Growth in tilapia, one of the main finfish cultured worldwide, is susceptible to changes in salinity. Specifically, the Mozambique tilapia, Oreochromis mossambicus, a species that can tolerate marked changes in salinity, is employed as a model to elucidate how gene transcriptional patterns that underlie growth and osmoregulation respond to salinity challenges. As the primary site for the active transport of Na+ and Cl-, the gill is critical to maintaining osmoregulatory balance of the whole organism. Here, we investigated the branchial expression of genes associated with tight junctions, hyperplasia, and immune/stress responses in tilapia residing under tidally-changing salinities.
Mozambique tilapia were reared in either fresh water (FW) or seawater (SW) and then transferred to a tidal regimen (TR), characterized by a change in salinity from FW to SW every 6 h for 15 days. Fish exposed to a TR were sampled at the end of either the FW- (TF) or the SW-phase (TS) of the tidal cycle. Using RNA-Seq, we then compared the gills of fish sampled in TF and TS versus those acclimated to steady-state FW and SW. Transcripts with the highest differential expression between salinity regime groups were followed up with qPCR analyses.
While plasma osmolality was inversely related to environmental salinity in fish acclimated to both steady-state and tidal salinities, plasma glucose was higher in fish at TF compared with all other salinity groups. The most abundant and differentially expressed tight junction protein (TJP) transcripts included claudins (cldn-4 like, cldn-like ZF-A89, cldn7b, and cldn23a), occludins (oclna and oclnb), and tjp1a and tjp3. In general, the expression of TJP genes varied to a greater extent between the FW- and SW-phases of the TR (TF and TS, respectively) than between steady-state FW and SW. A similar pattern was observed with the stress/immune response-associated branchial transcripts, il1b, gstr, and hsp40, but not cmc20. Lastly, in fish originally reared in SW, the hyperplasia-associated transcript, pcna, showed differential expression between TF and TS alongside an overall downregulation of npcc, mk167, and ets1 in fish sampled in both TF and TS. Taken together, these results indicate that branchial responses differ between tilapia residing in steady-state versus tidally-changing salinities and thereby provide insights into how cultured fish are impacted by marked and frequent salinity changes.
[Supported by HATCH (#HAW02051-H), NOAA (#NA18OAR4170347), and NSF (IOS-1755016 and -1755131)]