Transcriptomic analysis of Nibea albiflora skin in response to infection by Cryptocaryon irritans.

Massive infection caused by Cryptocaryon irritans is detrimental to the development of marine aquaculture. Recently, our lab found that Nibea albiflora has low sensitivity and low mortality to C. irritans infection. The present study was designed to investigate the mechanisms of the N. albiflora response to C. irritans infection by analyzing transcriptome changes in the skin. Skin samples of control and experimental groups with C. irritans infection were collected at 24 and 72 h (24 h control, 24 h post-infection, 72 h control, and 72 h post-infection). Three parallels were set for each group and sample time, and a total of 12 skin samples were collected for sequencing. Overall, 297,489,843 valid paired-end reads and 48,817 unigenes were obtained with an overall length of 59,010,494 nt. In pairwise comparisons, changes in expression occurred in 1621 (764 upregulated and 857 downregulated), 285 (180 upregulated and 105 downregulated), 993 (489 upregulated and 504 downregulated), and 37 (8 upregulated and 29 downregulated) genes at 24 h control vs 24 h post-infection, 72 h control vs 72 h post-infection, 24 h post-infection vs 72 h post-infection, and 24 h control vs 72 h control, respectively. Gene Ontology (GO) analysis of differentially expressed genes (DEGs) indicated that the number of genes enriched in GO sub-categories were ordered 24 h control vs 24 h post-infection > 24 h post-infection vs 72 h post-infection >72 h control vs 72 h post-infection > 24 h control vs 72 h control. Further analysis showed that immune-related GO terms (including immune system process, complement activation, and humoral immunity) were significantly enriched at both 72 h control vs 72 h post-infection and 24 h post-infection vs 72 h post-infection, but no immune-related GO terms were significantly enriched in the 24 h control vs 72 h control and at 24 h control vs 24 h post-infection, indicating that C. irritans infection mainly affected the physiological metabolism of N. albiflora at an early stage (24 h), and immune-related genes play an important role at a later stage (72 h) of infection. In KEGG pathway analysis, the complement and coagulation cascade pathway are involved in early infection. Hematopoietic cell lineage, natural killer (NK) cell-mediated cytotoxicity, and the intestinal immune network for IgA production are involved in later infection. Further analysis showed that the alternative pathway of complement and coagulation cascades plays an important role in the resistance of N. albiflora to early C. irritans infection. During late infection, CD34, IgM, and IgD were significantly upregulated in the hematopoietic cell lineage pathway. CCR9 was significantly downregulated, and IGH and PIGR were significantly upregulated in the intestinal immune network for IgA production. GZMB and IGH were significantly downregulated in NK cell-mediated cytotoxicity. These findings indicate that acquired immunity at the mRNA level was initiated during later infection. In addition, the IL-17 signaling pathway was enriched by downregulated DEGs at 24 h post-infection vs 72 h post-infection, suggesting the inflammatory response at 24 h was stronger than at 72 h and the invasion of the parasite has a greater impact on the host.