Photoperiod-dependent developmental reprogramming of the transcriptional response to seawater entry in Atlantic salmon (Salmo salar)

Abstract

The developmental transition of juvenile salmon from a freshwater resident morph (parr) to a seawater (SW) migratory morph (smolt) requires a range of physiological adaptations, including the capacity to hypo-osmoregulate. This process, known as smolting, involves both photoperiod-dependent preparative changes before SW is encountered, and activational changes stimulated by exposure to SW. To explore the relationship between these two aspects we undertook experiments in which physiological and transcriptomic responses to SW-challenge were assessed in fish that had experienced different histories of photoperiodic exposure. Compared to fish held on constant light (LL), exposure to short photoperiod (SP) dramatically impaired hypo-osmoregulation in SW, and was associated with extensive glucocorticoid-related changes in gill gene expression. Additionally, a major effect of photoperiodic history was observed in the transcriptional response of LL-acclimated fish to SW, with the response profiles of fish held on LL throughout life being quite distinctive from those of fish which had experienced an 8 week period of exposure to SP prior to return to LL (SPLL). These differences in profile likely reflect a diminishing role for NFAT-mediated responses in SPLL fish, as pathways linked to acute changes in cellular tonicity or intracellular calcium levels decline in importance with preparation for SW.

Comparison of anadromous and landlocked Atlantic salmon genomes reveals signatures of parallel and relaxed selection across the Northern Hemisphere

“…Heatmaps were made by first discarding genes that had normalized read counts <50 in all assayed tissues, before using J‐Express (v. 2012) (Dysvik & Jonassen, 2001) to generate heatmaps using high‐level mean and variance normalization, with complete linkage clustering and Euclidean distance measure. Gene expression in gills of salmon exposed to saltwater for 24 hr was examined using RNA‐Seq data obtained from Array Express (accession number E‐MTAB‐8276), described previously in Iversen et al (2020). Sequence reads were filtered using Cutadapt with parameters ‐q 20 ‐O 8 and ‐m 40 and mapped to the salmon gene models with Bowtie2 using default settings.…”

“…We also screened the sweeps for genes differentially expressed in juvenile fish exposed to saltwater by re-analysis of a recently published RNA-Seq dataset (Iversen et al, 2020) from salmon gills. This revealed that 14 of the genes in the sweeps were differentially expressed (p adj < .001) in at least one sampling point in fish challenged by saltwater for 24 hr at six sampling points over a 110-day period (File S3).…”