MicroRNAs and the Evolution of Metazoan Complexity

Kevin Peterson (Dartmouth College, USA)

As a historical science Evolutionary Biology is broadly studied by workers who usually focus on either evolutionary pattern or evolutionary process. This is because the types of data used to elucidate the patterns of life are largely divorced from the processes that evolve life. For example, the sorts of data now being used to build the metazoan phylogenetic tree, whether it be expressed sequence tags or mitochondrial genomes, are mute about how animals initially evolved, diversified, complexified and simplified over their near 800 million year evolutionary history. One new source of phylogenetic information though, microRNAs (miRNAs), may also shed insight into key questions surrounding the evolution of animals, and in particular animal complexity. miRNAs are small, ~22 nucleotide non-coding genes that regulate, usually negatively, the translation of protein-coding genes. But unlike the transcription factors and other components of the developmental tool kit that miRNAs regulate, new clade-specific miRNAs are continuously being acquired through time in all animal lineages studied to date so that miRNAs can be used as a source of phylogenetic information. In addition though, large increases to the miRNA repertoire are associated with increases to animal complexity, including at the base of the bilaterians and at the base of the vertebrates. Where studied, the most copiously expressed miRNAs in clade-specific tissues (e.g., the vertebrate liver) are clade-specific miRNAs (e.g., the vertebrate-specific miR-122), and whose mis-regulation often results in disease of that very same organ (e.g., liver cancer). On the other hand, simple animals (e.g., sponges) have few miRNAs, and losses of miRNAs within the bilaterians are associated with instances of secondary morphological simplification, as seen in, for example, rotifers and acoel flatworms. Therefore, miRNAs might be a unique source of evolutionary data in that they simultaneously shed insight into both the evolutionary pattern and at least some of the processes underlying animal evolutionary history.