Early sponges and toxic protists: possible sources of cryostane, an age diagnostic biomarker antedating Sturtian Snowball Earth
Early sponges and toxic protists: possible sources of cryostane, an age diagnostic biomarker antedating Sturtian Snowball Earth
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DOI：
10.1111/gbi.12165
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发表时间：
2016-03-01
期刊：
影响因子：
3.7
通讯作者：
Hope, J.
中科院分区：
文献类型：
--
作者：
Brocks, J. J.;Jarrett, A. J. M.;Hope, J.
8亿至7.17亿年前，在斯特阶雪球冰期之前，真核微化石的多样性增加。为了提供一个独立的和互补的观点，这一进化时期，本研究提出了分布的真核生物标志物，从三个前斯特阶继承整个超大陆Rodinia：ca。780Ma Kanpa Formation of the Western Australian Officer Basin，the ca. 800- 740 Ma瑞典Visingso群和740 Ma美国亚利桑那州Chuar群。真核生物甾烷的分布在这三个序列中是非常相似的，但不同于所有其他已知的年轻和古老的甾烷组合。胆甾烷是唯一的常规结构，而本土甾烷烷基化的位置C-24，如麦角甾烷，豆甾烷，地诺甾烷和异丙基胆甾烷，和正丙基胆甾烷，没有观察到。这种甾烷分布似乎是前Sturtian新元古代的年龄诊断。它证明了前雪球真核生物的独特进化状态，指出分类差异仍然低于埃迪卡拉纪（635- 541 Ma）。所有三个盆地还显示存在一种新的C-28甾烷，初步鉴定为26-甲基胆甾烷，此处命名为cryostane。唯一已知的现存生物，可以甲基化甾醇在26位是demosponges。这一分配是合理的，因为分子钟将最早的动物的出现置于前斯特阶新元古代。这种不寻常的26-甲基甾醇可能保护了海绵，但也保护了其他真核生物，使其免受自身膜溶解毒素的侵害。一些原生生物释放溶解性毒素来阻止捕食者并杀死真核生物的猎物。由于传统的膜固醇可以成为这些毒素的攻击位点，具有不寻常的侧链修饰的固醇保护细胞。这一解释cryostane支持的化石证据的捕食在楚阿群，促进假说的增殖真核吞噬在铅到Cryogenian。
The period 800-717million years (Ma) ago, in the lead-up to the Sturtian Snowball glaciation, saw an increase in the diversity of eukaryotic microfossils. To afford an independent and complementary view of this evolutionary period, this study presents the distribution of eukaryotic biomarkers from three pre-Sturtian successions across the supercontinent Rodinia: the ca. 780Ma Kanpa Formation of the Western Australian Officer Basin, the ca. 800-740Ma Visingso Group of Sweden, and the 740Ma Chuar Group in Arizona, USA. The distribution of eukaryotic steranes is remarkably similar in the three successions but distinct from all other known younger and older sterane assemblages. Cholestane was the only conventional structure, while indigenous steranes alkylated in position C-24, such as ergostane, stigmastane, dinosterane and isopropylcholestane, and n-propylcholestane, were not observed. This sterane distribution appears to be age diagnostic for the pre-Sturtian Neoproterozoic. It attests to the distinct evolutionary state of pre-Snowball eukaryotes, pointing to a taxonomic disparity that was still lower than in the Ediacaran (635-541Ma). All three basins also show the presence of a new C-28 sterane that was tentatively identified as 26-methylcholestane, here named cryostane. The only known extant organisms that can methylate sterols in the 26-position are demosponges. This assignment is plausible as molecular clocks place the appearance of the earliest animals into the pre-Sturtian Neoproterozoic. The unusual 26-methylsterol may have protected sponges, but also other eukaryotes, against their own membranolytic toxins. Some protists release lytic toxins to deter predators and kill eukaryotic prey. As conventional membrane sterols can be the site of attack for these toxins, sterols with unusual side-chain modification protect the cell. This interpretation of cryostane supports fossil evidence of predation in the Chuar Group and promotes hypotheses about the proliferation of eukaryophagy in the lead-up to the Cryogenian.