microRNAs in Drosophila regulate cell fate by repressing single mRNA targets

Regulation of gene expression governs all aspects of the lifespan of the organism, such as embryonic development, stem cell differentiation, reproduction and aging. Among the most important regulators of these extremely complex processes are microRNAs (miRNAs), small non-coding RNAs that repress gene expression by binding to primary sequences on the mRNA of their target. Theoretically, the mere existence of a miRNA recognition sequence on a given mRNA is sufficient to generate a functional response. Since these short sequences are abundant, one miRNA can potentially bind to multiple targets, thus generating endless possible biological outcomes. However, is this really the case? Bioinformatics and molecular biology tools provide theoretical interaction predictions, but the data obtained by these methods is often too general and is impaired by false identifications. Therefore, a better understanding of the biological role of miRNAs requires mapping of the exact miRNA-mRNA interactions that occur in vivo. Drosophila melanogaster provides several unique advantages over other model organisms in the study of miRNA functional targeting. The majority of its miRNAs are evolutionarily conserved up to humans, suggesting that they regulate similar pathways across organisms. Complete genome-wide collections make Drosophila the only organism that enables constitutive and inducible gain and loss-of function manipulations of all annotated miRNAs. These powerful tools led to several groundbreaking discoveries of the role that miRNAs play in regulation of development, stem-cell function and aging, and proved that although many outcomes are possible, most Drosophila miRNAs regulate a single phenotype through downregulation of a single major mRNA target.