Global profiling and annotation of templated isomiRs dynamics across <i>Caenorhabditis elegans</i> development

microRNAs (miRNAs) are small non-coding RNAs that regulate gene expression through translational repression and mRNA destabilization. During canonical miRNA biogenesis, several miRNA isoforms, or isomiRs, are produced from a single precursor miRNA. Templated isomiRs are generated through Drosha or Dicer cleavage at alternate positions on either the primary or the precursor miRNAs, generating truncated or extended 5’ and/or 3’ miRNA ends. As changes to the mature miRNA sequence can alter miRNA gene target repertoire, we investigated the extent of templated isomiR prevalence, providing a profiling map for templated isomiRs across stages of C. elegans development. While most miRNA loci did not produce abundant templated isomiRs, a substantial number of miRNA loci produced isomiRs were just as, or more, abundant than their annotated canonical mature miRNAs. 3’ end miRNA alterations were more frequent than the seed-altering 5’ end extensions or truncations. However, we identified several miRNA loci that produced a considerable amount of isomiRs with 5’ end alterations, predicted to target new, distinct sets of genes. Overall, the presented annotation of templated isomiR dynamics across C. elegans developmental stages provides a basis for further studies into miRNA biogenesis and the intriguing potential of functional miRNA diversification through isomiR production.