RNAi pathways in Mucor: A tale of proteins, small RNAs and functional diversity

•Regulation of gene expression by endogenous small RNAs in a basal fungus.•Dicer-dependent and Dicer-independent RNAi pathways.•Physiological and developmental processes regulated by RNAi.•A novel RNase III involved in a new RNA degradation pathway. The existence of an RNA-mediated silencing mechanism in the opportunistic fungal pathogen Mucor circinelloides was first described in the early 2000. Since then, Mucor has reached an outstanding position within the fungal kingdom as a model system to achieve a deeper understanding of regulation of endogenous functions by the RNA interference (RNAi) machinery. M. circinelloides combines diverse components of its RNAi machinery to carry out functions not only limited to the defense against invasive nucleic acids, but also to regulate expression of its own genes by producing different classes of endogenous small RNA molecules (esRNAs). The recent discovery of a novel RNase that participates in a new RNA degradation pathway adds more elements to the gene silencing-mediated regulation. This review focuses on esRNAs in M. circinelloides, the different pathways involved in their biogenesis, and their roles in regulating specific physiological and developmental processes in response to environmental signals, highlighting the complexity of silencing-mediated regulation in fungi.