Figure 10

SnoRNA evolution by constrained drift. In this model, snoRNAs are genomically mobile and may vary in copy number (through processes of retrotransposition or DNA-level duplication). A) An intronic snoRNA (dark red) may be integrated into a new genomic location (orange gene) through retrotransposition. The original copy is retained (light red gene) leading to a copy number increase. B) The genomic location (and copy-number) of snoRNAs is constrained only by the requirement for phenotype to be satisfied, and a range of genomic organisations may satisfy phenotype. Within these bounds, multiple genomic architectures may generate the same phenotypic outcome, so architecture and copy number may be free to drift within these bounds. The top three expression profiles satisfy phenotype, and are therefore viable. Cases 1 & 2 show different host genes with equivalent expression profiles, whereas case 3 exhibits subfunctionalisation, where combined expression from two host genes, each with limited expression profiles, satisfies the required phenotypic expression profile through expression of the equivalent snoRNA. Profile 4 shows two (non-intronic) copies of a snoRNA gene, where the full expression profile is not achieved. An individual with such an expression profile would be eliminated from the population under constrained drift. SnoRNA genes (black squares); exons (rectangles); snoRNA expression profiles (thin black bars). Primes (') indicate a functionally equivalent snoRNA copy. Note that proteins A & B need not be evolutionarily related.