Evolutionary comparisons of miRNA regulation system in six model organisms.

miRNAs are a class of endogenous small non-coding regulatory RNAs, that can mediate the transcriptional gene silencing as well as gene expression activation. miRNAs, which are found in a wide range of species, participate in cell differentiation, proliferation, development, apoptosis, tumorigenesis, metabolism, immune system, and signaling pathways. Here, we focused on the relationship between evolution and the miRNA system, with an emphasis on both miRNAs and their target genes. Six species from the evolutionary ladder were selected as a focus of this study. Public data were retrieved and combined to compare miRNAs abundance, miRNA families, molecular functions of target genes, biological processes of target genes, protein families of target gene products, transcription factors regulated by the miRNAs, signaling pathways and tissues across the six species. We found that the expansion rate of miRNAs was significantly higher compared to other genes in human evolution. Newborn miRNA families, which were quantitatively larger than dead miRNA families, seem to be closely related to the species complexity and tissue specificity. Additionally, miRNAs in higher order species were more likely to target genes related to signaling and the immune system, while miRNAs from lower order species preferred to target genes related to the embryonic development process, reproduction and growth. Meanwhile, miRNA systems displayed diversity in regulating transcription factors, signaling pathways and tissues. Our research suggested that the miRNA system might promote evolution, especially in higher species.