Abstract：
Contemporary glaciers are inhabited by streptophyte algae that balance photosynthesis and growth with tolerance of low temperature, desiccation and UV radiation. These same environmental challenges have been hypothesised as the driving force behind the evolution of land plants from streptophyte algal ancestors in the Cryogenian (720-635 million years ago). We sequenced, assembled and analysed the metagenome-assembled genome of the glacier alga Ancylonema nordenskiöldii to investigate its adaptations to life in ice, and whether this represents a vestige of Cryogenian exaptations. Phylogenetic analysis confirms the placement of glacier algae within the sister lineage to land plants, Zygnematophyceae. The metagenome-assembled genome is characterised by an expansion of genes involved in tolerance of high irradiance and UV light, while lineage-specific diversification is linked to the novel screening pigmentation of glacier algae. We found no support for the hypothesis of a common genomic basis for adaptations to ice and to land in streptophytes. Comparative genomics revealed that the reductive morphological evolution in the ancestor of Zygnematophyceae was accompanied by reductive genome evolution. This first genome-scale data for glacier algae suggests an Ancylonema-specific adaptation to the cryosphere, and sheds light on the genome evolution of land plants and Zygnematophyceae.
摘要：
当代冰川居住着链球菌藻类，它们平衡光合作用和生长以及对低温的耐受性，干燥和紫外线辐射。这些相同的环境挑战已被假设为在Cryogenian（720-6.35亿年前）从链球菌藻类祖先那里进化陆地植物的驱动力。我们测序了,组装并分析了冰川藻类Ancylonemanordenskiöldii的宏基因组组装基因组，以研究其对冰中生命的适应，以及这是否代表了Cryogenian灭绝的痕迹。系统发育分析证实了冰川藻类在姊妹谱系中与陆地植物的位置，Zygnematphyceae。宏基因组组装的基因组的特征是涉及高辐照度和紫外线耐受性的基因的扩展，而谱系特异性的多样化与冰川藻类的新型色素沉着筛选有关。我们没有发现支持适应冰和在链球菌中着陆的共同基因组基础的假设。比较基因组学表明，Zygnematphyceae祖先的还原形态进化伴随着还原基因组进化。冰川藻类的第一个基因组规模数据表明Ancylonema对冰冻圈的特异性适应，并阐明了陆地植物和Zygnematphyceae的基因组进化。
