Three models for the genesis of miRNA genes. (Blue bars) Ancestral miRNAs; (orange bars) novel miRNAs. (A) An example of subfunctionalization: a miRNA* acquires function; following gene duplication, one daughter copy maintains the function of the original miRNA, while the other maintains the function of the former miRNA*. Another example of subfunctionalization begins with heterologous 5′ processing. (B) Neofunctionalization: a miRNA gene duplicates; one daughter copy maintains the function of the original miRNA, while the other accumulates mutations that confer novel functionality to either the former miRNA or miRNA*. (C) De novo gene emergence: an unselected portion of a pre-existing transcript, such as an intron or part of a pri-miRNA, acquires the capacity to fold into a hairpin that can be processed into a mature miRNA. That product is selectively maintained because of the fortuitous benefit of gene silencing guided by its seed.
