Expression of two gonadotropin-releasing hormone (GnRH) precursor genes invarious tissues of the Japanese eel and evolution of GnRH

We isolated and characterized two distinct cDNAs for mammalian gonadotropin -releasing hormone (mGnRH) and chicken GnRH-II (cGnRH-II) precursors from t he Japanese eel by rapid amplification of cDNA ends. Each GnRH precursors w ere composed of a signal peptide, a GnRH decapeptide, a processing site and a GnRH-associated peptide. Northern blot and reverse transcription-polymer ase chain reaction analysis revealed that the mGnRH precursor gene is expre ssed in all tissues tested including the brain, pituitary, eye, olfactory e pithelium, ovary, testis, liver, kidney, spleen, heart, gill, intestine, pa ncreas, muscle, skin, fin and peripheral blood leukocyte. In contrast, the cGnRH-II precursor gene expression was detected only in the brain, pituitar y, olfactory epithelium, ovary and testis. These findings suggest unknown p hysiological function(s) for mGnRH besides the well-documented role in the pituitary gonadotropin synthesis and release. The eel mGnRH acid cGnRH-II p recursors have high amino add homologies with seabream GnRH (sbGnRH) precur sors of the Perciforms and cGnRH-II precursors of other teleosts, respectiv ely. Phylogenetic analysis showed the existence of three distinct evolution ary arms of GnRHs; multiple GnRH forms (mGnRH, guinea pig GnRH, chicken GnR H-I, sbGnRH and catfish GnRH (cfGnRH)) on the first, cGnRH-II on the second , and salmon GnRH (sGnRH) on the third arm. This analysis suggests that mGn RH progenitor has undergone sequence divergence to give rise to sbGnRH and cfGnRH, whereas sGnRH represents a separate evolutionary line.