Four alternative scenarios for the timing of exaptation of a coding sequence into a regulatory function exclusive to teleost fish. After whole-genome duplication (WGD; gray and red circles) in teleost fish (teleost), one copy of an ancestral coding sequence lost its coding function (gray branches). (a) The sequence was exapted into a regulatory function (red branches) within a window (t) of approximately 12.7 million years after non-functionalization, before significant sequence identity was lost as a result of neutral changes. (b) Exaptation was initiated after WGD, but before loss of coding function. Thus the sequence had a dual function for some time. (c) The regulatory function was acquired on top of the coding function before WGD, followed by differential loss of the two functions in the two sequence copies. (d) The exaptation took place earlier in evolution, and was followed by multiple losses: in one sequence copy following WGD in teleosts, and another on the mammalian (mouse) branch. The sequence identity has been retained due to selection on the new regulatory function (in one teleost copy), or on the coding function in the other teleost copy, in mammals and in elephant shark (shark).
Fredman et al. Genome Biology 2011 12:138 doi:10.1186/gb-2011-12-12-138