Characterization and Comparative Profiling of MiRNA Transcriptomes in Bighead Carp and Silver Carp

MicroRNAs (miRNAs) are small non-coding RNA molecules that are processed from large ‘hairpin’ precursors and function as post-transcriptional regulators of target genes. Although many individual miRNAs have recently been extensively studied, there has been very little research on miRNA transcriptomes in teleost fishes. By using high throughput sequencing technology, we have identified 167 and 166 conserved miRNAs (belonging to 108 families) in bighead carp (Hypophthalmichthys nobilis) and silver carp (Hypophthalmichthys molitrix), respectively. We compared the expression patterns of conserved miRNAs by means of hierarchical clustering analysis and log2 ratio. Results indicated that there is not a strong correlation between sequence conservation and expression conservation, most of these miRNAs have similar expression patterns. However, high expression differences were also identified for several individual miRNAs. Several miRNA* sequences were also found in our dataset and some of them may have regulatory functions. Two computational strategies were used to identify novel miRNAs from un-annotated data in the two carps. A first strategy based on zebrafish genome, identified 8 and 22 novel miRNAs in bighead carp and silver carp, respectively. We postulate that these miRNAs should also exist in the zebrafish, but the methodologies used have not allowed for their detection. In the second strategy we obtained several carp-specific miRNAs, 31 in bighead carp and 32 in silver carp, which showed low expression. Gain and loss of family members were observed in several miRNA families, which suggests that duplication of animal miRNA genes may occur through evolutionary processes which are similar to the protein-coding genes.