Fast-evolving microRNAs are highly expressed in the early embryo of Drosophila virilis

MicroRNAs are short non-protein-coding RNAs that regulate gene expression at the post-transcriptional level and are essential for the embryonic development of multicellular animals. Comparative genome-scale analyses have revealed that metazoan evolution is accompanied by the continuous acquisition of novel microRNA genes. This suggests that novel microRNAs may promote innovation and diversity in development. We determined the evolutionary origins of extant Drosophila microRNAs and estimated the sequence divergence between the 130 orthologous microRNAs in Drosophila melanogaster and Drosophila virilis, separated by 63 million years of evolution. We then generated small RNA sequencing data sets covering D. virilis development and explored the relationship between microRNA conservation and expression in a developmental context. We find that late embryonic, larval, and adult stages are dominated by conserved microRNAs. This pattern, however, does not hold for the early embryo, where rapidly evolving microRNAs are uniquely present at high levels in both species. The group of fast-evolving microRNAs that are highly expressed in the early embryo belong to two Drosophilid lineage-specific clusters: mir-310 similar to 313 and mir-309 similar to 6. These clusters have particularly complex evolutionary histories of duplication, gain, and loss. Our analyses suggest that the early embryo is a more permissive environment for microRNA changes and innovations. Fast-evolving microRNAs, therefore, have the opportunity to become preferentially integrated in early developmental processes, and may impact the evolution of development. The relationship between microRNA conservation and expression throughout the development of Drosophila differs from that previously observed for protein-coding genes.