Expression of gonadotropin and gonadotropin receptor genes during early sexual maturation in male Atlantic salmon parr.

Atlantic salmon males may mature already as small parr in freshwater. Sexual maturation in teleosts as in vertebrates is characterized by the activation of the brain-pituitary-gonad axis. The endocrine regulation of early puberty is still not well understood. In the present study, one-summer-old male Atlantic salmon parr were sampled regularly from December several months prior to the beginning of spermatogenesis until spawning in October. Pituitary expression levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) beta subunit genes were analyzed in parallel with testis expression of FSH receptor (FSHR) and LH receptor (LHR) genes by RT-PCR and plasma 11-ketostestosterone (11-KT) was measured. Expression levels of FSHbeta, low during winter and spring started to increase prior to the onset of gonadal growth at the end of May while LHbeta mRNA levels were hardly detectable. Both gonadotropin receptor genes were expressed in immature testis with FSHR transcripts being more abundant (8-fold). FSHR transcript levels increased in parallel to FSHbeta levels from early spermatogenesis onwards, while LHR mRNA started to increase prior to any large changes in LHbeta expression. Both transcript levels of LHbeta and LHR were highest during spermiation. Plasma 11-KT increased at the beginning of spermatogenesis reaching highest levels at spermiogenesis suggesting a possible role of FSH in inducing 11-KT production during early spermatogenesis while LH stimulates via its specific receptor 11-KT production at spermiogenesis. The commitment into sexual maturation appears to be dependant on both the presence of FSHR in immature testis and the increase of FSH expression.