Phylogenetic Relationships and Evolution of Extant Horsetails,Equisetum, Based on Chloroplast DNA Sequence Data (rbcLandtrnL‐F)

Abstract

Equisetum is 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 understanding Equisetum systematics, 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 of Equisetum, including all 15 species and two samples of one widespread hybrid, Equisetum #ferrissii, based on a combined analysis of two chloroplast markers, rbcL and trnL-F. Our robustly supported phylogeny identifies two monophyletic clades corresponding to the two subgenera recognized by earlier workers. The phylogenetic placement of Equisetum bogotense, however, is ambiguous. In maximum likelihood analyses, it allies with subg. Hippochaete as 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 the rbcL locus 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. The Equisetum crown 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.

Reconstruction and Phylogenetic Significance of a NewEquisetumLinnaeus Species from the Lower Jurassic of Cerro Bayo (Chubut Province, Argentina)

“…Molecular phylogenetic analyses recovered the monophyly of the Hippochaete clade, but not that of the Equisetum clade (Des Marais et al, 2003;Guillon, 2004Guillon, , 2007 due largely to the inconsistent placement of Equisetum bogotense Kunth, 1815 in different studies (see Guillon, 2007 and references therein).…”

“…It has been estimated, using molecular data, to have occurred in the Cenozoic (Des Marais et al, 2003). In contrast it also has been estimated as 136…”

“…nov. in the context of extant subgenera. The two modern Equisetum subgenera -Equisetum and Hippochaete-have been historically distinguished based on numerous morphological features (Hauke, 1963(Hauke, , 1978Des Marais et al, 2003). However, their morphological differentiation has been recently questioned on the basis of molecular phylogenetic inferences (Des Marais et al, 2003;Guillon, 2004Guillon, , 2007.…”

“…The two modern Equisetum subgenera -Equisetum and Hippochaete-have been historically distinguished based on numerous morphological features (Hauke, 1963(Hauke, , 1978Des Marais et al, 2003). However, their morphological differentiation has been recently questioned on the basis of molecular phylogenetic inferences (Des Marais et al, 2003;Guillon, 2004Guillon, , 2007. In these studies, Equisetum bogotense, a species classically assigned to the subgenus Equisetum based on morphological features, has been recovered as basal to subgenus Hippochaete (Guillon, 2007) or sister to both subgenera (Des Marais et al, 2003;Guillon, 2004 the genus (see Guillon, 2007).…”

A Tribute to Donald M. Britton (1923–2012), Canada’s Premier Pteridologist

“…On the occasion of his 80th birthday in 2003, two issues of BEN (Botanical Electronic Newsletter) were dedicated to his achievements (Brunton 2003;Catling 2003;Ceska and Ceska 2003;Pryer 2003;Reznicek 2003), and an upcoming paper concerning Cystopteris and Gymnocarpium, genera of which Don was a keen student, is dedicated to his memory (Pryer, in press).…”