Department of Arctic and Marine Biology, UiT -The Arctic University of Norway, Tromsø NO-9037, Norway.
Department of Animal and Aquaculture Sciences, Norwegian University of Life Sciences, Ås NO-1432, Norway.
G3 (Bethesda). 2021 Apr 15;11(4). doi: 10.1093/g3journal/jkab072.
The developmental transition of juvenile salmon from a freshwater resident morph (parr) to a seawater (SW) migratory morph (smolt), known as smoltification, entails a reorganization of gill function to cope with the altered water environment. Recently, we used RNAseq to characterize the breadth of transcriptional change which takes place in the gill in the FW phase of smoltification. This highlighted the importance of extended exposure to short, winter-like photoperiods (SP) followed by a subsequent increase in photoperiod for completion of transcriptional reprogramming in FW and efficient growth following transfer to SW. Here, we extend this analysis to examine the consequences of this photoperiodic history-dependent reprogramming for subsequent gill responses upon exposure to SW. We use RNAseq to analyze gill samples taken from fish raised on the photoperiod regimes we used previously and then challenged by SW exposure for 24 hours. While fish held on constant light (LL) throughout were able to hypo-osmoregulate during a 24 hours SW challenge, the associated gill transcriptional response was highly distinctive from that in fish which had experienced a 7-week period of exposure to SP followed by a return to LL (SPLL) and had consequently acquired the characteristics of fully developed smolts. Fish transferred from LL to SP, and then held on SP for the remainder of the study was unable to hypo-osmoregulate, and the associated gill transcriptional response to SW exposure featured many transcripts apparently regulated by the glucocorticoid stress axis and by the osmo-sensing transcription factor NFAT5. The importance of these pathways for the gill transcriptional response to SW exposure appears to diminish as a consequence of photoperiod mediated induction of the smolt phenotype, presumably reflecting preparatory developmental changes taking place during this process.
幼鲑从淡水居留型（幼鱼）向海水洄游型（幼鱼）的发育转变，即变态，需要重新组织鳃的功能以适应改变的水环境。最近，我们使用 RNA 测序来描述在变态的淡水阶段发生的广泛转录变化。这突出了在淡水阶段长时间暴露于短的、类似冬季的光周期（SP），然后随后增加光周期对完成转录重编程和在转移到海水后有效生长的重要性。在这里，我们扩展了这项分析，以研究这种光周期历史依赖性重编程对随后暴露于海水时的鳃反应的后果。我们使用 RNA 测序分析了从以前使用的光周期条件下饲养的鱼中采集的鳃样本，然后暴露于海水 24 小时。虽然在整个实验期间处于持续光照（LL）下的鱼在 24 小时海水挑战期间能够进行低渗调节，但相关的鳃转录反应与经历 7 周 SP 暴露然后返回 LL（SPLL）并因此获得完全发育的幼鱼特征的鱼有很大不同。从 LL 转移到 SP 的鱼，然后在 SP 下维持剩余的研究时间无法进行低渗调节，并且与海水暴露相关的鳃转录反应的许多转录物显然受糖皮质激素应激轴和渗透压感知转录因子 NFAT5 调节。这些途径对海水暴露时鳃转录反应的重要性似乎随着光周期介导的幼鱼表型诱导而降低，这可能反映了在此过程中发生的预备性发育变化。
