Science，Nature三个测序结果

　　

生物通报道：基因组测序已进入了全速前进的阶段，近期又有三种生物的测序结果公布。

11月13日的《Nature》公布了三角褐指藻（Phaeodactylum tricornutum）的完整基因组序列，这是由来自法国，意大利，美国，比利时，德国等多国组成的研究小组完成的，是第二个被测序的硅藻。

硅藻是一类最重要的浮游生物，分布极其广泛。在世界大洋中，只要有水的地方，一般都有硅藻的踪迹，尤其是在温带和热带海区。因为硅藻种类多、数量大，因而被称为海洋的"草原"。 硅藻是一类具有色素体的单细胞植物，常由几个或很多细胞个体连结成各式各样的群体。

此次测序的硅藻与第一个被测序的硅藻——假微型海链藻（Thalassiosira pseudonana）所做的对比表明，通过硅藻与细菌之间的双向基因转移，硅藻从细菌获得了数百个基因。基因转移在硅藻演化中似乎是普遍存在的，产生了基因的非正规组合（其中包括一些来自植物和动物的基因），很可能在营养管理和环境信号作用中起重要作用。

11月14日的《Science》公布了白蚁（Pseudotrichonympha grassii）的基因组序列。

左图为白蚁，右上图为原虫，右下图为细菌Pseudotrichonympha grassii在细胞内的照片


用于消化木材的生物化学能力即神秘又在加工生物燃料上有着很高的需求。科学家们如今描绘了一种成为白蚁以消化木材作为其唯一食物来源基础的复杂的寄生虫体内含有寄生虫的关系。


许多共生性微生物居住在白蚁的肠道内协助白蚁消化木材。其中的一种是原虫，它本身又是细菌Pseudotrichonympha grassii 的一个宿主。Yuichi Hongoh及其同僚对P. grassii 的完整的基因组进行了序列测定。他们报道说， 其独特的基因组序列披露了该种细菌能够固定大气中的氮，再循环废弃物中的氮并为它们自己以及它们的原虫宿主制造氨基酸的能力。

另英国路透社报道，澳大利亚政府资助的袋鼠基因组中心科学家表示，他们已首次勾勒出了袋鼠的基因图谱，发现其与人类基因组有诸多相似之处。该组织负责人珍妮-格雷夫斯说：“（人类和袋鼠的基因组）有一定的差别，人类有些地方多一些，有些地方少一些，但都是同样的基因，且很多基因排序相同。”她还称：“我们原以为它们的排序完全被打乱了，但事实不然，人类大批基因组如数存在于在袋鼠基金组中。”

研究人员发现，人类和袋鼠至少在1-5亿年前还拥有着共同的祖先，而鼠类与人类大约在7000万年前开始各自演化。他们还称，袋鼠最初的进化地在中国，后来才跨越美洲大陆来到澳洲和南极洲。

（生物通：万纹）

原文摘要：

The Phaeodactylum genome reveals the evolutionary history of diatom genomes

Diatoms are photosynthetic secondary endosymbionts found throughout marine and freshwater environments, and are believed to be responsible for around one-fifth of the primary productivity on Earth1, 2. The genome sequence of the marine centric diatom Thalassiosira pseudonana was recently reported, revealing a wealth of information about diatom biology3, 4, 5. Here we report the complete genome sequence of the pennate diatom Phaeodactylum tricornutum and compare it with that of T. pseudonana to clarify evolutionary origins, functional significance and ubiquity of these features throughout diatoms. In spite of the fact that the pennate and centric lineages have only been diverging for 90 million years, their genome structures are dramatically different and a substantial fraction of genes (40%) are not shared by these representatives of the two lineages. Analysis of molecular divergence compared with yeasts and metazoans reveals rapid rates of gene diversification in diatoms. Contributing factors include selective gene family expansions, differential losses and gains of genes and introns, and differential mobilization of transposable elements. Most significantly, we document the presence of hundreds of genes from bacteria. More than 300 of these gene transfers are found in both diatoms, attesting to their ancient origins, and many are likely to provide novel possibilities for metabolite management and for perception of environmental signals. These findings go a long way towards explaining the incredible diversity and success of the diatoms in contemporary oceans.

 

Genome of an Endosymbiont Coupling N2 Fixation to Cellulolysis Within Protist Cells in Termite Gut

Termites harbor diverse symbiotic gut microorganisms, the majority of which are as yet uncultivable and their interrelationships unclear. Here, we present the complete genome sequence of the uncultured Bacteroidales endosymbiont of the cellulolytic protist Pseudotrichonympha grassii, which accounts for 70% of the bacterial cells in the gut of the termite Coptotermes formosanus. Functional annotation of the chromosome (1,114,206 base pairs) unveiled its ability to fix dinitrogen and recycle putative host nitrogen wastes for biosynthesis of diverse amino acids and cofactors, and import glucose and xylose as energy and carbon sources. Thus, nitrogen fixation and cellulolysis are coupled within the protist's cells. This highly evolved symbiotic system probably underlies the ability of the worldwide pest termites Coptotermes to use wood as their sole food.