As a valuable commercial fish, Takifugu poecilonotus has high exploitability and aquaculture prospects. However, there were few studies regarding growth-related and sex-related genes of T. poecilonotus. In order to explore the mechanisms of sexual growth dimorphism and sex determination, we identified sex-related and growth-related genes and their biological pathways. It is first time that the transcriptome data of finepatterned puffer (Takifugu poecilonotus) has been reported. The result of annotation revealed that numerous sequences of finepatterned puffer could be annotated to well-known genes and biological pathways regarding growth and sex. In the present study, 149,814 unigenes were sequenced and assembled. In total, 110,737 (73.92%) unigenes were significantly matched to protein databases. In the present study, 10,385 DEGs were identified between different sexes of T. poecilonotus. Some of these DEGs play important roles in sex growth and determination. Our results may provide fundamental resources for further research on the molecular mechanisms of biological processes of T. poecilonotus.
In the present study, numerous growth- and sex-related genes were detected from the transcriptome data of T. poecilonotus. Obviously, it is widely accepted that genes encoding components of the somatotropic axis play critical roles in regulating the formation of skeletal muscles in finfish, including the growth hormone gene (Gh), insulin-like growth factors (Igf), somatostatin and their carrier proteins and receptors [41]. Jia et al. [42,43] showed that specific growth rate, body weight and plasma growth hormone of female tiger puffers (Takifugu rubripes) reared in an offshore sea cage aquaculture system (OSCS) were significantly higher than those of fish reared in other aquaculture systems. The mRNA expression results indicated that female individuals reared in the OSCS showed higher somatic growth axis-related gene (Igf1, Igf2, Igf1r, Igf2r, Ghr1, and Ghr2) expression levels. In the present study, compared to male T. poecilonotus, female T. poecilonotus showed significantly higher Igf1r, Igfbp1, Igfbp3 and Ghr expression levels.
Fatty acid-binding proteins (FABPs) belong to the protein superfamily of lipid-binding proteins [44]. They are 126-137 amino acids in length depending on species, with an average molecular mass of 14-16 kDa [45-47]. Fabps have been well studied for decades [44,48]. The first teleost FABPs were identified in the hearts of sea ravens (Hemitripterus americanus) and ocean pouts (Macrozoarces americanus) [49]. Subsequently, teleost FABPs/Fabps were identified in different tissues of many teleosts, including striped bass (Morone saxatilis) [50], rainbow trout (Oncorhynchus mykiss) [51], Atlantic salmon (Salmo salar L.) [52]. Fatty acid-binding protein genes are involved in many processes of cell physiology, including development, growth, and cell differentiation [53]. In many respects, Fabps participates in the binding, sequestration and metabolism of long-chain fatty acids, eicosanoids, bile salts, and other hydrophobic ligands [54-56]. Fabps may also be possible carriers of certain hydrophobic reactants in their passage from the cytosol to chromatin, and thus, they may have a direct or indirect effect on cell growth [57]. Previous studies identified and characterized the Fabps genes of pufferfish (Tetraodon nigroviridis), including Fabp1-3, Fabp6, Fabp7, Fabp10, and Fabp11 [58-59]. In the present study, compared to female T. poecilonotus, male T. poecilonotus exhibited higher Fabp1, Fabp6 and Fabp7 expression levels, showing that these sex-specific genes may play critical roles in reproduction and growth.
More importantly, we also found Fabp4 in the transcriptome of T. poecilonotus. This is the first time that the expression of Fabp4 has been found in fish. By binding with long-chain fatty acids, Fabp4 can affect the uptake, transportation, esterification, and b-oxidation of fatty acids and regulate the energy balance and lipid signalling within cells [60,61]. Furthermore, fabp4 has also been found to be associated with growth, fat deposition and carcass traits in mammals. It was reported that Fabp4 is a potential candidate gene for obesity, as it is located within a quantitative trait locus (QTL) region for serum leptin levels in mice [62]. In addition, FABP4 protein content may be a marker of intramuscular fat accretion in the longissimus thoracis muscle in pigs [63,64]. Obviously, Fabp4 has become a strong candidate gene for fat metabolism. However, the biological function of the Fabp4 gene in fish is unknown, and further work is required to fully characterize the Fabp4 gene in fish.
Prolactin (PRL) plays critical role in multiple biological function by binding to its receptor (PRLR) in fish [65]. In adult fish, the major action of PRL is freshwater osmoregulation. In addition, it has also been reported that PRL associated with reproduction, behaviour, growth, and immunoregulation [65-67]. Prolactin and PRLR are also present in embryos and exhibit widespread tissue distribution in fish larvae. Yang et al. [68] identified transcripts of PRL and PRLR in the early embryo of rainbow trout (O. mykiss) and indicated PRL and PRLR was associated with the post-hatching development of larvae. In early development and metamorphosis of amphibians and mammals, the role of PRL has been well understood. However, the role of PRL in fish is not clearly established. The potential roles of PRL in fish embryos and larvae are considered in relation to their physiological status, and the spectrum of activities differs by species (Power, 2005). Relatively few reports support somatotropic action for PRL in fish. Some researchers indicated that PRL influences the growth of Mozambique tilapia by stimulating liver IGF-I production [69]; however, the way in which PRL stimulates IGF-I production is unclear. Hence, further work is required to fully characterize the biological function of PRL and PRLR in fish.
Sex determination is the process of establishing individual gender and regulating the differentiation of sex characteristics [70,71]. In contrast to other vertebrates (birds or humans), the sex determination of teleost fishes is diverse [72-74]. According to the annotations, many sex-related genes have been identified in the T. poecilonotus transcriptome. As a sex-determining gene, doublesex and mab-3-related transcription factor 1 (Dmrt1) is highly conserved and has been identified in many teleost fish [72,75,76]. The dmrt1 gene plays an important role in the testis differentiation and maintenance of male-specified germ cells by encoding a transcription factor [77]. In the present study, Dmrt1 showed higher expression levels in male T. poecilonotus than in female T. poecilonotus, indicating that Dmrt1 may be crucial for the development and maturation of gonads in male T. poecilonotus.
As extracellular matrix surrounding oocytes, zona pellucida plays a protective role in fish oocytes, and it is important in sperm binding [78]. Zona pellucida comprises four kinds of glycoproteins (ZP1-4), they are incorporated into long filaments [79,80]. The zona pellucida proteins were originally detected in the egg envelope of mammalian, and have also been found in the inner layer of the fish chorion [81]. Previous study has shown that Zp2 plays an important role in the early formation of oocyte envelope, and Zp3 could be treated as a major class of female-specific reproductive molecules [82]. In the present study, Zp1, Zp2, and Zp3 showed higher expression levels in T. poecilonotus females than that in males, suggesting that these genes may also play critical roles in folliculogenesis and reproduction.