Signatures of Relaxed Selection in the CYP8B1 Gene of Birds and Mammals.

The CYP8B1 gene is known to catalyse reactions that determine the ratio of primary bile salts and the loss of this gene has recently been linked to lack of cholic acid in the bile of naked-mole rats, elephants and manatees using forward genomics approaches. We screened the CYP8B1 gene sequence of more than 200 species and test for relaxation of selection along each terminal branch. The need for retaining a functional copy of the CYP8B1 gene is established by the presence of a conserved open reading frame across most species screened in this study. Interestingly, the dietary switch from bovid to cetacean species is accompanied by an exceptional ten amino acid extension at the C-terminal end through a single base frame-shift deletion. We also verify that the coding frame disrupting mutations previously reported in the elephant are correct, are shared by extinct Elephantimorpha species and coincide with the dietary switch to herbivory. Relaxation of selection in the CYP8B1 gene of the wombat (Vombatus ursinus) also corresponds to drastic change in diet. In summary, our forward genomics-based screen of bird and mammal species identifies recurrent changes in the selection landscape of the CYP8B1 gene concomitant with a change in dietary lipid content.