Study On Transcriptomic Response To Salinity Challenge And Antioxidant Response To Copper Stress Of Hybrid Sturgeon Acipenser Schrenckii?× Acipenser Gueldenstaedtii?

Acipenseriformes is the only existing species of Chondrostei,which have the physiological characteristics between Chondrichthyes and Osteichthyes.It is a group of medium or large fish with tremendous economic importance and also one of the most endangered species.As the most primitive taxa in Actinopterygii,Acipenseriformes maintaine a large number of primitive biological traits,are the living fossil in the aquatic animals of the Earth.Salinity adaptation is a complex process involving a set of physiological responses to environments with differing ionoregulatory requirements.Sturgeon,as a euryhaline fish,has developed specific osmoregulation mechanisms to meet the significant osmotic challenge of high salinity waters.Unfortunately,the molecular mechanism of this species in response to salinity stress remains broadly unclear.Aiming to elucidate the molecular adaption mechanism of sturgeons to salt stress,the transcriptomic responses of gills in juvenile hybrid sturgeons Acipenser schrenckii?×Acipenser gueldenstaedtii?under high salinity stress were detected using the digital gene expression system of Illumina.The transcriptional expression databases of the sea water group and fresh water groups bybrid sturgeon gill were constructed based on this data.In total,167,501,278 clean reads with an average length of 520 bp,an N50 of 630 bp and a sequence read length of 101 bp.were obtained and assembled into 280,238 unigenes.The N50 value of unigenes was 630bp,the minimum length was 201 bp,the maximum length was 25214 bp,and the average length was 520 bp.Besides Unigenes were annotated by comparing against NR,Swiss-Prot,KEGG,KOG,GO,and Pfam databases,and 62,242 unigenes?22.21%?were annotated in at least one database.Under high salinity stress,6,969and 4,411 genes were identified as remarkably up-or down-regulated,respectively.The relevant KEGG pathways were obtained by mapping all DEGs to the KEGG database.These pathways included seveal classical osmoregulation-related pathways,suchasglycerophospholipidmetabolism,fatty-acidbiosynthesis,glycolysis/gluconeogenesis,and oxidative phosphorylation.In addition,eight of the differently expressed genes were confirmed using quantitative real-time PCR.The unigene data obtained from high-throughput sequencing of hybrid sturgeon gill was used in the futher study,the waterborne copper induced oxidative stress response of hybrid sturgeon.We performed a 96-hour acute copper exposure experiment with Cu²⁺concentration of 1.0 mg.L^-1 in seawater.The effects of copper on the physiological behavior of the hybrid sturgeon such as the change of respiratory frequency were recorded.We found that during the copper exposure process,the respiratory frequency was firstly increased and then decreased,besides,their swim ability was negatively affected.In addition,we measured the activity of superoxide dismutase?SOD?,catalase?CAT?activity and total antioxidant capacity?T-AOC?of gill and liver tissues from 24,48,72 and 96 hours groups.The quantitative real-time PCR was employed to test the Cu/Zn SOD and CAT gene expression levels in gill and liver tissues.Through comprehensive studying of physiological behaviors,tissue enzyme activities and related gene expressions results of hybrid sturgeon exposed to copper,it was found that copper exposure would destroy the ability of the hybrid sturgeon to regulate oxygen expenditure,induce ROS overproduction,affect the antioxidant enzyme activity and cause tissue damage.The expression of Cu/Zn SOD and CAT genes was inhibited in part.The toxicity of copper to hybrid sturgeon was obvious,and it significantly inhibited the SOD activity in gill and liver.In the copper treatment group,the results of SOD activity in gill and liver were significantly lower than those in the control group,unlike SOD,copper had a certain induction effect on CAT activity.The T-AOC level showed a trend of rising first and then decreasing as the copper exposure time increases.In the gill,the level of T-AOC decreased more significantly,indicating that tissue damage appeared in both the gill and the live under the copper exposure.Moreover,tissue damage in gill are even more serious compared with liver.The toxicity of copper is more evident in its inhibitory effect on the expression of related genes in tissues.The expression level of Cu/Zn SOD gene is significantly inhibited by the influence of copper toxicity,although the expression level of CAT gene was found to increase in gill.The level of CAT gene expression in liver tissue was lower than in the control group and decreased with the increase of exposure time.Accordingly,these results could help us to have a better understanding of the osmoregulation mechanisms and heavy metal oxidative stress coping strategies of sturgeon,make great contribution to develop diversified marine aquaculture,and have better protection and utilization of wild sturgeon resources.