Hybridization and polyploid speciation are well documented evolutionary mechanisms in plants. Although taxa derived through such reticulate processes occur in nearly every major lineage of pteridophytes, including the homosporous lycopods and heterosporous quillworts, to date there have been no confirmed cases of hybridization or allopolyploid speciation in the related genus Selaginella. Recent fieldwork in the southwestern US has revealed examples of hybridization among taxa within S. subg. Tetragonostachys. Hybrids between the strongly divergent species S. arizonica and S. rupincola initially were identified based on intermediate morphologies and abortive sporangia. Confirmatory isozyme data support at least two independent hybridization events and suggest that the hybrids are triploid, resulting from crosses between tetraploid S. rupincola and diploid S. arizonica. In addition, nrITS sequences diagnostic for both S. arizonica and S. rupincola are present in the hybrid, and were obtained via sequencing multiple clones of the hybrid nrITS PCR product. Whereas the two nrITS sequences in the hybrid are clearly derived from the parental nrITS sequences, they exhibit moderate degrees of divergence from the parental sequences. Hybrids between S. arizonica and S. eremophila, closely related members of section Eremophilae, also have been documented. Some of these plants produce well-formed spores, and isozyme banding patterns suggest that certain populations entirely consist of stabilized tetraploids. Owing to the similarity in nrITS sequences for the putative parents, nrITS sequence data do not fully aid in resolving parentage in this hybrid. If confirmed by additional markers, the S. arizonica X S. eremophila hybrid would represent the first documented example of allotetraploid speciation in Selaginella.

Key words: hybridization, ITS, polyploidy, Selaginella