Logares Ramiro, Rengefors Karin, Kremp Anke, Shalchian-Tabrizi Kamran, Boltovskoy Andrés, Tengs Torstein, Shurtleff Aaron, Klaveness Dag
Limnology Division, Ecology Department, Lund University, Lund, Sweden.
Microb Ecol. 2007 May;53(4):549-61. doi: 10.1007/s00248-006-9088-y. Epub 2007 Apr 5.
The agents driving the divergence and speciation of free-living microbial populations are still largely unknown. We investigated the dinoflagellate morphospecies Scrippsiella hangoei and Peridinium aciculiferum, which abound in the Baltic Sea and in northern temperate lakes, respectively. Electron microscopy analyses showed significant interspecific differences in the external cellular morphology, but a similar plate pattern in the characteristic dinoflagellate armor. Experimentally, S. hangoei grew in a wide range of salinities (0-30), whereas P. aciculiferum only grew in low salinities (0-3). Despite these phenotypic differences and the habitat segregation, molecular analyses showed identical ribosomal DNA sequences (ITS1, ITS2, 5.8S, SSU, and partial LSU) for both morphospecies. Yet, a strong interspecific genetic isolation was indicated by amplified fragment length polymorphism (F (ST) = 0.76) and cytochrome b (cob) sequence divergence ( approximately 1.90%). Phylogenetic reconstructions based on ribosomal (SSU, LSU) and mitochondrial (cob) DNA indicated a recent marine ancestor for P. aciculiferum. In conclusion, we suggest that the lacustrine P. aciculiferum and the marine-brackish S. hangoei diverged very recently, after a marine-freshwater transition that exposed the ancestral populations to different selective pressures. This hypothetical scenario agrees with mounting data indicating a significant role of natural selection in the divergence of free-living microbes, despite their virtually unrestricted dispersal capabilities. Finally, our results indicate that identical ITS rDNA sequences do not necessarily imply the same microbial species, as commonly assumed.
驱动自由生活的微生物种群分化和物种形成的因素在很大程度上仍然未知。我们研究了分别在波罗的海和北温带湖泊中大量存在的甲藻形态种汉氏斯克里普藻（Scrippsiella hangoei）和针状多甲藻（Peridinium aciculiferum）。电子显微镜分析显示，两种藻在外部细胞形态上存在显著的种间差异，但在典型的甲藻外壳的板片模式上相似。实验表明，汉氏斯克里普藻能在广泛的盐度范围（0 - 30）内生长，而针状多甲藻仅在低盐度（0 - 3）下生长。尽管存在这些表型差异和栖息地隔离，但分子分析显示这两种形态种的核糖体DNA序列（ITS1、ITS2、5.8S、SSU和部分LSU）相同。然而，扩增片段长度多态性（F(ST)=0.76）和细胞色素b（cob）序列分歧（约1.90%）表明存在强烈的种间遗传隔离。基于核糖体（SSU、LSU）和线粒体（cob）DNA的系统发育重建表明，针状多甲藻有一个近代的海洋祖先。总之，我们认为，在经历了使祖先种群面临不同选择压力的海水 - 淡水转变之后，淡水的针状多甲藻和海洋 - 咸淡水的汉氏斯克里普藻是在非常近的时期内分化的。这一假设情景与越来越多的数据一致，这些数据表明自然选择在自由生活微生物的分化中起重要作用，尽管它们具有几乎不受限制的扩散能力。最后，我们的结果表明，与通常假设的情况不同，相同的ITS rDNA序列不一定意味着是同一微生物物种。
