Tetrapod phylogeny inferred from 18S and 28S ribosomal RNA sequences and a review of the evidence for amniote relationships [published erratum appears in Mol Biol Evol 1991 May;8(3):398]

The 18S ribosomal RNAs of 21 tetrapods were sequenced and aligned with fivepublished tetrapod sequences. When the coelacanth was used as an outgroup,Lissamphibia (living amphibians) and Amniota (amniotes) were found to bestatistically significant monophyletic groups. Although little resolutionwas obtained among the lissamphibian taxa, the amniote sequences support asister-group relationship between birds and mammals. Portions of the 28Sribosomal RNA (rRNA) molecule in 11 tetrapods also were sequenced, althoughthe phylogenetic results were inconclusive. In contrast to previousstudies, deletion or down- weighting of base-paired sites were found tohave little effect on phylogenetic relationships. Molecular evidence foramniote relationships is reviewed, showing that three genes(beta-hemoglobin, myoglobin, and 18S rRNA) unambiguously support abird-mammal relationship, compared with one gene (histone H2B) that favorsa bird- crocodilian clade. Separate analyses of four other genes (alpha-crystallin A, alpha-hemoglobin, insulin, and 28S rRNA) and a combinedanalysis of all sequence data are inconclusive, in that different groupsare defined in different analyses and none are strongly supported. It issuggested that until sequences become available from a broader array oftaxa, the molecular evidence is best evaluated at the level of individualgenes, with emphasis placed on those studies with the greatest number oftaxa and sites. When this is done, a bird-mammal relationship is moststrongly supported. When regarded in combination with the morphologicalevidence for this association, it must be considered at least as plausibleas a bird-crocodilian relationship.