MicroRNAs and RNA interference in zebrafish development.

Posttranscriptional regulation of gene activity has been a somewhat neglected subject in developmental genetics. With the discovery of RNA-mediated silencing mechanisms, however, insights into how targeted transcript inactivation integrates with developmental processes have changed radically. The number of studies in zebrafish that take advantage of techniques to manipulate the activity of microRNAs (miRNAs)--a group of short, noncoding RNAs that suppress translation of target genes--is on a steady rise, and the studies are starting to provide unique insights into the diversity of developmental processes that are controlled by transcript inhibition. Here I review recent studies in the zebrafish that demonstrate roles for miRNAs in the fine-tuning of neural crest cell migration, regulation of neural Hox gene expression, and regeneration after tissue amputation. New discoveries on the involvement of miRNAs in regulating red blood cell maturation also shed light on how miRNA gene activity itself is controlled. Because experimental suppression of single miRNAs often results in surprisingly specific phenotypes, it will have to be considered whether novel mutants identified in genetic screens should also be assayed for lesions in miRNA genes or their target sequences, rather than in protein-coding genes alone.