Phylogeny of Selaginellaceae

Sammanfattning: The phylogeny of Selaginellaceae, a small, but historically important family of heterosporous lycopods, has been the focus of this thesis. The approximately 700 extant species are herbaceous and distributed all over the world, with most species in the tropics and subtropics. Lycopods constituted a dominant element of the Upper Carboniferous flora, but Selaginellaceae were probably established already in the Lower Carboniferous some 345 million years ago, as revealed by macrofossil data. Major patterns of relationships were investigated based on a representative sample of global diversity and molecular (plastid gene rbcL, nuclear region 26S rDNA) and morphological data. Analyses were performed using parsimony and Bayesian inference. A survey of megaspore surface and wall structures was carried out for living species and included in the phylogenetic analyses. The resulting phylogenetic trees were used to evaluate various hypotheses on the evolution of the group, including the origins of tropical and temperate species diversity, as well as the evolution of xerophytism. Results showed that Selaginellaceae are monophyletic, and many subclades were identified. In a basal dichotomy two species, Selaginella selaginoides (L.) Link and S. deflexa Brackenridge, appear in a strongly supported clade as sister group to a clade comprising all other species (rhizophoric clade). The rhizophoric clade is recognised by the presence of rhizophores, which are highly characteristic root-like organs, and on the presence of decussately arranged sporophylls. Within the rhizophoric clade a basal dichotomy is most often found resulting in two more or less equally sized sister groups. These and many other groupings within these clades are new and have not previously been recognized in any other systematic study. Some of the new groups seem to have corresponding morphological synapomorphies, such as aspects of rhizophore development and megaspore characteristics. Others share distinctive ecological traits (e.g., xerophytism). For many groups, however, no morphological, ecological, or physiological markers are yet known.The inclusion of megaspore fossils allowed for tentative ages to be assigned to certain clades within the family. The phylogenetic tree is inconclusive with regard to a tropical or temperate origin of modern species diversity, but there is clear evidence for multiple independent origins of xerophytic strategies. Besides the phylogenetic results, this study reveals exceptionally high levels of substitution rates and rate heterogeneity in Selaginellaceae.