putative parents. For example, intron sequences from both I.X eatonii (a known diploid hybrid) and I. riparia (an allotetraploid) have virtually identical sequences with the putative parental species (I. engelmannii and I. echinospora). The sequences of the allotetraploid, I. louisianensis, are not similar to any of the North American species. One sequence is weakly supported as sister to a clade consisting of I. tegetiformans, and I. lithophila. The other sequence is sister to a clade consisting of I. virginica, I. melanopoda, and I. echinospora.

Key words: hybrid origins, IsoŽtes, LEAFY, species delimitation