An Evolutionary Perspective of Animal MicroRNAs and Their Targets
Genomic organization of miRNA genes and their expression. The number of miRNA genes correlates with chromosome length (a) and the number of protein-coding genes (b). Outliers chromosomes 14, 19, and are indicated. When removing these three chromosomes the number of miRNA genes aligns well on the regression line ( indicated). (c) Proportion of miRNA genes hosted in Introns—Intronic (based on Refseq genes), Clustered on the same genomic strand, or Stand-alone miRNAs. Overlapping intronic and clustered miRNAs are also indicated. Each row refers to 50 kb, 10 kb, and 1 kb distance between paired genes on the same strand. It was shown that clusters of size 3 kb give a large proportion of clusters (27%) with little change when increasing pairwise distance to 10 kb . Diagrams are based on data from Refseq. (d) Human miRNA copy number was plotted against the average miRNA expression level of 40 Human tissues . A similar plot of another dataset  gave comparable results (data not shown).
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