Diploid and triploid Chinook salmon (Oncorhynchus tshawytscha) have altered microRNA responses in immune tissues after infection with Vibrio anguillarum
Production of sterile fishes through artificial retention of a third set of chromosomes (triploidy) is a sustainable alternative for aquaculture since it reduces pressure on wild populations. However, these fishes have reduced survival in stressful conditions and in response to pathogens. In this study, the impact of Vibrio anguillarum infection on diploid and triploid Chinook salmon (Oncorhynchus tshawytscha) was investigated to identify if there was any significant regulation by microRNAs (miRNA) in immune responsive tissues. Small RNAs from hindgut, head kidney, and spleen were sequenced to determine miRNA transcript abundance was altered due to ploidy and infection in nine-month old full-sibling diploids and triploids. Diploids had higher percent survival at the end of the infection, but no difference was found in mortality between the two ploidies.  All three tissues had differentially expressed miRNA prior to infection, indicating that subtle changes in epigenetic regulation due to increased ploidy. Additionally, miRNA were altered by infection, but only in spleen was there a difference in miRNA expression between diploids and triploids after three days of infection. Furthermore, one miRNA (ssa-miR-2188-3p) was confirmed as having an altered response to infection in triploids compared to diploids, implicating potential immune dysregulation due to increased ploidy. The miRNAs identified in this study are predicted to target immune pathways, providing evidence for their importance in regulating responses to pathogens. This study is the first to investigate how increased ploidy alters miRNA expression in response to infection. Additionally, it provides evidence for epigenetic dysregulation in triploid fishes, which may contribute to  their  poor performance in response to stress. Overall design: Diploid and triploid full-sibling Chinook salmon hindgut, head kidney, and spleen were sampled before (Day0) and after (Day3) being exposed to Vibrio anguillarum