Abstract

Smoltification in Atlantic salmon (Salmo salar) is a critical developmental transition involving extensive physiological remodeling to enable the migration from freshwater to seawater. Among the most significant changes are those occurring in the gills, where ionoregulatory machinery is upregulated to support salt secretion. While the transcriptional dynamics of gill remodeling are well characterized, the upstream regulatory mechanisms driving these changes remain poorly understood. Here, we investigated whether histone acetylation—a key epigenetic mechanism regulating transcription—is changing in gills of Atlantic salmon during smoltification in an industrial setting. We performed immunohistochemistry using an antibody against acetylated histone H3 at lysine 9 (H3K9ac) to assess chromatin states across smoltification stages. We detected consistent nuclear H3K9ac immunoreactivity in gill tissue, which significantly declined in post-smolt fish following seawater transfer. To explore the regulatory basis of this pattern, we quantified the expression of genes encoding histone-modifying enzymes. Notably, the decrease in H3K9ac coincided with a significant upregulation of HDAC1, a histone deacetylase associated with transcriptional repression. Additional changes were observed in the expression of HDAC3, Sin3A, and selected histone acetyltransferases, particularly in fish with delayed seawater transfer. This study provides the first evidence that histone acetylation, specifically H3K9ac, is dynamically regulated during smoltification in Atlantic salmon. Our findings suggest that epigenetic modifications may contribute to transcriptional reprogramming in the gills during this critical physiological transition. Further studies targeting the epigenetic regulation of ion transporter gene promoters are needed to elucidate the mechanisms underlying osmoregulatory adaptation.