Figure 2.

Pathways of co-option leading to gains in floral anthocyanins. Solid boxes represent coding regions of R2R3-MYB transcription factors, and horizontal lines represent their cis-regulatory elements. Within these regions, “F” denotes a floral cis-regulatory element, while “V” indicates a vegetative element. Vertical lines denote mutation locations and asterisks the resulting substitution. An “X” indicates a loss of function mutation. (A) Gene duplication followed by co-option via neofunctionalization. An R2R3-MYB that regulates some other product in flowers (such as flavonoids) duplicates. A coding mutation in the DNA binding domain of one copy results in a transition to ABP regulation. (B) Gene duplication followed by co-option and specialization. Duplication of an R2R3-MYB that regulates vegetative anthocyanins acquires a cis-regulatory mutation in one copy that introduces a floral specific element. Specialization occurs if the copy with the floral cis-element loses its vegetative expression. (C) Co-option occurs without gene duplication when a mutation introduces a floral cis-element into a pre-existing R2R3-MYB that regulates vegetative anthocyanins. (D) Co-option of a pre-existing R2R3-MYB followed by gene duplication. After the co-option of the ABP for floral anthocyanins, vegetative and floral anthocyanins are favored differently by selection. In these cases, gene duplication may be advantageous to reduce this constraint. Post duplication deletions of the alternate cis-elements then produce specialization.