MicroRNAs (miRNAs) are a class of endogenous small non-coding RNAs of 20–25 nucleotides that play a key role in diverse biological processes. Japanese flounder undergo dramatic metamorphosis in their early development. The metamorphosis is characterized by morphological transformation from a bilaterally symmetrical to an asymmetrical body shape concomitant with extensive morphological and physiological remodeling of organs. So far, only a few miRNAs have been identified in fish and there are very few reports about the Japanese flounder miRNA. Solexa sequencing technology was used to perform high throughput sequencing of the small RNA library from the metamorphic period of Japanese flounder. Subsequently, aligning these sequencing data with metazoan known miRNAs, we characterized 140 conserved miRNAs and 57 miRNA: miRNA* pairs from the small RNA library. Among these 57 miRNA: miRNA* pairs, twenty flounder miRNA precursors were amplified from genomic DNA. We also demonstrated evolutionary conservation of Japanese flounder miRNAs and miRNA* in the animal evolution process. Using miRNA microarrays, we identified 66 differentially expressed miRNAs at two metamorphic stages (17 and 29 days post hatching) of Japanese flounder. The results show that miRNAs might play a key role in regulating gene expression during Japanese flounder metamorphosis. We identified a large number of miRNAs during flounder metamorphosis, some of which are differentially expressed at two different metamorphic stages. The study provides an opportunity for further understanding of miRNA function in the regulation of flounder metamorphosis and gives us clues for further studies of the mechanisms of metamorphosis in Japanese flounder.

The complete mitochondrial genome of Siberian sturgeon Acipenser baerii had been determined. The total length of the mitogenome was 16,763 bp and had 13 protein-coding, 22 tRNA, and 2 rRNA genes. Except for the eight tRNA and Nd6 genes, all other mitochondrial genes are encoded on the heavy strand. The overall base composition of the heavy strand is 30.3% A, 24.3% T, 29.1% C, 16.3% G, with an AT content of 54.6%. The DNA sequence of A. baerii shared 97.8, 97.6, 93.5, 91.6, 93.6, and 87.1% sequence homology with that of Acipenser sinensis, Acipenser gueldenstaedtii, Acipenser transmontanus, Acipenser dabryanus, Acipenser stellatus, and Polyodon spathula. Molecular data presented here provide a useful tool for evolutionary as well as population genetics studied.