Catfish Gonadotrophins: Cellular Origin, Structural Properties and Physiology

Gonadotrophins ( GTH s) play a central role in the regulation of gametogenesis and spawning. The structural duality of the GTH s [luteinising hormone ( LH) and follicle‐stimulating hormone ( FSH )] is established in fishes with the exception of ancestral vertebrates. Most studies indicate that, in teleosts, the GTH s are secreted in separate cells. Phylogenetic analysis shows that the common α‐subunit of the GTH s (and also of thyroid‐stimulating hormone ) and LH β are highly conserved in fishes, as in tetrapods. However, FSH β shows considerable divergence in teleosts. There may be 12 or 13 cysteine residues, with an additional one near the N‐terminus. There may be one or two N‐linked glycolsyation sites. In catfishes, there are 13 cysteine residues and one N‐linked glycosylation site. In an extreme situation, a potential glycosylation site is lacking in some fishes. Both FSH and LH receptors are characterised in teleosts. The FSH receptor is promiscuous and can be cross‐activated by LH . By contrast, the LH receptor is highly selective, being activated by its natural ligand or by heterologous ligands (e.g. human chorionic gonadotrophin ). Consequently, teleosts show different patterns of LH and FSH secretion. In catfishes, in the absence of native FSH protein, LH controls all aspects of reproduction, from early gametogenesis to spawning.