Identification and analyses of miRNA genes in allotetraploid Gossypium hirsutum fiber cells based on the sequenced diploid G. raimondii genome

The plant genome possesses a large number of microRNAs (miRNAs) mainly 21-24 nucleotides in length. They play a vital role in regulation of target gene expression at various stages throughout the whole plant life cycle. Here we sequenced and analyzed ≈ 10 million non-coding RNAs (ncRNAs) derived from fiber tissue of the allotetraploid cotton (Gossypium hirsutum) 7 days post-anthesis using ncRNA-seq technology. In terms of distinct reads, 24 nt ncRNA is by far the dominant species, followed by 21 nt and 23 nt ncRNAs. Using ab initio prediction, we identified and characterized a total of 562 candidate miRNA gene loci on the recently assembled D(5) genome of the diploid cotton G. raimondii. Of all the 562 predicted miRNAs, 22 were previously discovered in cotton species and 187 had sequence conservation and homology to homologous miRNAs of other plant species. Nucleotide bias analysis showed that the 9th and 1st positions were significantly conserved among different types of miRNA genes. Among the 463 putative miRNA target genes, most significant up/down-regulation occurred in 10-20 days post-anthesis, indicating that miRNAs played an important role during the elongation and secondary cell wall synthesis stages of cotton fiber development. The discovery of new miRNA genes will help understand the mechanisms of miRNA generation and regulation in cotton.