Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany.
PLoS One. 2010 Feb 16;5(2):e9235. doi: 10.1371/journal.pone.0009235.
Cyanobacterial morphology is diverse, ranging from unicellular spheres or rods to multicellular structures such as colonies and filaments. Multicellular species represent an evolutionary strategy to differentiate and compartmentalize certain metabolic functions for reproduction and nitrogen (N(2)) fixation into specialized cell types (e.g. akinetes, heterocysts and diazocytes). Only a few filamentous, differentiated cyanobacterial species, with genome sizes over 5 Mb, have been sequenced. We sequenced the genomes of two strains of closely related filamentous cyanobacterial species to yield further insights into the molecular basis of the traits of N(2) fixation, filament formation and cell differentiation. Cylindrospermopsis raciborskii CS-505 is a cylindrospermopsin-producing strain from Australia, whereas Raphidiopsis brookii D9 from Brazil synthesizes neurotoxins associated with paralytic shellfish poisoning (PSP). Despite their different morphology, toxin composition and disjunct geographical distribution, these strains form a monophyletic group. With genome sizes of approximately 3.9 (CS-505) and 3.2 (D9) Mb, these are the smallest genomes described for free-living filamentous cyanobacteria. We observed remarkable gene order conservation (synteny) between these genomes despite the difference in repetitive element content, which accounts for most of the genome size difference between them. We show here that the strains share a specific set of 2539 genes with >90% average nucleotide identity. The fact that the CS-505 and D9 genomes are small and streamlined compared to those of other filamentous cyanobacterial species and the lack of the ability for heterocyst formation in strain D9 allowed us to define a core set of genes responsible for each trait in filamentous species. We presume that in strain D9 the ability to form proper heterocysts was secondarily lost together with N(2) fixation capacity. Further comparisons to all available cyanobacterial genomes covering almost the entire evolutionary branch revealed a common minimal gene set for each of these cyanobacterial traits.
蓝藻形态多样，从单细胞球体或杆状到多细胞结构，如菌落和丝状。多细胞物种代表了一种进化策略，将某些代谢功能分化和分隔开来，以进行繁殖和氮（N2）固定到专门的细胞类型（例如异形胞、异形胞和 Diazocytes）。只有少数具有超过 5 Mb 基因组大小的丝状分化蓝藻物种已被测序。我们对两种密切相关的丝状蓝藻物种的基因组进行了测序，以进一步深入了解 N2 固定、丝状形成和细胞分化的分子基础。Cylindrospermopsis raciborskii CS-505 是一种来自澳大利亚的产生圆柱鱼腥藻素的菌株，而 Raphidiopsis brookii D9 来自巴西，合成与麻痹性贝类中毒（PSP）相关的神经毒素。尽管它们的形态、毒素组成和分离的地理分布不同，但这些菌株形成一个单系群。它们的基因组大小约为 3.9（CS-505）和 3.2（D9）Mb，这是描述的自由生活丝状蓝藻中最小的基因组。尽管重复元件含量差异很大，但我们观察到这些基因组之间存在显著的基因顺序保守性（同线性）。我们在这里表明，这些菌株具有特定的 2539 个基因，具有 >90%的平均核苷酸同一性。CS-505 和 D9 基因组与其他丝状蓝藻物种相比非常小且精简，而且菌株 D9 缺乏异形胞形成的能力，这使得我们能够定义一组负责丝状物种每个特征的核心基因。我们推测，在菌株 D9 中，形成适当异形胞的能力与 N2 固定能力一起被次要丢失。进一步与几乎涵盖整个进化分支的所有可用蓝藻基因组进行比较，揭示了每个蓝藻特征的共同最小基因集。
