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In aquaculture production, fish phenotypes are influenced by a combination of external and internal factors, while the underlying mechanisms often involve distinct DNA methylation patterns. These play a fundamental role in regulating gene expression in response to environmental shifts. The Senegalese sole (Solea senegalensis) holds significant potential in aquaculture due to its high commercial importance. However, its production faces several challenges, including the inability of F1 soles to reproduce. In our study, we analysed the methylation patterns in the testicular DNA of 12 male S. senegalensisindividuals from four distinct groups, which differed in rearing origins (wild and F1 soles) and sexual maturity stages (mature and immature). We employed reduced representation bisulfite sequencing to identify significant methylation differences among studied groups. The differential methylation analysis showed a substantial disparity between the F1 and wild groups. This difference was evident in both the number of different methylated CpGs (DMCpGs) and methylation levels, highlighting a distinction in groups reared under different conditions. The comparison between immature and mature wild groups revealed notable differences, suggesting that the testes methylation profile undergoes few changes in wild individuals during sexual maturity. These differences in methylation between wild and hatchery-born and reared offspring imply epigenetic modifications triggered by rearing in captivity and by the absence of natural stimulisuch as temperature fluctuations. We further annotated the DMCpGs based on their position and co-localization with genes in the Senegalese sole genome. Our analysis revealed that the methylation differences observed in DMCpGs were more pronounced in intron regions, with higher methylation levels. Conversely, the methylation differences in exon and promoter areas were more subtle. Upon annotation, we discovered that these DMCpG sites were located near or within various transcription factors. Our comprehensive analysis sheds light on the intricate methylation profile of S. senegalensis, focusing on genes associated with reproductive traits, particularly those related to sex determination (SD) systems and spermatogenesis. Overall, we observe a trend toward higher methylation levels in DMCpGs when comparing F1 individuals to wild groups, as well as immature to mature wild groups. Hence, sexual maturity and place of origin resulted in marked methylation differences, which could potentially affect the expressions of genes involved in both development and reproduction of farmed males. This analysis explores the gene-regulating effects of this epigenetic mechanism providing valuable insights into the variations in methylation found in the testes of Senegalese sole.

Individual identification of fish is often desirable for the smallest possible size, but it is crucial that tagging does not interfere with fish survival, physiology, or behavior. We evaluated radio-frequency identification (RFID) tags (10 mg) and PIT tags (PIT; 32 mg) in fish of two different size-classes of Nile Tilapia Oreochromis niloticus: 200–500 mg and 640–1,600 mg, wet mass (WM). This produced four categories of tag load for each type of tag: 5.0, 3.3, 2.5, and 2.0% of WM. We tested 30 fish per category. Survival averaged 95.8% for RFID tags and 98.3% for PIT tags. Tag retention after 35 d was 99.1% for RFID tags and 96.6% for PIT tags. Tagged fish grew more slowly than controls. Growth penalty was proportional to tag load, but restricted to the first 4 d after tagging and compensated by catch-up growth, except in fish <300 mg presumably due to greater difficulties of handling and tagging. Small PIT tags can thus be used confidently in tilapia of about 1.3 g and RFID tags in tilapia of about 0.4 g. If growth is not a premium, the corresponding minimal sizes are 1.0 (for PIT) and 0.3 g (for RFID).

AbstractIndividual identification of fish is often desirable for the smallest possible size, but it is crucial that tagging does not interfere with fish survival, physiology, or behavior. We evaluated radio-frequency identification (RFID) tags (10 mg) and PIT tags (PIT; 32 mg) in fish of two different size-classes of Nile Tilapia Oreochromis niloticus: 200–500 mg and 640–1,600 mg, wet mass (WM). This produced four categories of tag load for each type of tag: 5.0, 3.3, 2.5, and 2.0% of WM. We tested 30 fish per category. Survival averaged 95.8% for RFID tags and 98.3% for PIT tags. Tag retention after 35 d was 99.1% for RFID tags and 96.6% for PIT tags. Tagged fish grew more slowly than controls. Growth penalty was proportional to tag load, but restricted to the first 4 d after tagging and compensated by catch-up growth, except in fish <300 mg presumably due to greater difficulties of handling and tagging. Small PIT tags can thus be used confidently in tilapia of about 1.3 g and RFID tags in tilapia of about 0.4 g. If growth is not a premium, the corresponding minimal sizes are 1.0 (for PIT) and 0.3 g (for RFID).Received February 11, 2014; accepted March 30, 2014

Increases in branchial Na+/K+ ATPase activity during seawater adaptation of euryhaline fish species, have been well documented. During the parr-smolt transformation of salmonids this activity increases two to five fold and is used as an indicator of the transformation. In order to improve the understanding of differences in enzyme activity found between Atlantic salmonSalmo salar parr and smolt fish, we investigated the gene expression of the Na+/K+ ATPase a-subunit(s) in gill tissue. Gill mRNAs were analyzed and quantified at distinct time points using Northern and Dot blot techniques. We amplified by PCR, a conserved region of the cDNA encoding the Na+/K+ ATPase a-subunit of the rainbow troutOncorhynchus mykiss. The PCR products (670 bp) were cloned and all independent clones showed a sequence corresponding to the a subunit of the Na+/K+ ATPase. The fragments obtained appeared as a heterogenous population of three sequences showing, when compared between each other, 86 to 93% identity. This suggests that different allelic forms of the a-subunit are expressed in gill tissue. Hybridization studies performed with these PCR probes revealed two mRNA species, a major 3.7 kb transcript and a minor transcript of 1.8 kb. Enhanced 3.7 kb transcript levels are concurrent with elevated enzyme activity in smolts during the March and April parrsmolt transformation of Atlantic salmon. Interestingly, our study disclosed that smolt fish only displayed a two-fold increase in transcript levels when compared to parr whereas enzyme activity showed a 4 to 5 fold increase. This suggests that the increase in the 3.7 kb mRNA content of gill tissue is probably not the only mediator leading to the rise in enzyme activity during parr-smolt transformation.