Evaluating the radiation of the POMC gene in teleosts: characterization of American eel POMC.

A distinctive feature of the pituitary hormone precursor, proopiomelanocortin (POMC), is the presence of multiple melanocortin core sequences (HFRW), and one copy of the opioid, beta-endorphin. In the older lineages of ray-finned fish (i.e., orders Acipenseriformes and Semionotiformes), certain extant lobe-finned fish (Australian lungfish and African lungfish), and the tetrapods there are three melanocortin regions in POMC: ACTH/alphaMSH, beta-MSH, and gamma-MSH. However, among the teleosts, the most recent radiation of the ray-finned fishes, the gamma-MSH sequence is absent from the POMC genes of euteleosts like the carp, tilapia, chum salmon, sockeye salmon, and rainbow trout. The objective of this study was to determine whether the gamma-MSH sequence still may be present in the POMC gene of a more basal lineage of the teleosts such as a representative from subdivision Elopomorpha. To this end, a POMC cDNA was cloned and sequenced from the pituitary of the American eel, Anguilla rostrata (order Anguilliformes, family Anguillidae). The open reading frame of the eel POMC cDNA was 648 nucleotides in length and encoded 216 amino acids. As predicted, eel POMC contained the deduced amino acid sequences for beta-endorphin, ACTH/alpha-MSH, and beta-MSH. These end-products displayed primary sequence features that are common to ray-finned fish. Eel POMC lacks a gamma-MSH sequence and a large portion of the joining peptide region. In this regard, the eel POMC gene thus displays features very similar to the POMC genes that have been sequenced from euteleosts. Although it is conceivable that the gamma-MSH sequence may be present in representatives from the other basal extant lineages of teleosts (i.e., subdivisions Osteoglossomorpha or Clupeomorpha), it is also possible that the deletion that resulted in the loss of the gamma-MSH sequence occurred in the ancestral neopterygian that gave rise to the teleosts. In this case, the gamma-MSH sequence should be absent in all extant teleosts.