Structure, evolution and function of the bi-directionally transcribed iab-4/iab-8 microRNA locus in arthropods

In Drosophila melanogaster, the iab-4/iab-8 locus encodes bi-directionally transcribed microRNAs that regulate the function of flanking Hox transcription factors. We show that bi-directional transcription, temporal and spatial expression patterns and Hox regulatory function of the iab-4/iab-8 locus are conserved between fly and the beetle Tribolium castaneum. Computational predictions suggest iab-4 and iab-8 microRNAs can target common sites, and cell-culture assays confirm that iab-4 and iab-8 function overlaps on Hox target sites in both fly and beetle. However, we observe key differences in the way Hox genes are targeted. For instance, abd-A transcripts are targeted only by iab-8 in Drosophila, whereas both iab-4 and iab-8 bind to Tribolium abd-A. Our evolutionary and functional characterization of a bi-directionally transcribed microRNA establishes the iab-4/iab-8 system as a model for understanding how multiple products from sense and antisense microRNAs target common sites.