Molecular characterization of sea bass gonadotropin subunits (α, FSHβ, and LHβ) and their expression during the reproductive cycle

Abstract

Reproduction is controlled by two pituitary gonadotropin hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). This study reports the cloning, sequence analysis, and gene expression of gonadotropin (GTH) subunits from the European sea bass ( Dicentrarchus labrax). The GTH subunits were cloned from a sea bass brain–pituitary cDNA library. The nucleotide sequences of the common α, the FSHβ, and the LHβ subunit cDNAs were 625, 521, and 591 base pair (bp) long, respectively, encoding for mature peptides of 94, 105, and 115 amino acids (aa), respectively. Sequence analysis showed that sea bass FSHβ is more similar to higher vertebrate FSHβ’s (35–37%) than to LHβ’s (26–30%), whereas sea bass LHβ is more similar to LHβ’s (40–53%) than to FSHβ’s (26–41%). Phylogenetic analysis of fish GTH sequences grouped the β subunits into two groups, FSH and LH, distributed into four classes, corresponding to the accepted divisions of Elopomorphs, Ostariophysis, Salmonids, and Percomorphs. A dot-blot technique was developed to analyze GTH pituitary mRNA levels during the reproductive cycle of male sea bass. From October (initiation of gametogenesis) to February (spermiation), the expression of all three subunits in the pituitary increased in parallel, concomitantly with the gonadosomatic index (GSI) and the accumulation of LH protein in the pituitary, all values declining sharply at post-spermiation. This study demonstrates that the pituitary of sea bass contains two gonadotropin hormones and that both gonadotropins are probably involved in the control of gametogenesis, gamete maturation, and spermiation.