Observations on the radiation of lobe-finned fishes, ray-finned fishes, and cartilaginous fishes: Phylogeny of the opioid/orphanin gene family and the 2R hypothesis

At the close of the Devonian Period the rapid decline in the diversity of the lobe-finned fishes was countered by the emergence and diversification of the ray-finned fishes and the cartilaginous fishes that now dominate marine and freshwater ecosystems. All of these jawed vertebrates were derived from the ancestral gnathostomes; a chordate lineage that had experienced two genome duplication events during the evolution of the phylum. This review analyzes trends in the phylogeny of the opioid/orphanin gene family (four prohormone/neuropeptide precursor-coding genes) in the major classes of gnathostomes that survived the extinction events at the close of the Devonian Period and focuses on some features of this gene family that appear to set the cartilaginous fishes (class Chondrichthyes) apart from class Sarcopterygii (lobe-finned fishes and tetrapods) and class Actinopterygii (the ray-finned fishes).

Research highlights
► The opioid/orphanin gene family has undergone three distinct radiations in the major gnathostome lineages.
► The proorphanin gene may have emerged after the chordate 2R genome duplication event.
► The cartilaginous fish may have diverged from the bony fish before the proorphanin gene emerged.