Expression of let-7 microRNAs that are involved in Japanese flounder (Paralichthys olivaceus) metamorphosis.

The let-7 microRNAs (miRNAs), a class of small noncoding RNAs, are phylogenetically conserved and temporally expressed and control the proper timing of events during development as heterochronic genes in many animals. Japanese flounder (Paralichthys olivaceus) undergoes a metamorphosis from the larval to juvenile form. Here, we identified 21 let-7 miRNA precursors from different genome loci in Japanese flounder. P. olivaceus let-7 miRNAs are widely expressed in adult tissues, highly expressed during metamorphosis, but weakly during embryonic development. Exogenous thyroid hormone (0.1 mg/L), which induces premature metamorphosis, significantly promotes the expression of let-7 miRNAs, while thiourea (30 mg/L), which affects metamorphic arrest, inhibits the expression of let-7 miRNAs in metamorphosis in P. olivaceus. These results show that let-7 miRNAs widely participate in tissue development and metabolism during development and are also involved in regulation of temporal transitions associated with cell proliferation and differentiation during metamorphosis, in P. olivaceus.