The Gain of Three Mitochondrial Introns Identifies Liverworts As the Earliest Land Plants

The first evidence for the emergence of land plants (embryophytes) consists of mid-Ordovician spore tetrads (approximately 476 Myr old). The identity of the early plants that produced these spores is unclear; they are sometimes claimed to be liverworts, but there are no associated megafossils, and similar spores can be produced by a diversity of plants. Indeed, the earliest unequivocal megafossils of land plants consist of early vascular plants and various plants of uncertain affinity. Different phylogenetic analyses have identified liverworts, hornworts and bryophytes as each being the first lineage of land plants; the consensus of these conflicting topologies yields an unresolved polychotomy at the base of land plants. Here we survey 352 diverse land plants and find that three mitochondrial group II introns are present, with occasional losses, in mosses, hornworts and all major lineages of vascular plants, but are entirely absent from liverworts, green algae and all other eukaryotes. These results indicate that liverworts are the earliest land plants, with the three introns having been acquired in a common ancestor of all other land plants, and have important implications concerning the early stages of plant evolution.