Phylogenetic relationships and evolution of extant Horsetails, Equisetum, based on chloroplast DNA sequence data (rbcL and trnL-F)

• Published in: International journal of plant sciences 2003-09, Vol.164 (5), p.737-751
• Main Authors: Des Marais, D.L, Smith, A.R, Britton, D.M, Pryer, K.M
• Format: Article
• Language: English
• Subjects: Biological taxonomies; biomarkers; chloroplasts; Equisetum; Equisetum bogotense; Evolution; Flowers & plants; Fossils; Gametophytes; Genetics; Hippochaete; Horsetails; Hybridity; molecular sequence data; molecular systematics; nucleotide sequences; Parsimony; Phylogeny; plant proteins; rbcL gene; rbcL protein; ribulose-bisphosphate carboxylase; sequence alignment; sequence analysis; Stomata; Taxa; trnF gene; trnL gene; trnL protein; We they distinction
• ISSN: 1058-5893; 1537-5315
• DOI: 10.1086/376817

Equisetumis a small and morphologically distinct genus with a rich fossil record. Two subgenera have been recognized based principally on stomatal position and stem branching: subg.Equisetum(eight species; superficial stomates; stems branched) and subg.Hippochaete(seven species; sunken stomates; stems generally unbranched). Prior attempts at understandingEquisetumsystematics, phylogeny, and character evolution have been hampered by the high degree of morphological plasticity in the genus as well as by frequent hybridization among members within each subgenus. We present the first explicit phylogenetic study ofEquisetum, including all 15 species and two samples of one widespread hybrid,Equisetum×ferrissii, based on a combined analysis of two chloroplast markers,rbcLandtrnL‐F. Our robustly supported phylogeny identifies two monophyletic clades corresponding to the two subgenera recognized by earlier workers. The phylogenetic placement ofEquisetum bogotense, however, is ambiguous. In maximum likelihood analyses, it allies with subg.Hippochaeteas the most basal member, while maximum parsimony places it as sister to the rest of the genus. A consensus phylogeny from the two analyses is presented as a basal trichotomy (E. bogotense, subg.Hippochaete, subg.Equisetum), and morphological character evolution is discussed. We detected rate heterogeneity in therbcLlocus between the two subgenera that can be attributed to an increased rate of nucleotide substitution (transversions) in subg.Hippochaete. We calculated molecular‐based age estimates using the penalized likelihood approach, which accounts for rate heterogeneity and does not assume a molecular clock. TheEquisetumcrown group appears to have diversified in the early Cenozoic, whereas the Equisetaceae total group is estimated to have a Paleozoic origin. These molecular‐based age estimates are in remarkable agreement with current interpretations of the fossil record.