Evolution of a New Function by Degenerative Mutation in Cephalochordate Steroid Receptors
A) Sequence similarity of BfER and BfSR to each other and human steroid receptors. The percent of identical amino acid residues for each pairwise comparison is shown. DBD and LBD, DNA-binding and ligand-binding domains. B) Reduced phylogeny of the steroid receptor gene family. Numbers in parentheses refer to the number of sequences in each group used in the analysis. Estrogen-sensitive receptors, including the ancestral steroid receptor (AncSR1) are in blue; ketosteroid receptors are in yellow. Black triangle, loss of transcriptional activation; yellow triangle, gain of ketosteroid sensitivity in the vertebrate AR/PR/GR/MR clade. Support for each branch is shown as the approximate likelihood ratio (the ratio of the likelihood of the best tree with that node to the best tree without it), the chi-square likelihood confidence statistic for the node, and the Bayesian posterior probability. Nodes that place B. floridae receptors as orthologs of the ERs and kSRs are in red. Scale bar shows expected per-site substitution rate for branch lengths. Complete phylogenies and a list of genes, species, and accessions are in Figures S6, S7, S8 and Table S1. C) Conserved synteny between BfER-containing scaffold 42 and human chromosomes 14 and 6, which contain ERβ (ESR2) and ERα (ESR1), respectively. Green boxes with connecting lines indicate reciprocal BLAST best-hits between scaffold 42 in the B. floridae genome and human chromosome 14; purple boxes, orthologs between Bf scaffold 42 and human chromosome 6. Genes are shown in order with spacing approximately proportional to physical distance. BfER, ESR1, and ESR2 all share orthologous nearest neighbors (red lines).