Functional characterization of a "plant-like" HYL1 homolog in the cnidarian Nematostella vectensis indicates a conserved involvement in microRNA biogenesis

While the biogenesis of microRNAs (miRNAs) in both animals and plants depends on Dicer, a conserved RNAse III enzyme, its helping partner proteins are considered distinct for each kingdom. Nevertheless, recent discovery of homologs of Hyponastic Leaves1 (HYL1), a plant-specific Dicer partner, in the metazoan phylum Cnidaria challenges the view that miRNAs evolved convergently in animals and plants. Here we show that the HYL1 homolog Hyl1-like a (Hyl1La) is crucial for proper development and miRNA biogenesis in the cnidarian model Nematostella vectensis. Inhibition of Hyl1La resulted in arresting of metamorphosis in Nematostella embryos. Moreover, most miRNAs are significantly downregulated in Hyl1La knockdown animals. These results support the participation of cnidarian HYL1 homologs in miRNA biogenesis and points towards the function of this pathway in cnidarian development. Further, it suggests that the last common ancestor of animals and plants carried a HYL1 homolog that took essential part in miRNA biogenesis.