Molecular Cloning and Temporal Expression Pattern of hsp70 Gene During the Early Life Stages of Seriola aureovittata

Seriola aureovittata is a long-distance migratory oceanic species belonging to the Carangidae family; it is also known as yellowtail kingfish. It inhabits temperate and subtropical marine waters worldwide. We reached a significant breakthrough in S. aureovittata seedlings production in 2017, enabling current massively juvenile rearing in China, which became a promising candidate for the farming industry, especially for the rapidly growing open ocean aquaculture. However, sudden larvae death caused by environmental changes-related stresses during the early life stages of S. aureovittata leads to great losses. Thus, exploring the physiological mechanism of larvae responses to environmental stresses during seedlings' production under artificial breeding conditions is urgently needed. Heat shock proteins (HSPs) play an important function in the physiological regulation of stress and immune responses in vertebrates, including fish. HSP70 is a member of the HSPs family studied in dozens of fish species, with physiological roles in protein homeostasis, DNA protection, and stress tolerance enhancement, among others. To investigate the possible physiological effects of HSPs on early growth and development of S. aureovittata, we cloned and obtained the full-length cDNA sequence of the hsp70 gene. Furthermore, the structure and spatial and temporal expression patterns of hsp70 during embryonic development, larval and juvenile growth and development of S. aureovittata were determined. The results showed that the full-length cDNA sequence of the hsp70 gene contains 2 332 bp, wherein the 5'-UTR is 187 bp, the ORF is 1 920 bp, and the 3'-UTR is 225 bp in length. A 639 amino acids protein with a molecular weight of 70.1 kDa is encoded by hsp70. The hsp70 spatial expression exhibited a sex dimorphism pattern, with significantly high expression levels in the gill, heart, spleen, and ovary in females, whereas in males, the significantly high expression levels were found in the pituitary, gill, head kidney, and testis. Notably, the highest hsp70 expression levels were observed in the ovary of females and the testis of males. During the S. aureovittata embryonic development, the hsp70 mRNA was detected in fertilized eggs before cleavage, indicating that hsp70 is parentally inherited. Additionally, the hsp70 mRNA could be detected at all stages of embryonic development, wherein the lowest expression levels were observed before the low blastula stage, with a significant increase at the early gastrula stage maintained until the hatching stage. The high levels of hsp70 mRNA were detected in one-day-old larva, which decreased in four-day-old larva, followed by an increase in 15-day-old larva. Remarkably, the hsp70 mRNA level once again decreased in 20-day-old larva, maintaining an average level until 60-day-old juveniles. The results from the present study may provide insights into the origin and physiological function of hsp70 during the early life stages of S. aureovittata.