Figure 1. Evolutionary expansion of the steroid receptors and their ligands.

A, Pathway for synthesis of vertebrate steroid hormones. The main pathway – synthesis of estrogens (red) via progestagens (blue) and androgens (green) – is at least as ancient as the chordate ancestor. Yellow box, synthesis pathway to corticosteroids (purple), is a later evolutionary novelty found only in vertebrates. The numbering system on the steroid backbone is shown in black. B, Phylogeny of the SR gene family. Receptors are color-coded by the classes of ligands to which they are most sensitive. Ancestral steroid receptors (AncSR1 and AncSR2) resurrected in this study are marked as circles. The number of sequences in each clade is shown in parentheses. Branch supports show approximate likelihood ratios and chi-square confidence metrics for each clade compared to the best phylogeny without that clade. Estrogen-responsive receptors are shown in red. For unreduced phylogenies and a list of sequences, see Figures S10, S11 and Table S7. C, Maximum likelihood reconstruction of ligand-contacting amino acids in AncSR1 and AncSR2, along with residues at homologous sites in extant human SRs. The steroid rings are labeled; circled R indicates polar functional groups at which the major steroid classes differ from each other; arrows indicate residues within hydrogen bonding distance. Residues that differ between AncSR1 and AncSR2 are highlighted in yellow.