In Atlantic cod aquaculture, precocious puberty is a major challenge, as it negatively impacts growth rate and fillet quality. The application of continuous light (CL) from the summer solstice prior to maturation has been shown to delay gonadal development to some extent and promote weight gain. The liver is most likely involved in these responses to photoperiod, since it is a key organ in overall metabolism. In this study, we aimed to investigate the effects of photoperiod on gene expression in Atlantic cod liver. Juvenile cod were divided into two groups and exposed to either ambient light (AL) or continuous light (CL) in triplicate tanks for 6 months. A total of 36 RNA-seq libraries (9 males and 9 females from each photoperiod condition) were prepared with the NEBNext Ultra II Directional kit. Using the Limma package, 2,542 significantly differentially expressed genes (DEGs, |Log2 fold change| ≥ 1, Benjamini-Hochberg adjusted p-value ≤ 0.05) were identified. In females, 1,044 DEGs were found between CL and AL groups (704 up-regulated and 340 down-regulated), and in males there were 1,184 DEGs (800 up-regulated and 384 down-regulated). A total of 314 genes were differentially expressed between males and females under ambient light conditions (168 up-regulated and 146 down-regulated), whereas no DEGs were detected between males and females under continuous light conditions. Among the top DEGs, photoperiod manipulation affected the expression of mogat2 (monoacylglycerol O-acyltransferase 2, LogFC 3.526312) with down-regulation of LOC115551355 ( pellucid zone sperm-binding protein 3-like, LogFC -5.735752), and LOC115556237 (vitellogenin-2-like, LogFC -4.749765). Gene Ontology (GO) terms enriched included “response to external stimulations” and “response to external biotic stimulus” in CL vs AL females. In males, enriched GO terms on “DNA replication” were associated with up-regulated genes, including mcm5 (LogFc 2.719089), and mcm2 (LogFC 2.719463) and “small molecule catabolic process” such as tat (LogFC -2.795083 tyrosine aminotransferase) were observed comparing CL vs AL. In addition, a significant down-regulation of vitellogenesis-related genes (LOC115556237, LogFC -10.737861) was observed in males compared to females under AL conditions, along with enrichment in heme binding and tetrapyrrole binding terms. These results provide novel insights into the regulation of gene networks responsible for growth and puberty in this commercially important fish species and this fundamental knowledge may contribute to more sustainable cod farming.
Acknowledgements:
This study has received funding by the Research Council of Norway under the Researcher Project for Scientific Renewal (project No. 336112) and FPI contract PRE2022-101795 associated to grant CEX2019-000928-S-20-5 funded by AEI 10.13039/501100011033 (Spain).