Data from: Specificity in diversity: single origin of a widespread ciliate-bacteria symbiosis

Symbioses between eukaryotes and sulfur-oxidizing (thiotrophic) bacteria have convergently evolved multiple times. Although well described in at least eight classes of metazoan animals, almost nothing is known about the evolution of thiotrophic symbioses in microbial eukaryotes (protists). In this study, we characterized the symbioses between mouthless marine ciliates of the genus Kentrophoros, and their thiotrophic bacteria, using comparative sequence analysis and fluorescence in situ hybridization. Ciliate small-subunit rRNA sequences were obtained from 17 morphospecies collected in the Mediterranean and Caribbean, and symbiont sequences from 13 of these morphospecies. We discovered a new Kentrophoros morphotype where the symbiont-bearing surface is folded into pouch-like compartments, illustrating the variability of the basic body plan. Phylogenetic analyses revealed that all investigated Kentrophoros belonged to a single clade, despite the remarkable morphological diversity of these hosts. The symbionts were also monophyletic and belonged to a new clade within the Gammaproteobacteria, with no known cultured representatives. Each host morphospecies had a distinct symbiont phylotype, and statistical analyses revealed significant support for host–symbiont codiversification. Given that these symbioses were collected from two widely separated oceans, our results indicate that symbiotic associations in unicellular hosts can be highly specific and stable over long periods of evolutionary time.真核生物与硫氧化（sulfur-oxidizing, thiotrophic）细菌之间的共生关系已多次趋同演化。尽管此类共生关系已在至少8个门类的后生动物（metazoan）中得到详尽阐释，但学界对微生物真核生物（protists，原生生物）中的化能硫营养共生演化机制几乎一无所知。本研究采用比较序列分析与荧光原位杂交（fluorescence in situ hybridization）技术，对Kentrophoros属无口海洋纤毛虫与其化能硫营养共生细菌之间的共生关系展开了系统表征。研究人员从地中海与加勒比海采集的17个纤毛虫形态种中获取了小亚基rRNA（small-subunit rRNA）序列，并从其中13个形态种中获得了共生菌序列。本研究发现了一种全新的Kentrophoros形态型，其携带共生菌的体表折叠形成囊状隔间，直观展现了该类群基础躯体构型的变异多样性。系统发育分析结果显示，尽管这些宿主存在显著的形态多样性，但所有被研究的Kentrophoros类群均隶属于单一演化支。共生菌同样为单系群（monophyletic），隶属于γ-变形菌纲（Gammaproteobacteria）下的一个全新演化支，目前尚无已知可培养的代表菌株。每个宿主形态种均拥有独特的共生菌系统型（phylotype），统计学分析证实宿主与共生菌之间存在显著的协同分化（codiversification）信号。鉴于此类共生关系采集自两个地理隔离甚远的大洋，本研究结果表明，单细胞宿主的共生关联可具备高度的特异性，并能在漫长的进化时间尺度上保持稳定。