Figure 3. Evolution of minimal specificity.

A, Evolution of ligand-recognition criteria on the SR phylogeny. For each ancient and extant receptor, the criteria that distinguish activating ligands from other endogenous steroids are shown in brackets. Rules labeled “not” indicate significantly strongly reduced sensitivity when the specified moiety is present; other rules indicate strongly increased sensitivity when the moiety is present. The structures of representative endogenous hormones – estrogens (E), androgens (A), progestagens (P) and corticosteroids (C) – that were synthesized at each point in time are shown. Green portions of each hormone show moieties that satisfy the receptor's rules; red portions violate rules. Each receptor's rules are sufficient to allow activation by only a single class of hormones (gray boxes). The evolution of corticosteroid synthesis is indicated; AncSR2's criteria would not have been sufficient to distinguish corticosteroids from progestagens. Inset: common steroid structure with A-ring and key carbons labeled. Dose-response curves for extant receptors are shown in Figure S7. B, AncSR1 is activated/antagonized by xenoestrogens in a luciferase reporter assay. IC50, concentration at which half-maximal inhibition was achieved in the presence of estradiol (EC₈₀ = 200 nM). Each point shows the mean and SEM of three replicates.