How complex body plans evolved in animals such as fruit flies and vertebrates, as compared to the relatively simply jellyfish and sponges, is not known given the similarity of developmental genetic repertoires shared by all these taxa. PIs have shown that a core set of 18 microRNAs (miRNAs), non-coding RNA molecules that negatively regulate the expression of protein-coding genes, are found only in protostomes and deuterostomes and not in sponges or cnidarians. Because many of these miRNAs are expressed in specific tissues and/or organs, miRNA-mediated regulation could have played a fundamental evolutionary role in the origins of organs such as brain and heart - structures not found in cnidarians or sponges - and thus contributed greatly to the evolution of complex body plans. Furthermore, the continuous acquisition and fixation of miRNAs in various animal groups strongly correlates both with the hierarchy of metazoan relationships and with the non-random origination of metazoan morphological innovations through geologic time. To test these ideas, PIs will make miRNA libraries from a sponge, a cnidarian, an acoel flatworm, an annelid, and a mollusc, and they predict that sponges will not have any miRNAs, whereas the cnidarian and the acoel flatworm will have very few beyond the set of phylogenetically conserved miRNAs. In addition, they expect to find new miRNAs shared only between the annelid and mollusc, and new miRNAs specific to each taxon. Unraveling the phylogenetic history of non-coding RNA molecules may shed a very bright light on the mechanisms underlying the evolution of complex animal body plans.
