Data from: Closely coupled evolutionary history of ecto- and endosymbionts from two distantly related animal phyla

The level of integration between associated partners can range from ectosymbioses to extracellular and intracellular endosymbioses, and this range has been assumed to reflect a continuum from less intimate to evolutionarily highly stable associations. In this study, we examined the specificity and evolutionary history of marine symbioses in a group of closely related sulphur-oxidizing bacteria, called Candidatus Thiosymbion, that have established ecto- and endosymbioses with two distantly related animal phyla, Nematoda and Annelida. Intriguingly, in the ectosymbiotic associations of stilbonematine nematodes, we observed a high degree of congruence between symbiont and host phylogenies, based on their ribosomal RNA (rRNA) genes. In contrast, for the endosymbioses of gutless phallodriline annelids (oligochaetes), we found only a weak congruence between symbiont and host phylogenies, based on analyses of symbiont 16S rRNA genes and six host genetic markers. The much higher degree of congruence between nematodes and their ectosymbionts compared to those of annelids and their endosymbionts was confirmed by cophylogenetic analyses. These revealed 15 significant codivergence events between stilbonematine nematodes and their ectosymbionts, but only one event between gutless phallodrilines and their endosymbionts. Phylogenetic analyses of 16S rRNA gene sequences from 50 Cand. Thiosymbion species revealed seven well-supported clades that contained both stilbonematine ectosymbionts and phallodriline endosymbionts. This closely coupled evolutionary history of marine ecto- and endosymbionts suggests that switches between symbiotic lifestyles and between the two host phyla occurred multiple times during the evolution of the Cand. Thiosymbion clade, and highlights the remarkable flexibility of these symbiotic bacteria.

共生伙伴间的整合程度跨度可从外共生（ectosymbiosis）延伸至细胞外内共生与细胞内内共生（endosymbiosis），此前学界普遍认为这一跨度对应着从松散结合到演化上高度稳定的共生连续体。本研究针对一类亲缘关系紧密的硫氧化细菌类群——*Candidatus Thiosymbion*（候选属）展开分析，该类群与线虫动物门（Nematoda）、环节动物门（Annelida）这两个亲缘关系较远的动物门分别建立了外共生与内共生关系，借此探讨了这类海洋共生体系的特异性与演化历史。值得注意的是，基于核糖体RNA（rRNA）基因分析，我们在丝唇线虫类（stilbonematine nematodes）的外共生关系中观察到共生菌与宿主的系统发育关系存在高度一致性。与之相反，在无肠带丝蚓类环节动物（寡毛类oligochaetes）的内共生关系中，通过对共生菌16S rRNA基因以及6个宿主遗传标记的分析，我们仅发现共生菌与宿主的系统发育关系存在微弱的一致性。协同系统发育分析（cophylogenetic analyses）进一步证实，线虫与其外共生菌之间的系统发育一致性程度远高于环节动物与其内共生菌。该分析显示，丝唇线虫类与其外共生菌之间存在15个显著的共分化事件（codivergence events），而无肠带丝蚓类与其内共生菌之间仅存在1个共分化事件。对50株*Candidatus Thiosymbion*菌株的16S rRNA基因序列进行系统发育分析，结果显示存在7个支持度较高的演化支（clade），这些演化支同时包含丝唇线虫类外共生菌与带丝蚓类内共生菌。这类海洋外共生与内共生菌紧密耦合的演化历史表明，在*Candidatus Thiosymbion*演化支的演化过程中，共生生活方式以及宿主门类之间的转换曾多次发生，这也凸显了这类共生细菌极强的演化灵活性。