Museo Nacional de Ciencias Naturales-CSIC, C/Serrano 115 bis, 28006, Madrid, Spain.
Microb Ecol. 2013 Aug;66(2):335-50. doi: 10.1007/s00248-013-0223-2. Epub 2013 Apr 23.
Global warming is causing ice retreat in glaciers worldwide, most visibly over the last few decades in some areas of the planet. One of the most affected areas is the region of Tierra del Fuego (southern South America). Vascular plant recolonisation of recently deglaciated areas in this region is initiated by Gunnera magellanica, which forms symbiotic associations with the cyanobacterial genus Nostoc, a trait that likely confers advantages in this colonisation process. This symbiotic association in the genus Gunnera is notable as it represents the only known symbiotic relationship between angiosperms and cyanobacteria. The aim of this work was to study the genetic diversity of the Nostoc symbionts in Gunnera at three different, nested scale levels: specimen, population and region. Three different genomic regions were examined in the study: a fragment of the small subunit ribosomal RNA gene (16S), the RuBisCO large subunit gene coupled with its promoter sequence and a chaperon-like protein (rbcLX) and the ribosomal internal transcribed spacer (ITS) region. The identity of Nostoc as the symbiont was confirmed in all the infected rhizome tissue analysed. Strains isolated in the present study were closely related to strains known to form symbioses with other organisms, such as lichen-forming fungi or bryophytes. We found 12 unique haplotypes in the 16S rRNA (small subunit) region analysis, 19 unique haplotypes in the ITS region analysis and 57 in the RuBisCO proteins region (rbcLX). No genetic variability was found among Nostoc symbionts within a single host plant while Nostoc populations among different host plants within a given sampling site revealed major differences. Noteworthy, interpopulation variation was also shown between recently deglaciated soils and more ancient ones, between eastern and western sites and between northern and southern slopes of Cordillera Darwin. The cell structure of the symbiotic relationship was observed with low-temperature scanning electron microscopy, showing changes in morphology of both cyanobiont cells (differentiate more heterocysts) and plant cells (increased size). Developmental stages of the symbiosis, including cell walls and membranes and EPS matrix states, were also observed.
全球变暖导致世界各地的冰川消融，在过去几十年中，地球上的一些地区冰川消融尤为明显。受影响最严重的地区之一是火地岛地区（南美洲南部）。在该地区，刚毛藻属（Gunnera magellanica）植物开始重新殖民最近冰川消退的地区，它与蓝细菌属（Nostoc）形成共生关系，这一特征可能使其在这一殖民过程中具有优势。刚毛藻属中的这种共生关系引人注目，因为它代表了被子植物和蓝细菌之间唯一已知的共生关系。这项工作的目的是研究三个不同嵌套尺度水平（标本、种群和区域）下，刚毛藻属中 Nostoc 共生体的遗传多样性。在研究中检查了三个不同的基因组区域：小亚基核糖体 RNA 基因（16S）的一个片段、与启动子序列偶联的 RuBisCO 大亚基基因和一种伴侣样蛋白（rbcLX）以及核糖体内部转录间隔区（ITS）。在所分析的所有受感染的根茎组织中都证实了 Nostoc 是共生体。本研究中分离出的菌株与已知与其他生物体（如地衣形成真菌或苔藓植物）形成共生关系的菌株密切相关。在 16S rRNA（小亚基）区域分析中发现了 12 个独特的单倍型，在 ITS 区域分析中发现了 19 个独特的单倍型，在 RuBisCO 蛋白区域（rbcLX）发现了 57 个。在单个宿主植物内的 Nostoc 共生体中未发现遗传变异，而在给定采样点内不同宿主植物之间的 Nostoc 种群则显示出较大差异。值得注意的是，最近冰川消退的土壤与更古老的土壤之间、东、西采样点之间以及达尔文山脉北、南坡之间也存在种群间的变异。通过低温扫描电子显微镜观察到共生关系的细胞结构，显示蓝藻共生体细胞（异形胞分化更多）和植物细胞（尺寸增大）的形态发生变化。还观察到共生关系的发育阶段，包括细胞壁和细胞膜以及 EPS 基质状态。
