3.4 Functional enrichment
Gene ontology (GO) analysis for biological processes shows very similar enrichment of GO terms within tissue types regardless of sampling method. Among tissue types, certain GO terms are common such as RNA processing and RNA modification, which is similar to a previous fish gene expression study (Passow et al. 2018). There is also variation in the number and diversity of enriched GO terms depending on the tissue type (Supporting Information Figure S4). There are many more enriched GO terms associated with genes that are highly expressed in muscles (6 to 14 categories) compared to gills (0 to 5 categories), while the number of enriched GO categories associated with comparisons of gills (14 to 15 categories) versus blood (7 to 11 categories), and blood (9 to 11 categories) versus muscle (6 to 10 categories), are similar. Gills in particular are enriched for many GO categories related to major cell cycle processes, which fits with their active growth and turnover as epithelial cells at the aquatic interface (Stolper et al. 2019). Regardless of the sampling method, genes associated with cellular stress appear to be more highly expressed in blood and gills compared to muscle, potentially indicating tissue-specific physiological effects of the sampling process.