| The allotetraploid produced by distant hybridization (red crucian carp,♀×common carp,♂), is the first case artificial polyploid fish with the bisexual fertile and stable inheritance in vertebrate in the wold. The triploid crucian carp was produced by mating the males of allotetraploids with the females of Japanese crucian carp(Carassius auratus cuvieri), which have shown several enhanced performances such as sterility, faster growth rate, good flesh quality and higher anti-disease ability. Both tetraploids and triploids provided a biological platform for the comparative studies of different ploidy level fishes.Somatic growth of fish is an integral biological process. A number of hormones are essential for stimulating and modulating growth. In teleosts, as in other vertebrates, somatic growth is regulated by growth hormone/insulin-like growth factor (GH/IGF) axis, which includes growth hormone (GH), growth hormone receptors (GHR), and insulin-like growth factors. The actions of GH are initiated by its binding to a special receptor, growth hormone receptor (GHR), localized on the cell membrane of target tissues, which induces a phosphorylation cascade for signaling and insulin-like growth factor1(IGF-1) gene expression. IGF-1arrives different tissues to promote tissues growth and cellular differentiation. In this study, we firstly cloned the cDNAs and comparatively analyzed the expressions of GH/IGF aixs related genes, such as GH, GHR, IGF-1and IGF-2in different ploidy fishes. The results provided molecular endocrinology evidences for the fast growth of triploids. The main topics in this paper were presented as follows:1) By RT-PCR, we obtained the cDNA sequences of GH, GHR and IGF-1of triploids and tetraploids. The animo acid sequences of these genes had conserved domains, such as cysteine residues and N-glycosylation sites. Sequnece alignment and phylogenetic analysis revealed that triploids and tetraploids GH, GHR and IGF-1putative proteins had higher amino acid similarities with the same proteins of other fish, while lower similarities with mammalian species, which confirmed their identities. These results indicted that the related genes of GH/IGF AXIS is conservative in evolution.2) The expressions of the GH/IGF aixs related genes were investigated in different ploidy fishes. GH mRNA was virtually restricted to the pituitaries with a very strong signal. While GHR and IGF-1expressed in all the examined tissues in triploids and tetraploids, with a higher expression in liver. In spawning and prespawning season, GH transcripts in pituitaries, GHR in liver and IGF-1in liver and muscle in triploids were significantly higher than those in diploids red crucian carp and tetraploids. The higher expression of GH/IGF axis in triploids may be the reason for the fast growth of them.4) The expressions of the GH/IGF aixs related genes in different conditions were investigated in different ploidy fishes. In our experiment, the temperature had no significantly effect on the expression of these genes in diploids crucian carp and triploids. Compared to feeding status, diploids and triploids had lower expressions of these genes in fasted. Anywhere, in different temperature and nutrition state, the triploids had higher expressions of GH, GHR and IGF-1than those in diploids, which may result in fast growth of triploids. We speculate that the differences among the different ploidy fishes were not made by the environment, but by the genetic characteristics of their own.4) We used the Real-time PCR and immunohistochemistry to study the IGF-1mRNA level and IGF-1peptide location in the ovary of the different ploidy fishes. In spawning and prespawning season, the expression in triploids was the highest; the expression of tetraploids was the lowest. In the ovary, IGF-1immunoreactivity was found in early oocytes, granulosa cells, follicle cells and outside layer of zona radiate. For ovarian and liver, hCG and E2treatment in vivo depressed IGF-1transcripts. The triploids are sterile, and most of the ovary cells are early oocytes. Thus, the expression of triploids was higher than the other fishes. In addition, the HPG axis related gene expressions in triploids are disorder, which can not inhibit the expression of IGF-1, resulting in a rise of IGF-1in triploids ovary.5) We obtain the IGF-2cDNA sequences of different ploidy fishes and analyzed the sequence homology. There were six conserved cysteine residues in mature polypeptides to form disulfide linkage in different ploidy fishes. The IGF-2shared high amino acid sequence identities with fish and a relatively low amino acid sequence identity with mammal. High sequence identity in the mature polypeptides of the obtained IGF-2implied their functional conservation during evolution. The IGF-2gene was observed in all the studied tissues, and the highest level was found in liver. In the spawning season, the expression of IGF-2gene in triploids was higher than that of the other two fishes. The result suggested that IGF-2may have the positive correlation with the fast growth of triploids.We presume that a large amount of STH cells prone to secret excess GH transcripts which induce more GH secrete; while the higher expression of GH induce higher expression of GHR in liver. Subsequently, it made higher expression of IGF-1in liver and muscle, which promote somatic growth. The elevated expressions of the GH, GHR and IGF-1in triploids therefore produce a significantly effect in the growth performance. In addition, as the expression of HPG axis in triploid is disorder, it can not activate the singnal in cell and inhibit the expression of IGF-1as normal, which may results in the rapid growth of the triploid.
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