Zebra Fish Embryos That Lack Mature Mirnas Undergo Abnormal Morphogenesis

that act as posttranscriptional repressors by binding to the 3′UTR of target mRNAs (Lee et al. 1993; Reinhart et al. 2000; Bartel 2004; Kloosterman and Plasterk 2006). In animals, mature miRNAs are generated from a primary transcript (pri-miRNA) through sequential cleavage by nucleases belonging to the RNAse III family. Initially, Drosha cleaves the pri-miRNA and excises a stem-loop precursor (pre-miRNA), which is then cleaved by Dicer into an RNA duplex (Bernstein et al. 2001; Grishok et al. 2001; Hutvagner et al. 2001; Ketting et al. 2001; Knight and Bass 2001). One strand of the duplex constitutes the mature miRNA, which is incorporated into a silencing complex (RISC) and guides it to target mRNAs (Hammond et al. 2000; Hutvagner and Zamore 2002; Khvorova et al. 2003; Schwarz et al. 2003). The 5′ region of the miRNA (seed) is the main determinant of target recognition (Lai 2002; Lewis et al. 2003, 2005; Doench and Sharp 2004; Brennecke et al. 2005). Hundreds of miRNAs have been identified and thousands of targets have been predicted (Lagos-Quintana et al. 2001; Lau et al. 2001; Lee and Ambros 2001; Enright et al. 2003; Stark et al. 2003, 2005; Ambros 2004; Bartel 2004; Rehmsmeier et al. 2004; Berezikov et al. 2005; Farh et al. 2005; Krek et al. 2005; Lai 2005; Lewis et al. 2005; Miranda et al. 2006; Rajewsky 2006; Sood et al. 2006). However, the developmental roles of microRNAs are largely elusive (Alvarez-Garcia and Miska 2005; Kloosterman and Plasterk 2006), and it is still controversial how target mRNAs are repressed (Pillai 2005; Valencia-Sanchez et al. 2006). We have used the zebra fish embryo as a model system to investigate these issues.