| Summary: | <p>Abstract</p> <p>Background</p> <p>MicroRNAs play a vital role in the regulation of gene expression and have been identified in every animal with a sequenced genome examined thus far, except for the placozoan <it>Trichoplax</it>. The genomic repertoires of metazoan microRNAs have become increasingly endorsed as phylogenetic characters and drivers of biological complexity.</p> <p>Results</p> <p>In this study, we report the first investigation of microRNAs in a species from the phylum Ctenophora. We use short RNA sequencing and the assembled genome of the lobate ctenophore <it>Mnemiopsis leidyi</it> to show that this species appears to lack any recognizable microRNAs, as well as the nuclear proteins Drosha and Pasha, which are critical to canonical microRNA biogenesis. This finding represents the first reported case of a metazoan lacking a Drosha protein.</p> <p>Conclusions</p> <p>Recent phylogenomic analyses suggest that <it>Mnemiopsis</it> may be the earliest branching metazoan lineage. If this is true, then the origins of canonical microRNA biogenesis and microRNA-mediated gene regulation may postdate the last common metazoan ancestor. Alternatively, canonical microRNA functionality may have been lost independently in the lineages leading to both <it>Mnemiopsis</it> and the placozoan <it>Trichoplax</it>, suggesting that microRNA functionality was not critical until much later in metazoan evolution.</p>
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