Molecular phylogeny of horsetails (Equisetum) including chloroplast atpB sequences.

Equisetum is a genus of 15 extant species that are the sole surviving representatives of the class Sphenopsida. The generally accepted taxonomy of Equisetum recognizes two subgenera: Equisetum and Hippochaete. Two recent phylogenetical studies have independently questioned the monophyly of subgenus Equisetum. Here, I use original (atpB) and published (rbcL, trnL-trnF, rps4) sequence data to investigate the phylogeny of the genus. Analyses of atpB sequences give an unusual topology, with E. bogotense branching within Hippochaete. A Bayesian analysis based on all available sequences yields a tree with increased resolution, favoring the sister relationships of E. bogotense with subgenus Hippochaete.