Molecular characterization and genetic analysis of Gnrh2 and Gthβ in different ploidy level fishes

The gonadotropin-releasing hormone (GnRH) and the gonadotropin hormone (GTH) play crucial roles in regulating the gonadal development of the vertebrate. In this study, the Gnrh2, Fshβ and Lhβ cDNAs were cloned and characterized in red crucian carp, triploids and tetraploids, and their phylogenetic relations were comparatively analyzed. All the Gnrh2 cDNAs in different ploidy fishes encoded proteins of 86 amino acids, which consisted of a signal peptide, a GnRH2 decapeptide and a GnRH-associated peptide (GAP) linked by a proteolytic cleavage site (Gly-Lys-Arg). The GnRH2 decapeptide and proteolytic cleavage site were absolutely consistent among the three ploidy fishes, but the differences in signal peptide and GAP between diploids and tetraploids were fewer than those between diploids and triploids. It was presumed that the red crucian carp was the original maternal parent of tetraploids, so they had closer relationship. In addition, the Fshβ and Lhβ cDNAs of these three fishes encoded proteins of 130 and 140 amino acids, respectively. Compared with the molecules of other teleosts, the cysteine residues and potential glycosylation sites of Lhβ in these three fishes are fully conserved. However, most teleosts of Fshβ had 12 cysteine residues, while those of these three fishes were 13, and 12 of which might form six conserved disulfide bridges by utilizing the cleavage sites between the first and the second cysteine residues. Moreover, the lack of the second glycosylation site in Fshβ of these three fishes might influence the special structure and biological activities. On the other hand, the phylogenic tree analyses revealed that Gnrh2, Fshβ and Lhβ had similar phylogeny relationships among the cyprinids, which indicated that they were conserved in molecular structure and function during the evolution.