Hagfish, Genome Duplications, and RFamide Neuropeptide Evolution

Abstract

As a phylum, the chordates are remarkably similar in terms of body plan design compared with many of the other major phyla of metazoans. The minimal features for placement within the phylum are a dorsal nerve cord and a notochord and pharyngeal gills slits at some point in the developmental life history of the organism (1). As shown in Fig. 1A, chordates can be initially segregated based on whether the organism lacks an internal cartilaginous or bony skeleton (protochordates; such as Amphioxus or the tunicates) or have an internal skeleton (subphylum Vertebrata). The vertebrates can be further separated into those organisms that lack a hinged jaw (superclass Agnatha; such as the hagfishes or lampreys) and those organisms that have a hinged jaw (superclass Gnathostoma; such as the cartilaginous fishes, bony fishes, amphibians, reptiles, birds, and mammals). As indicated from the fossil record, the ancestral protochordates most likely emerged during the Cambrian Period, some 550 million years ago (MYA) (2), and were followed in relatively quick succession by the ancestral agnathan vertebrates (late Ordovician Period; 450 MYA), and later the ancestral gnathostomes (Silurian Period 420 MYA) (3). Although the external and internal morphological design of the vertebrates (agnathans and gnathostomes) is based on common themes, there are features of the chordates that clearly have undergone considerable change during the evolution of the phylum. Two of these features, the diversity of gene families, and the interactions between the brain and pituitary, are considered in the article by Osugi et al. (4) in this issue of Endocrinology on the evolution, characterization, and localization of RFamide peptides in the central nervous system (CNS) of a hagfish. Why study hagfish, and what information on neuropeptide gene family evolution and brain/pituitary interactions can be gleaned from these organisms?