THERRIEN, JAMES P.*, MICHAEL D. WINDHAM, AND CHRISTOPHER H. HAUFLER. Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS 66045, Utah Museum of Natural History, University of Utah, Salt Lake City, UT 84112. - Hybridization and putative allotetraploid speciation in Selaginella subg. Tetragonostachys.
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
Key words: hybridization, ITS, polyploidy, Selaginella