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The CYP8B1 gene is known to catalyse reactions that determine the ratio of primary bile salts 2 and the loss of this gene has recently been linked to lack of cholic acid in the bile of naked-2 mole rats, elephants and manatees using forward genomics approaches. We screened the 2 CYP8B1 gene sequence of more than 200 species and test for relaxation of selection along 3 each terminal branch. The need for retaining a functional copy of the CYP8B1 gene is 3 established by the presence of a conserved open reading frame across most species screened 3 in this study. Interestingly, the dietary switch from bovid to cetacean species is accompanied 3 by an exceptional ten amino-acid extension at the C-terminal end through a single base 3 frame-shift deletion. We also verify that the coding frame disrupting mutations previously 3 reported in the elephant are correct, are shared by extinct Elephantimorpha species and 3 coincide with the dietary switch to herbivory. Relaxation of selection in the CYP8B1 gene of 3 the wombat (Vombatus ursinus) also corresponds to drastic change in diet. In summary, our 3 forward genomics based screen of bird and mammal species identifies recurrent changes in 3 the selection landscape of the CYP8B1 gene concomitant with a change in dietary lipid 4 content.

3 1 established by the presence of a conserved open reading frame across most species screened 3 2 in this study. Interestingly, the dietary switch from bovid to cetacean species is accompanied 3 3by an exceptional ten amino-acid extension at the C-terminal end through a single base 3 4 frame-shift deletion. We also verify that the coding frame disrupting mutations previously 3 5 reported in the elephant are correct, are shared by extinct Elephantimorpha species and 3 6 coincide with the dietary switch to herbivory. Relaxation of selection in the CYP8B1 gene of 3 7 the wombat (Vombatus ursinus) also corresponds to drastic change in diet. In summary, our 3 8 forward genomics based screen of bird and mammal species identifies recurrent changes in 3 9 the selection landscape of the CYP8B1 gene concomitant with a change in dietary lipid 4 0 content. 4 1