Figure 8.

Model of evolution of the ligand binding ability of ERs. On a classical phylogenetic tree of bilaterians, data available on the binding ability of all known ERs have been indicated. Two hypotheses are compared in terms of parsimony, whether the ancestral ER was liganded (in blue) or not (ie an orphan receptor) (in red). This result displays different costs in terms of parsimony: one unique event of gain specifically in vertebrate for the "ancestral ER orphan" hypothesis against at least two parallel events of loss of binding for the "ancestral ER binding E₂" hypothesis. In addition, three events of loss of the ER gene in urochordates, echinoderms and insects+nematodes are implied by the current distribution of the gene across metazoans.