Hines Hunter N, Onsbring Henning, Ettema Thijs J G, Esteban Genoveva F
Bournemouth University, Faculty of Science and Technology, Department of Life and Environmental Sciences, Poole, Dorset BH12 5BB, UK; Harbor Branch Oceanographic Institute, Florida Atlantic University, Fort Pierce, FL 34946, USA.
Department of Cell and Molecular Biology, Science for Life Laboratory, Uppsala University, SE-75123 Uppsala, Sweden.
Protist. 2018 Dec;169(6):875-886. doi: 10.1016/j.protis.2018.08.001. Epub 2018 Aug 20.
The ciliate Spirostomum semivirescens is a large freshwater protist densely packed with endosymbiotic algae and capable of building a protective coating from surrounding particles. The species has been rarely recorded and it lacks any molecular investigations. We obtained such data from S. semivirescens isolated in the UK and Sweden. Using single-cell RNA sequencing of isolates from both countries, the transcriptome of S. semivirescens was generated. A phylogenetic analysis identified S. semivirescens as a close relative to S. minus. Additionally, rRNA sequence analysis of the green algal endosymbiont revealed that it is closely related to Chlorella vulgaris. Along with the molecular species identification, an analysis of the ciliates' stop codons was carried out, which revealed a relationship where TGA stop codon frequency decreased with increasing gene expression levels. The observed codon bias suggests that S. semivirescens could be in an early stage of reassigning the TGA stop codon. Analysis of the transcriptome indicates that S. semivirescens potentially uses rhodoquinol-dependent fumarate reduction to respire in the oxygen-depleted habitats where it lives. The data also shows that despite large geographical distances (over 1,600km) between the sampling sites investigated, a morphologically-identical species can share an exact molecular signature, suggesting that some ciliate species, even those over 1mm in size, could have a global biogeographical distribution.
纤毛虫半绿旋口虫是一种大型淡水原生生物，体内密集地充满了内共生藻类，并且能够利用周围的颗粒构建一层保护涂层。该物种鲜有记录，且缺乏任何分子研究。我们从在英国和瑞典分离出的半绿旋口虫中获取了此类数据。通过对来自这两个国家的分离株进行单细胞RNA测序，生成了半绿旋口虫的转录组。系统发育分析确定半绿旋口虫是微小旋口虫的近亲。此外，对绿色藻类内共生体的rRNA序列分析表明，它与普通小球藻密切相关。除了进行分子物种鉴定外，还对纤毛虫的终止密码子进行了分析，结果揭示了一种关系，即TGA终止密码子频率随着基因表达水平的升高而降低。观察到的密码子偏好表明，半绿旋口虫可能正处于重新分配TGA终止密码子的早期阶段。转录组分析表明，半绿旋口虫可能利用依赖于视黄醌的富马酸还原作用在其生存的缺氧栖息地进行呼吸。数据还表明，尽管所研究的采样地点之间地理距离很远（超过1600公里），但形态相同的物种可以共享完全相同的分子特征，这表明一些纤毛虫物种，即使是那些体长超过1毫米的物种，可能具有全球生物地理分布。
