The ADAR Family in Amphioxus: RNA Editing and Conserved Orthologous Site Predictions.

RNA editing is a relatively unexplored process in which transcribed RNA is modified at specific nucleotides before translation, adding another level of regulation of gene expression. Cephalopods use it extensively to increase the regulatory complexity of their nervous systems, and mammals use it too, but less prominently. Nevertheless, little is known about the specifics of RNA editing in most of the other clades and the relevance of RNA editing from an evolutionary perspective remains unknown. Here we analyze a key element of the editing machinery, the ADAR (adenosine deaminase acting on RNA) gene family, in an animal with a key phylogenetic position at the root of chordates: the cephalochordate amphioxus. We show, that as in cephalopods, ADAR genes in amphioxus are predominantly expressed in the nervous system; we identify a number of RNA editing events in amphioxus; and we provide a newly developed method to identify RNA editing events in highly polymorphic genomes using orthology as a guide. Overall, our work lays the foundations for future comparative analysis of RNA-editing events across the metazoan tree.

Origin and evolution of the Trk family of neurotrophic receptors.

Among the numerous tyrosine kinase receptors, those belonging to the Trk family are distinctively involved in the development of complex traits within the vertebrate nervous system. Until recently, the lack of a proper Nt/Trk system in invertebrates has lead to the belief that they were a vertebrate innovation. Recent data, however, have challenged the field, and proved that bona fide Trk receptors do exist in invertebrates. Here, we review and discuss the evolutionary history of the Trk receptor family, and draw a comprehensive scenario that situates the origin of the Nt/Trk signalling prior to the origin of vertebrates. Probably, a ProtoTrk receptor was invented by means of domain and exon shuffling from pieces of ancient genes, generating the unique combination of domains found in extant Trk receptors. It is suggestive to propose that subtle protein mutations, gene duplications, and co-options in particular territories of a primitive Nt/Trk system were instrumental to the development of a complex vertebrate nervous system.