Histone post-translational modifications (PTMs) are epigenetic marks that can be induced by environmental stress and elicit heritable patterns of gene expression. To investigate this process in an ecological context, we characterized the influence of salinity stress on histone PTMs within the gills, kidney, and testes of Mozambique tilapia (Oreochromis mossambicus). A total of 503 histone PTMs were quantified in each tissue sample and compared between freshwater-adapted fish exposed to salinity treatments that varied in intensity and duration. Three salinity-responsive histone PTMs were identified in this study. When freshwater-adapted fish were exposed to seawater for two hours, the relative abundance of H1K16ub significantly increased in the gills. Long-term salinity stress elicited changes in an additional two histone PTMs in the testes. When freshwater-adapted fish were exposed to a pulse of severe salinity stress, where salinity reached a maximum of 82.5 g/kg, the relative abundance of H3K14ac and H3K18ub decreased significantly in the testes. This study demonstrates that specific types of salinity stress can alter histone PTMs in Mozambique tilapia, both in an osmoregulatory organ and in the germ line. These results signify a potential for histone PTMs to be involved in salinity acclimation and adaptation in euryhaline fishes, thereby adding to a growing body of evidence that epigenetic mechanisms are involved in such processes.

A set of short-term salinity treatments and a set of long-term salinity treatments were delivered to a total of 42 adult Mozambique tilapia. The short-term and long-term salinity treatments were conducted as separate experiments, where 18 fish were used for the short-term salinity treatments, and 24 fish were used for the long-term salinity treatments. For the short-term salinity treatments, each treatment group was composed of six fish as biological replicates. Fish in the treatment group Cn were only ever exposed to freshwater (0 g/kg). In treatment group G1, fish were transferred directly from freshwater to seawater (30 g/kg) and kept there for exactly two hours. Finally, the fish in the treatment group G2 were transferred directly from freshwater to seawater, kept there for exactly two hours, then transferred directly back to freshwater and kept there for an additional two hours. Following exposures, all fish were euthanized and dissected for their gills, kidney, and in males, testes. Due to the sexes of the fish randomly selected for this experiment, dissections yielded n = 6 gill samples, n = 6 kidney samples, and n = 3 testes samples per treatment group. For the long-term salinity treatments, we exposed freshwater-adapted fish to either 1) freshwater (treatment group S0), 2) one “pulse” of severe salinity stress (treatment group S1), where salinity gradually increased from freshwater to 82.5 g/kg, or 3) three pulses of severe salinity stress (treatment group S3). Following salinity treatments, all fish were euthanized, and their gills, kidney, and testes (if male) were collected for histone PTM analysis. These dissections yielded n = 8 gill samples, n = 6 kidney samples, and n = 6 testes samples per treatment group.

Following salinity treatments, we sampled the gills, kidney, and testes of Mozambique tilapia for histone PTM analysis. Samples are labeled in corresponding files by their 1) salinity treatment, 2) fish number, and 3) tissue. Therefore, the sample name"G1F3K" indicates it was 1) from a fish that was in the treatment group G1 (a short-term salinity treatment where fish were transferred directly from freshwater to seawater and kept there for two hour), 2) the third fish (F3) sampled from that treatment group, and 3) a kidney (K) sample.