4.2 Freshwater transfer overall induces DNA hypomethylation inD. labrax gills
Studies on the effect of salinity on DNA methylation dynamics has been
performed in plants (Rajkumar et al., 2019; Skorupa et al., 2021) and
also in animals, but only in few teleost species (Artemov et al., 2017;
Metzger and Schulte, 2018; Heckwolf et al., 2020, Zhang et al., 2022;
Yang et al., 2023).
Our data showed that a freshwater transfer in fish gills overall induced
hypomethylation of DNA across all chromosomes. Parallelly, a
downregulation of the expression of two paralogs of DNA
methyltransferase 3a was observed, which is consistent with this overall
decrease in DNA methylation. Predominant DNA hypomethylation in fresh
water vs seawater was observed in exons as well as in introns and
all over the genome, where DMRs were detected. A salinity effect on DNA
methylation level was also observed in threespine stickleback (G.
aculeatus ) of a marine ecotype, where overall hypomethylation was
observed in whole fish that were reared for 1 month at high salinity
relative to those reared at low salinity (Metzger and Schulte, 2018),
which is an inverse tendency than what we observed in D. labraxgills. It therefore seems that a medium-term (2 weeks in this study) as
well as a long term salinity acclimation (1 month) can affect DNA
methylation dynamics in fish tissues. In sea bass, the distribution of
DMRs across genomic features (e.g., promoters, exons, introns and
intergenic regions) did not differ from the relative proportions of
these features in the genome (Tine et al., 2014) which means that fresh
water induces methylation changes in all genomic features without
targeting specific regions in the genome. Global hypomethylation has
previously been shown following several stressors such as metal stress
in zebrafish embryos (Bian and Gao., 2021), and salinity stress in the
crustacean Daphnia magna (Jeremias et al., 2018). Global
hypomethylation could be considered as a global response to a stressor,
potentially playing a role in the modulation of transcription activity.