MicroRNA expression profiling during the life cycle of the silkworm (Bombyx mori)

Our results show that miRNAs display a wide variety of expression profiles over the whole life of the silkworm, including continuous expression from embryo to adult (miR-184), up-regulation over the entire life cycle (let-7 and miR-100), down-regulation over the entire life cycle (miR-124), expression associated with embryogenesis (miR-29 and miR-92), up-regulation from early 3rd instar to pupa (miR-275), and complementary pulses in expression between miR-34b and miR-275. Stage-by-stage examinations revealed further expression patterns, such as emergence at specific time-points during embryogenesis and up-regulation of miRNA groups in late embryos (miR-1 and bantam), expression associated with stage transition between instar and molt larval stages (miR-34b), expression associated with silk gland growth and spinning activity (miR-274), continuous high expression from the spinning larval to pupal and adult stages (miR-252 and miR-31a), a coordinate expression trough in day 3 pupae of both sexes (miR-10b and miR-281), up-regulation in pupal metamorphosis of both sexes (miR-29b), and down-regulation in pupal metamorphosis of both sexes (miR-275).We present the full-scale expression profiles of miRNAs throughout the life cycle of Bombyx mori. The whole-life expression profile was further investigated via stage-by-stage analysis. Our data provide an important resource for more detailed functional analysis of miRNAs in this animal.MiRNAs are an abundant class of small (~22 nucleotides) noncoding RNAs expressed by a variety of eukaryotic organisms and viruses [1,2], which represent at least 1% of predicted genes within the genomes of individual species [3]. A mammalian genome may contain >500 genes encoding miRNAs [4,5]. Accumulating evidence shows that miRNAs function in a broad range of biological processes, including development, cellular differentiation, proliferation, metabolism and apoptosis [1,6-8]. Organisms devoid of miRNAs undergo arrest during development [9,10].