Viral infections affect the transcription of biological clock components in lymphoid organs and lymphoid-associated tissues of common carp (Cyprinus carpio L.).

The circadian clock is a mechanism that allows organisms to adapt to the changing environment in a diurnal manner. It receives external cues (time givers), which synchronize the internal circadian clock with the external environment. A crucial aspect of the circadian clock involves the positive and negative feedback loops of clock genes. In the present study, we examined the diurnal changes in the expression of clock genes in the lymphoid organs (thymus, spleen, head kidney, trunk kidney), lymphoid-associated tissues (gills and gut), and peripheral blood leukocytes (PBLs) of common carp. Moreover, we investigated how the expression of these clock genes changes in these tissues/organs upon viral infections. Our findings showed that clock genes are constitutively expressed in the lymphoid organs/tissues, gills, gut and PBLs of common carp, and that different light regimes (LD, DD, and LL) altered their expression patterns. We also demonstrated that the two studied viruses, cyprinid herpesvirus 3 (CyHV-3) and spring viremia of carp virus (SVCV), affect the expression of clock genes (per1, per2, cry1,2, clock and bmal1,2) in the head kidney, trunk kidney, gill, and skin of common carp under the LD regime. Clock gene expression was negatively correlated with viral copy numbers. Overall, this study shows that the immune system and biological clock are interconnected and suggests that proper lighting conditions are crucial for the well-being and functionality of the circadian clock in fish. This consideration is important both in experimental settings and especially in aquaculture.