Fig 1. Regulatory evolution in the retention and divergence of gene duplicates.

The cells represent a mother and a daughter cell to illustrate different phases of development. The top row shows these cells between the duplication event and the divergence of the duplications. The same concept applies to multicellular organisms with different cell types. Colored dots represent gene products from the ancestral gene (gray, in the cells at the top) and from the duplicated genes (blue circles and orange triangles, in the cells at the bottom). Retention by dosage effects refers to a gain in fitness caused by a larger amount of gene product. The dosage change does not need to correspond to an exact doubling (as illustrated) but could be higher [2] or lower than that if, for instance, some mechanisms of attenuation are present [3]. Retention by subfunctionalization refers to the maintenance of the two copies by the splitting of the ancestral function, here illustrated by the different localizations. Dosage subfunctionalization refers to a special case in which the total expression is maintained, but the abundance of each duplicate can change. Neofunctionalization refers to the evolution of a new function by a paralog, here shown by the new localization. In the absence of sufficient selection pressure to maintain two copies, the system can revert to a single gene system through pseudogenization (or simple loss by other mechanisms) of one copy.