Figure 3.

Mitochondrial genetic codes mapped onto a likely phylogeny of the Metazoa. The trees are rooted by the Fungi as an outgroup. (A) It is most parsimonious (three changes: 1, 2, 3) to assume that both the Echinodermata and Rhabditophora independently evolved AAA = Asn, which must therefore be synapomorphies of the groups. Assuming AAA primitively codes for Asn in the Metazoa requires four changes (1′, 2′, 3′, 4′). AAA is not seen in hemichordate mitochondria. (B) In ignorance of the position of the Acoelomorpha and Catenulida, it is equally parsimonious to presume that, in the Metazoa, AUA primitively coded for Ile [with changes to Met in Chordata (1′), Lophotrochozoa (2′) and Ecdysozoa (3′)] or that AUA changed to coding for Met in a basal metazoan (1) and reverted to Ile in Echinodermata/Hemichordata (2) and Rhabditophora (3). It is hence impossible to polarize the changes based on this knowledge. (C) Positioning the Acoelomorpha and/or Catenulida anywhere on the thickened line forces us to conclude that AUA = Ile in Rhabditophora is a synapomorphy (1, 2, 3 changes) rather than primitive (1′ 2′, 3′, 4′ changes). If Acoela are indeed basal (ref. 8), this also suggests the AUA = Ile in Echinodermata/Hemichordata is a synapomorphy.