Small RNAs in germ cell development.

The identification of small RNA molecules and the elucidation of their functions in the cell enhanced our understanding of the mechanisms controlling gene expression at the posttranscriptional level. Here, we review findings concerning the role small RNA molecules play in the development of the germline in various organisms. In this context, microRNAs (miRNAs) function predominantly in regulating mRNA expression and turnover and direct the expression of specific proteins to founders of the germline, the primordial germ cells (PGCs). miRNA function is then important for the maintenance of germline stem cells (GSCs) in their niche and for coordinating GSC differentiation into the gametes. The second family of small RNA molecules, the short endo-siRNAs, regulate gene expression posttranscriptionally as well but are also important for transposable elements regulation and thus for genome stability. Last, the Piwi-interacting small RNAs (piRNAs) are specifically expressed in the germline, where they primarily control transposon activity, guarding the genome from potential damage associated with excessive transposition. Together, the function of these three groups of small RNAs ensures proper development and continuation of the germline through the generations.