Phylogenetic relationships of land plants using mitochondrial small-subunit rDNA sequences

Phylogenetic relationships among embryophytes (tracheophytes, mosses, liver worts, and hornworts) were examined using 21 newly generated mitochondrial small-subunit (19S) rDNA sequences. The "core" 19S rDNA contained more phyl ogenetically informative sites and lower homoplasy than either nuclear 18S or plastid 16S rDNA. Results of phylogenetic analyses using parsimony (MP) and likelihood (ML) were generally congruent. Using MP, two trees were obta ined that resolved either liverworts or hornworts as the basal land plant c lade. The optimal ML tree showed hornworts as basal. That topology was not statistically different from the two MP trees, thus both appear to be equal ly viable evolutionary hypotheses. High bootstrap support was obtained for the majority of higher level embryophyte clades named in a recent morpholog ically based classification, e.g., Tracheophyta, Euphyllophytina, Lycophyti na, and Spermatophytata. Strong support was also obtained for the following monophyletic groups: hornworts. liverworts, mosses, lycopsids, leptosporan giate and eusporangiate ferns, gymnosperms and angiosperms. This molecular analysis supported a sister relationship between Equisetum and leptosporang iate ferns and a monophyletic gymnosperms sister to angiosperms. The topolo gies of deeper clades were affected by taxon inclusion (particularly hornwo rts) as demonstrated by jackknife analyses. This study represents the first use of mitochondrial 19S rDNA for phylogenetic purposes and it appears wel l-suited for examining intermediate to deep evolutionary relationship among embryophytes.