Duality of gonadotropins in gnathostomes

The glycoprotein hormone alpha subunit and two beta subunits were cloned from the ventral lobe of the pituitary gland of an elasmobranch fish, Scyliorhinus canicula. The mature alpha subunit was 96 amino acids long and showed 64-76 amino acid residues in common with alpha subunit sequences of representatives of sarcopterygians (tetrapods and dipnoi) and actinopterigyans (chondrostei and teleostei). The Scyliorhinus beta 1 subunit was 115 amino acid long and had characteristics specific to FSH beta subunits and, in particular, the two potential N-linked glycosylation sites in conserved positions. The beta 2 sequence was 112 amino acids long. The Scyliorhinus beta 2 subunit had only one potential N-linked glycosylation site at the same position as that in LH. None of the two beta subunits from Scyliorhinus displayed the two amino acid insertions shared by TSH beta subunit sequences between the fifth and the sixth cysteines as compared to actinopterygian and sarcopterygian gonadotropins. These data indicate that Scyliorhinus beta 1 and beta 2 subunits are orthologous to FSH and LH beta subunits, respectively. It is concluded that the two FSH and LH lineages were already individualized at the emergence of chondrichthyans.