Integration of Transcriptome, Gross Morphology and Histopathology in the Gill of Sea Farmed Atlantic Salmon (Salmo salar): Lessons from Multi-site Sampling

The experiment focused on the transcriptomic changes associated with gill inflammation in sea farmed Atlantic salmon (Salmo salar). To ensure the multifactorial aspect of gill inflammation, fish were sampled at three marine production sites (A on Isle of Mull, B in Shetland and C in Shetland) between October 2017 and March 2018. All fish were of strain Fanad and originated from the same egg fertilisation batch. They were reared in different hatcheries (Couldoran, Pettigo-Damph and Knock-Frisa for sites A, B and C, respectively) for one year and entered the sea in spring 2017. The resultant gill tissues (44 samples in total with 1 gill sample per fish) were first scored for proliferative gill disease (PGD), using gross morphology PGD scores from 0 with no visual pathology to 5 with severe visual pathology, and then subjected to RNA-seq and histopathological (microscopic) examination. One RNA-seq sample (fish 95) was identified as an outlier and removed from the subsequent analysis. As a result, the analysis aiming to integrate gill transcriptome, gross morphology and histopathology was performed on 43 gill samples, classified either as PGD score 1 (n = 26) or PGD score 3 (n = 17). In total, 20 gill samples originated from site A (10 with PGD1 and 10 with PGD 3, 10 samples from site B (7 with PGD1 and 3 with PGD 3) and 13 samples from site C (9 with PGD1 and 4 with PGD 3).