The Antarctic Krill Euphausia superba Shows Diurnal Cycles of Transcription under Natural Conditions

BackgroundPolar environments are characterized by extreme seasonal changes in day length, light intensity and spectrum, the extent of sea ice during the winter, and food availability. A key species of the Southern Ocean ecosystem, the Antarctic krill (Euphausia superba) has evolved rhythmic physiological and behavioral mechanisms to adapt to daily and seasonal changes. The molecular organization of the clockwork underlying these biological rhythms is, nevertheless, still only partially understood.Methodology/Principal FindingsThe genome sequence of the Antarctic krill is not yet available. A normalized cDNA library was produced and pyrosequenced in the attempt to identify large numbers of transcripts. All available E. superba sequences were then assembled to create the most complete existing oligonucleotide microarray platform with a total of 32,217 probes. Gene expression signatures of specimens collected in the Ross Sea at five different time points over a 24-hour cycle were defined, and 1,308 genes differentially expressed were identified. Of the corresponding transcripts, 609 showed a significant sinusoidal expression pattern; about 40% of these exibithed a 24-hour periodicity while the other 60% was characterized by a shorter (about 12-hour) rhythm. We assigned the differentially expressed genes to functional categories and noticed that those concerning translation, proteolysis, energy and metabolic process, redox regulation, visual transduction and stress response, which are most likely related to daily environmental changes, were significantly enriched. Two transcripts of peroxiredoxin, thought to represent the ancestral timekeeping system that evolved about 2.5 billion years ago, were also identified as were two isoforms of the EsRh1 opsin and two novel arrestin1 sequences involved in the visual transduction cascade.ConclusionsOur work represents the first characterization of the krill diurnal transcriptome under natural conditions and provides a first insight into the genetic regulation of physiological changes, which occur around the clock during an Antarctic summer day.

研究背景：极地环境以昼夜时长、光照强度与光谱、冬季海冰覆盖范围以及食物可获得性的极端季节性变化为典型特征。作为南大洋生态系统的关键物种，南极磷虾（Antarctic krill, Euphausia superba）已演化出节律性的生理与行为机制，以适应每日与季节性的环境变化。然而，支撑这些生物节律的生物钟分子调控机制，目前仍仅部分为人所知。
研究方法与主要结果：南极磷虾的全基因组序列目前尚未公开。为大规模鉴定转录本，研究人员构建了归一化cDNA文库并进行焦磷酸测序。随后，所有已公开的南极磷虾（E. superba）序列被整合，构建出当前最完整的寡核苷酸微阵列平台，共计包含32217个探针。对罗斯海海域采集的、在24小时周期内5个不同时间点获取的样本进行基因表达特征分析，共鉴定出1308个差异表达基因。在对应的转录本中，609个呈现出显著的正弦表达模式；其中约40%表现为24小时周期节律，其余60%则以更短的（约12小时）节律为特征。研究人员将差异表达基因归类至不同功能类别，发现与翻译、蛋白水解、能量与代谢过程、氧化还原调控、视觉转导以及应激反应相关的基因显著富集——这些功能类别极有可能与每日环境变化相关。此外，研究还鉴定出两种过氧化物还原蛋白（peroxiredoxin）转录本，该蛋白被认为是约25亿年前演化出的祖先计时系统的代表；同时发现了EsRh1视蛋白的两种同工型，以及参与视觉转导级联反应的两种新型抑制蛋白1（arrestin1）序列。
研究结论：本研究首次在自然条件下对磷虾的日间转录组进行了表征，并首次揭示了南极夏季日间持续发生的生理变化的遗传调控机制。