Molecular phylogeny of brachiopods and phoronids based on nuclear–encoded small subunit ribosomal RNA gene sequences

B. L. Cohen, A. Gawthrop, T. Cavalier–Smith

Abstract

Brachiopod and phoronid phylogeny is inferred from SSU rDNA sequences of 28 articulate and nine inarticulate brachiopods, three phoronids, two ectoprocts and various outgroups, using gene trees reconstructed by weighted parsimony, distance and maximum likelihood methods. Of these sequences, 33 from brachiopods, two from phoronids and one each from an ectoproct and a priapulan are newly determined. The brachiopod sequences belong to 31 different genera and thus survey about 10% of extant genus–level diversity. Sequences determined in different laboratories and those from closely related taxa agree well, but evidence is presented suggesting that one published phoronid sequence ( GenBank accession UO12648) is a brachiopod–phoronid chimaera, and this sequence is excluded from the analyses. The chiton, Acanthopleura, is identified as the phenetically proximal outgroup; other selected outgroups were chosen to allow comparison with recent, non–molecular analyses of brachiopod phylogeny. The different outgroups and methods of phylogenetic reconstruction lead to similar results, with differences mainly in the resolution of weakly supported ancient and recent nodes, including the divergence of inarticulate brachiopod sub–phyla, the position of the rhynchonellids in relation to long– and short–looped articulate brachiopod clades and the relationships of some articulate brachiopod genera and species. Attention is drawn to the problem presented by nodes that are strongly supported by non–molecular evidence but receive only low bootstrap resampling support.

Overall, the gene trees agree with morphology–based brachiopod taxonomy, but novel relationships are tentatively suggested for thecideidine and megathyrid brachiopods. Articulate brachiopods are found to be monophyletic in all reconstructions, but monophyly of inarticulate brachiopods and the possible inclusion of phoronids in the inarticulate brachiopod clade are less strongly established. Phoronids are clearly excluded from a sister–group relationship with articulate brachiopods, this proposed relationship being due to the rejected, chimaeric sequence ( GenBank UO12648) . Lineage relative rate tests show no heterogeneity of evolutionary rate among articulate brachiopod sequences, but indicate that inarticulate brachiopod plus phoronid sequences evolve somewhat more slowly. Both brachiopods and phoronids evolve slowly by comparison with other invertebrates. A number of palaeontologically dated times of earliest appearance are used to make upper and lower estimates of the global rate of brachiopod SSU rDNA evolution, and these estimates are used to infer the likely divergence times of other nodes in the gene tree. There is reasonable agreement between most inferred molecular and palaeontological ages. The estimated rates of SSU rDNA sequence evolution suggest that the last common ancestor of brachiopods, chitons and other protostome invertebrates (Lophotrochozoa and Ecdysozoa) lived deep in Precambrian time. Results of this first DNA–based, taxonomically representative analysis of brachiopod phylogeny are in broad agreement with current morphology–based classification and systematics and are largely consistent with the hypothesis that brachiopod shell ontogeny and morphology are a good guide to phylogeny.