We employed the transcriptome analysis to investigate the different gene expression of fin tissues of pearl white red tilapia. The results showed that the tyrosine protein kinase STYK1 HLF7, HSP 70, HSP 30, HSP 90 and Transcription factor Sp6 expression increased significantly, while MC1R, Transcription factor (MafB, Jun-D, AP − 1, E2F5, ETV6, Sp9, Sp7, E2F1, Sp4) expression significantly decreased in low temperature group, indicating that the color change of red tilapia at low temperature resulted from the co-regulation of multiple genes.
Heat shock protein (HSP70) is one of the most conservative and important proteins in HSPP family which is synthesized by the reaction of organism to the physical, chemical and biological stress agents in the environment [21]. HSP70 gene can produce abundant heat stress proteins, remove the denatured proteins, protect the cells from the damage of denatured proteins, and reduce the impact of external stress environment on the body of fish when the cells are under stress [22]. Studies have found that the HSP70 mRNA expression in the liver was significantly increased after 12 hrs of low-temperature stress on the fish [23]. It has been demonstrated that both low temperature and high temperature caused the rapid expression of HSP70 gene in the body cells of platy bream and the synthesis of heat stress protein [23]. In our study, the expression of HSP70 in HLF7 cultured at low temperature was significantly higher than that in normal temperature group, suggesting that the low temperature environment outside led to the changes in the physiological function of red tilapia. Moreover, HSP70 in the body of red tilapia would be highly expressed to protect the normal physiological function of fish, thus enhancing the adaptability of red tilapia to the environment.
The biological basis of body color change in fish is the number, morphology and distribution of different pigment cells on skin and scales [24]. Compared with only a kind of pigment cell in the mammalian (melanocytes), bony fishes were found to have 6 kinds of pigment cells, including the melanocytes, red pigment cells, yellow pigment cells, rainbow, white and blue pigment cells, making the formation and changes of the fish body color more complicated and different red tilapia showing different body colors [25]. It has also been reported that MC1R gene, ASIP gene and TYRP1 gene are the major candidate genes of fish body color formation [26]. MC1R gene, as a key gene to produce melanin, expressed slightly in HLF7 of the low temperature group, and it was not the main reason for the formation of black spots [27]. It is speculated that the formation of melanin by MC1R gene was affected by the albino gene. Meanwhile, the MC1R gene, which was related to the synthesis of melanin, represents a critical role in the formation of fish body color. However, MC1R expression was low in HLF7 in black-spotted red tilapia of the low-temperature group in our study, probably because it was an acute regulator and made pigments significantly deposited in the short term. And hence the surface of the skin was colored with streaks, and then black markings or red markings, but the expression level was stable.
It has been revealed that the body color of Mozambique tilapia became black in the dark after 25 hrs, but there was no significant change in MC1R expression in the epidermis and the α-MSH level in the serum. Therefore, the MC1R gene could also act as an acute regulator in the regulation of fish pigmentation, but the pigment is significantly deposited in a short period sine this activity expression cannot be maintained for a long time [28]. So even if the red tilapia epidermis exhibited a certain color, there was no significant change in MC1R mRNA expression in fish tissues.
Tyrosinase is the rate-limiting enzyme of melanin synthesis vital for the rate and quantity of eumelanin and pheomelanin [29]. Under the action of tyrosinase, tyrosine forms eumelanin through a series of oxidation and catalytic polymerization. It turns to the synthetic pathway of brown melanin when the activity of tyrosine is reduced [30]. Among them, eumelanin is responsible for producing black and brown phenotypes, and brown melanin for producing yellow and red phenotypes [31]. During the formation of melanin, hormones, inorganic ions which are in vivo and external light, UV light physicochemical factors may affect the activity of TYR. Besides, the of the eumelanin to pheomelanin by ASIP gene was linked to the expression of the tyrosine family gene (TYR, TYRP1, DCT) and reduced its expression [32]. Currently, the expression of gene related to the tyrosinase synthesis becomes an important indirect indicator of the energy of melanin formation in fish due to the absence of an effective method for determining the melanin content and body color index of fish [33].