Identification of members of the gonadotropin-releasing hormone (GnRH), corticotropin-releasing factor (CRF) families in the genome of the holocephalan, Callorhinchus milii (elephant shark)

The gonadotropin-releasing hormone (GnRH) and corticotropin-releasing family (CRF) are two neuropeptides families that are strongly conserved throughout evolution. Recently, the genome of the holocephalan, Callorhinchus milii (elephant shark) has been sequenced. The phylogenetic position of C. milii, along with the relatively slow evolution of the cartilaginous fish suggests that neuropeptides in this species may resemble the earliest gnathostome forms. The genome of the elephant shark was screened, in silico, using the various conserved motifs of both the vertebrate CRF paralogs and the insect diuretic hormone sequences to identify the structure of the C. milii CRF/DH-like peptides. A similar approach was taken to identify the GnRH peptides using conserved motifs in both vertebrate and invertebrate forms. Two CRF peptides, a urotensin-1 peptide and a urocortin 3 peptide were found in the genome. There was only about 50% sequence identity between the two CRF peptides suggesting an early divergence. In addition, the urocortin 2 peptide seems to have been lost and was identified as a pseudogene in C. milii. In contrast to the number of CRF family peptides, only a GnRH-II preprohormone with the conserved mature decapeptide was found. This confirms early studies about the identity of GnRH in the Holocephali, and suggests that the Holocephali and Elasmobranchii differ with respect to GnRH structure and function.

Research highlights
► Only a single form of GnRH was found in the Callorhinchus milii genome.
► Two forms of CRF were present, which appears to be the result of an ancient gene duplication in this taxon.
► No evidence for a functional urocortin 2 gene.
► These findings suggest that the interaction of the GnRH and CRF system in the Holocephali differ from other lineages.