Characterization And Sex-dependent Expression Of GH/IGF Axis Genes In Yellow Catfish

Sexual size dimorphism has been observed in many aquaculture fish species including yellow catfish, in which male fish grow much faster than female fish. In vertebrates, body growth is greatly regulated by the growth hormone(GH) / insulin-like growth factor(IGF) axis which consists of several pivotal signaling factors including GH, IGF-1 and IGF-2. In order to investigate the molecular mechanism of sexual size dimorphism in yellow catfish(Pelteobagrus fulvidraco), Tissue distribution of GH, IGF-1 and IGF-2 will be studied. To assess whether GH/IGF axis genes are involved in the process of body growth, the expression pattern of GH, IGF-1 and IGF-2 during larval growth will be investigated. Additionally, the direct effect of MT on the expression of GH, IGF-1 and IGF-2 mRNAs will be investigated in yellow catfish. The main results are respresented as follow:1、The GH cDNA of yellow catfish is 1088 bp that contained a 603 bp coding sequence. The GH protein has a signal peptide and predicted functional domain. The mRNA sequence of yellow catfish IGF-I is 547 bp that encodes a protein with 145 amino acids. The deduced amino acid sequence of yellow catfish IGF-I is composed of a signal peptide and predicted IIGF domain. IGF-II cDNA of yellow catfish is 876 bp and contains a 759 bp coding region encoding a protein with 252 amino acids. The deduced amino acid sequence of yellow catfish IGF-2 is composed of a signal peptide, predicted IGF-2 domain and predicted IGF II E-peptide domains. Phylogenetic tree analysis showed that the GH tree showed four main clades including mammals, birds, amphibians and teleosts. In the clade of teleosts, yellow catfish is grouped with high bootstrap support in the branch of Siluriformes, which is closed to the Cypriniformes. The IGF-I and IGF-II phylogenetic tree displayed two respective branch, in which either IGF-I or IGF-II sequence shared high similarities, respectively. In the clade of teleosts, yellow catfish and other Siluriformes species were clustered together with high support.2、These three gene were widely expressed in the tested tissues. Pituitary was the main tissue for GH mRNA expression with very low expression in other tissues. IGF-1 was most expressed in liver, followed with hypothalamus, pituitary and gonad, whereas IGF-2 was mainly expressed in liver with relative low expression in hypothalamus and pituitary. These indicated that pituitary gland is the main location of GH secretion, while IGF-1 and IGF-2 are mostly produced in hypothalamus and liver.3、GH had a relative stable expression in female larval fish between 1 to 3 weeks post hatching(wph) and increased at 4 wph. In contrast, GH was gradually increased between 1-4 wph. For IGF-1, its expression was gradually increased in both males and females between 1-4 wph. The expression of IGF-2 was slightly increased at 2wph and decreased at 3wph, and then was recovered at 4wph similar to its expression at 2wph. However, IGF-2 was consistently increased between 1-3 wph, and slightly decreased at 4wph. Interestingly, all the expression of GH, IGF-1 and IGF-2 was significantly higher in male fish than the female fish during larval growth. These results indicate that different levels of key genes in the GH/IGF axis can explain the sexual size dimorphism in multiple fish species.4、In female larval fish, methyltestosterone(MT) treatment activated the expression level of GH/IGF axis genes compared with the control groups. GH was activated at one week post treatment(wpt) and increased at 3wpt to a more high level. IGF-1 was activated at 2wpt and decreased at 3wpt and then keep a stable level at 4wpt. In addition, IGF-2 was activated at 2wpt and consistently increased at 3wpt and then followed down at 4wpt. Moreover, all the expression of GH, IGF-1 and IGF-2 mRNAs was activated between 1-3 wpt. In male larval fish, both GH and IGF-1 are gradually increased in control groups during larval development. GH was also gradually increased in the MT treated group whereas its expression was much lower than the control at all stages. IGF-1 was increased at 2wpt, and decreased at 3wpt and then increased at wpt. But its expression was reduced by MT compared with the control. The expression of IGF-2 was increased in both the MT treated and control groups at 2wpt. while it was reduced by MT at 3wpt. Our results suggest that sex difference in growth performance are probably caused by the expression difference of GH/IGF signaling between males and females which are regulated by the sex hormone.