Selaginella subg. Tetragonostachys is a group of ca. 50 widely distributed species that exhibit a number of putative adaptations to xeric environments. Morphologically, the subgenus includes taxa with thickened, isophyllous microphylls, tetrastichous sporophylls, and in several species, the presence of vessels. Previous phylogenetic analyses based on rbcL sequence data strongly supported the monophyly of this subgenus, but failed to adequately resolve relationships within the group. To address open questions of phylogenetic relationships, nuclear ribosomal internally transcribed spacer (nrITS) sequences were obtained from 44 ingroup taxa and 6 outgroups. Maximum parsimony analysis of the data yielded several well supported clades. The resulting tree topology exhibits patterns that are consistent with classic Tertiary relictual distributions. There is strong support for a clade comprised of taxa from southern Africa, Madagascar, and India - land masses previously united in a larger assemblage. Further vicariant relationships include an eastern US, western US, Asian clade, again with strong bootstrap support. Moderate support is also obtained for a clade of species within section Arenicolae, comprised of taxa distributed in the southwestern US, southeastern US, and northern Africa. In addition, several of the clades correspond well with morphological characters traditionally considered indicative of sister group relationships. These include such features as red pigmented microphylls, readily fragmenting stems, dimorphic sporophylls, and the prostrate, incurling habit. The overall pattern from the nrITS phylogeny suggests that speciation in S. subg. Tetragonostachys may be attributed primarily to vicariant events, rather than to long distance dispersal.

Key words: ITS, Selaginella, Tertiary relict, vicariance