Fig. 1.

Proposed hypothesis for the evolution of snoRNAs. snoRNAs have been described as mobile genetic elements that can copy themselves to other genomic locations, whether on the same or different chromosomes [62,63]. These snoRNA retrogenes and their parental copies are subjected to evolutionary forces and might either retain snoRNA functionality (if they do not gain deleterious mutations and are in the right genomic context), become inactive (due to deleterious mutations or improper expression) or gain new functionality. Here, chromosomes are represented by blue lines, snoRNAs by pink boxes, mutated snoRNAs by pink and grey boxes and non-coding RNA elements with new functionality with green boxes.