The evolution of evolvability in microRNA target sites in vertebrates

The lack of long-term evolutionary conservation of microRNA (miRNA) target sites appears to contradict many analyses of their functions. Several hypotheses have been offered, but an attractive one-that the conservation may be a function of taxonomic hierarchy (vertebrates, mammals, primates, etc.)-has rarely been discussed. For such an analysis, we cannot use evolutionary conservation as a criterion of target identification, and hence, have used high confidence target sites in the cross-linking immunoprecipitation (CLIP) data. Assuming that a proportion, p, of target sites in the CLIP data are conserved, we define the evolvability of miRNA targets as 1-p. Genomic data from vertebrate species show that the evolvability between human and fish is very high, at more than 90%. The evolvability decreases to 50% between birds and mammals, 20% among mammalian orders, and only 6% between human and chimpanzee. Within each taxonomic hierarchy, there is a set of targets that are conserved only at that level of evolution. Extrapolating the evolutionary trend, we find the evolvability in any single species to be close to 0%. Thus, all miRNA target sites identified by the CLIP method are evolutionarily conserved in one species, but the conservation is lost step by step in larger taxonomic groups. The changing evolvability of miRNA targets suggests that miRNA-target interactions may play a role in the evolution of organismal diversity.