The Caenorhabditis elegans intermediate-size transcriptome shows high degree of stage-specific expression

Earlier studies have revealed a substantial amount of transcriptional activity occurring outside annotated protein-coding genes of the Caenorhabditis elegans genome. One important fraction of this transcriptional activity relates to intermediate-size (70-500 nt) transcripts (is-ncRNAs) of mostly unknown function. Profiling the expression of this segment of the transcriptome on a tiling array through the C. elegans life cycle identified 5866 hitherto unannotated transcripts. The novel loci were distributed across intronic and intergenic space, with some enrichment toward protein-coding gene termini. The majority of the putative is-ncRNAs showed either stage-specific expression, or distinct developmental variation in their expression levels. More than 200 loci showed male-specific expression, and conserved loci were significantly enriched on the X chromosome, both observations strongly suggesting involvement of is-ncRNAs in sex-specific functions. Half of the novel loci were conserved in other nematodes, and numerous loci showed significant conservational correlations to nearby coding genes. Assuming functional roles for most of the novel loci, the data imply a nematode is-ncRNA tool kit of considerable size and variety.