Resolution of Prochlorococcus and Synechococcus Ecotypes by Using 16S-23S Ribosomal DNA Internal Transcribed Spacer Sequences

Cultured isolates of the marine cyanobacteria Prochlorococcus and Synechococcus vary widely in their pigment compositions and growth responses to light and nutrients, yet show greater than 96% identity in their 16S ribosomal DNA (rDNA) sequences. In order to better define the genetic variation that accompanies their physiological diversity, sequences for the 16S-23S rDNA internal transcribed spacer (ITS) region were determined in 32 Prochlorococcus isolates and 25 Synechococcus isolates from around the globe. Each strain examined yielded one ITS sequence that contained two tRNA genes. Dramatic variations in the length and G+C content of the spacer were observed among the strains, particularly among Prochlorococcus strains. Secondary-structure models of the ITS were predicted in order to facilitate alignment of the sequences for phylogenetic analyses. The previously observed division of Prochlorococcus into two ecotypes (called high and low-B/A after their differences in chlorophyll content) were supported, as was the subdivision of the high-B/A ecotype into four genetically distinct clades. ITS-based phylogenies partitioned marine cluster A Synechococcus into six clades, three of which can be associated with a particular phenotype (motility, chromatic adaptation, and lack of phycourobilin). The pattern of sequence divergence within and between clades is suggestive of a mode of evolution driven by adaptive sweeps and implies that each clade represents an ecologically distinct population. Furthermore, many of the clades consist of strains isolated from disparate regions of the world's oceans, implying that they are geographically widely distributed. These results provide further evidence that natural populations of Prochlorococcus and Synechococcus consist of multiple coexisting ecotypes, genetically closely related but physiologically distinct, which may vary in relative abundance with changing environmental conditions.

海洋蓝细菌原绿球藻（Prochlorococcus）与聚球藻（Synechococcus）的培养分离株在色素组成及对光照、营养的生长响应方面存在显著差异，但其16S核糖体DNA（rDNA）序列同源性却高于96%。为更清晰地阐明伴随其生理多样性的遗传变异机制，研究人员对全球范围内32株原绿球藻分离株与25株聚球藻分离株的16S-23S rDNA内部转录间隔区（ITS）序列进行了测定。所有受试菌株均得到一段包含两个转运RNA基因的ITS序列。研究发现，各菌株间的间隔区长度与G+C含量存在显著变异，其中尤以原绿球藻菌株间的差异最为突出。为便于序列比对以开展系统发育分析，研究人员预测了ITS的二级结构模型。此前报道的原绿球藻分为两类生态型（根据叶绿素含量差异分别命名为高B/A型与低B/A型）的结论得到了验证，同时高B/A生态型可细分为4个遗传上独立的进化枝这一划分也获得支持。基于ITS序列构建的系统发育树将海洋簇A聚球藻划分为6个进化枝，其中3个进化枝可与特定表型关联：运动性、色适应以及缺失藻尿胆素。进化枝内与进化枝间的序列分歧模式暗示了一种由适应性清扫驱动的进化模式，且表明每个进化枝均代表一个具有独特生态位的种群。此外，诸多进化枝包含从全球各大洋不同区域分离得到的菌株，这说明它们具有广泛的地理分布范围。本研究结果进一步证实，原绿球藻与聚球藻的自然种群由多个共存的生态型构成：这些生态型在遗传层面紧密相关，但生理特征截然不同，且其相对丰度可能随环境条件的变化而发生改变。