Diverse Transcriptome Responses to Salinity Change in Atlantic Cod Subpopulations

Abstract

Adaptation to environmental variation caused by global climate change is a significant aspect of fisheries management and ecology. A reduction in ocean salinity is visible in near-shore areas, especially in the Baltic Sea, where it is affecting the Atlantic cod population. Cod is one of the most significant teleost species, with high ecological and economical value worldwide. The population of cod in the Baltic Sea has been traditionally divided into two subpopulations (western and eastern) existing in higher- and lower-salinity waters, respectively. In recent decades, both Baltic cod subpopulations have declined massively. One of the reasons for the poor condition of cod in the Baltic Sea is environmental factors, including salinity. Thus, in this study, an oligonucleotide microarray was applied to explore differences between Baltic cod subpopulations in response to salinity fluctuations. For this purpose, an exposure experiment was conducted consisting of salinity elevation and reduction, and gene expression was measured in gill tissue. We found 400 differentially expressed genes (DEGs) involved in the immune response, metabolism, programmed cell death, cytoskeleton, and extracellular matrix that showed a subpopulation-dependent pattern. These findings indicate that osmoregulation in Baltic cod is a complex process, and that western and eastern Baltic cod subpopulations respond differently to salinity changes.

Keywords: Gadus morhua; gene expression; gill tissue; microarray; osmoregulation.

Figure 1

Sampling locations. The fish of the Kiel Bight (KIEL) and the Gulf of Gdańsk (GDA) represent the eastern and western Baltic cod subpopulations, respectively.

Figure 2

Experimental design using Gadus morhua individuals of two geographical origins: the Gulf of Gdańsk (GDA) and the Kiel Bight (KIEL), representing eastern and western Baltic cod, respectively.

Figure 3

Measurements of the (a) length and (b) weight of the individuals used in this study (n = 81). The Gulf of Gdańsk (GDA) represents EBC; the Kiel Bight (KIEL) represents WBC. Two asterisks indicate a p-value < 0.001.

Figure 4

Number of DEGs in the each experimental group. The Gulf of Gdańsk (GDA) represents EBC; the Kiel Bight (KIEL) represents WBC. (a) Number of DEGs in each experimental group. (b) No. of DEGs in time course. An asterisk represents a p-value < 0.05.

Figure 5

Differences in expression between the GDA and KIEL groups (Wilcoxon rank sum test). (a) Expression after 1 h of exposure to salinity change. (b) Expression after 12 and 72 h of exposure to the change in salinity. An asterisk represents a p-value < 0.05; two asterisks denotes a p-value < 0.001. The Gulf of Gdańsk (GDA) represents EBC; the Kiel Bight (KIEL) represents WBC.

Figure 6

Principal component analysis (PCA) performed for all DEGs. Blue color: the Gulf of Gdańsk (GDA) represents EBC; red color: the Kiel Bight (KIEL) represents WBC.

Figure 7

Bubble plot of pathway enrichment analysis using subpopulation-dependent genes. The plot presents the top five pathways, with the highest p-value divided into the main KEGG categories. CP: cellular processes; EIP: environmental information processing; and OS: organismal systems.

Figure 8

Changes in the enrichment of the top pathways of subpopulation-dependent genes. (a) Time course and (b) salinity change.