Evolution of Fish Let-7 MicroRNAs and Their Expression Correlated to Growth Development in Blunt Snout Bream.

The lethal-7 () miRNA, known as one of the first founding miRNAs, is present in multiple copies in a genome and has diverse functions in animals. In this study, comparative genomic analysis of miRNAs members in fish species indicated that miRNA is a sequence conserved family in fish, while different species have the variable gene copy numbers. Among the ten members including let-7a/b/c/d/e/f/g/h/i/j, the let-7a precursor sequence was more similar to ancestral sequences, whereas other miRNA members were separate from the late differentiation of let-7a. The mostly predicted target genes of miRNAs are involved in biological process, especially developmental process and growth through Gene Ontology (GO) enrichment analysis. In order to identify the possible different functions of these ten miRNAs in fish growth development, their expression levels were quantified in adult males and females of , as well as in 3-, 6-, and 12-months-old individuals with relatively slow- and fast-growth rates. These ten miRNAs had similar tissue expression patterns between males and females, with higher expression levels in the brain and pituitary than that in other tissues ( < 0.05). Among these miRNAs, the relative expression level of was the highest among almost all the tested tissues, followed by , and . As to the groups with different growth rates, the expression levels of miRNAs in pituitary and brain from the slow-growth group were always significantly higher than that in the fast-growth group ( < 0.05). These results suggest that miRNA members could play an important role in the regulation of growth development in through negatively regulating expression of their target genes.