Transcriptomic Profiling of the Immune Response to Crowding Stress in Juvenile Turbot ( Scophthalmus maximus )

In this study, juvenile turbot Scophthalmus maximus were vaccinated with attenuated Edwardsiella tarda and reared in two different densities, low density (LD, 5.25 ± 0.02 kg/m2) as control groups and high density (HD, 20.53 ± 0.05 kg/m2) as experimental groups, and only density as variable was considered. Five weeks after vaccination, the transcriptomes of spleen and head kidney from the turbot in the two density groups were analyzed with RNA-Seq technology. A total of 447 million reads were assembled into 41,136 genes with an average length of 1274 bp and N50 size of 2295 bp. A comparison of gene expression between HD and LD revealed 1155 differentially expressed genes (DEGs), including 246 significantly upregulated unigenes (fold-change>2) and 909 significantly downregulated unigenes (fold-change>2). Enrichment and pathway analysis of the immune-related DEGs showed the centrality of the Toll-like receptor signaling pathway, cytosolic DNA-sensing pathway and platelet activation in the host immune responses. The overexpressed inflammatory cytokines and downregulated signal-regulated cytokines are involved in these pathways. We inferred that overexpressed inflammatory cytokines indicate that cells suffer damage and downregulated signal pathway-related cytokines indicate the immune response is restrained in turbot under crowding stress. Our results increase understanding of the effect of crowding stress on immunosuppression and provide valuable information on healthful aquaculture strategies in juvenile turbot.