DES MARAIS, DAVID L.1*, KATHLEEN M. PRYER2, and ALAN R. SMITH3. 1Department of Integrative Biology, University of California, Berkeley, CA 94720; 2Department of Biology, Duke University, Durham, NC 27708; 3University Herbarium, University of California, Berkeley, CA 94720. - Phylogeny, character evolution, and biogeography of extant horsetails (Equisetum).
Equisetum is a small (15 extant species), easily recognized,
and highly distinctive genus of vascular plants with a cosmopolitan
distribution. Only Australia, New Zealand, and Antarctica lack native
representatives. Most species are found between 40 and 60 degrees
north latitude and they are generally confined to seasonally wet
ground. Two subgenera have been recognized based on stomatal position
and growth form: subg. Equisetum (8 species; superficial
stomates; stems branched) and subg. Hippochaete (7 species;
sunken stomates; stems unbranched). Fossils assigned to
Equisetum or Equisitites date back to the Triassic
Period or later, with possible records extending to the Carboniferous.
Prior attempts at understanding Equisetum systematics,
phylogeny, and character evolution have been hampered by the high
degree of variability in the genus, as well as by rampant
hybridization among members within each subgenus. Carefully avoiding
hybrids, we present the first explicit phylogenetic study of
Equisetum, including all 15 species, based on a combined
analysis of two chloroplast markers, trnL-F and rbcL,
and demonstrate robust support for two monophyletic groups that
correspond to the two subgenera recognized by earlier workers. The
species relationships resolved within each of these groups, however,
are mostly in disagreement with previous views. In addition, the South
American species E. bogotense is not a member of either
of these two groups, but rather it is isolated as sister to the other
14 species. With a robust phylogeny in hand, we explore questions
related to morphological character evolution and biogeography in this
ancient genus. Contrary to earlier interpretations, unisexual
gametophytes and a diminutive stature appear to be ancestral
conditions. Several taxonomic characters such as dimorphism,
previously considered to be good indicators of species relationships,
are shown to be homoplastic. Using fossil evidence and geological
vicariance of continents to calibrate divergence times, we also
investigate possible biogeographic hypotheses.
Key words: biogeography, character evolution, Equisetum, fossils, horsetails, phylogeny, rbcL, trnL-F