The symbiosis between bacteria and sponges has arguably the longest evolutionary history for any extant metazoan lineage, yet little is known about bacterial evolution or adaptation in this process. An example of often dominant and widespread bacterial symbionts of sponges is a clade of uncultured and uncharacterised Proteobacteria. Here we set out to characterise this group using metagenomics, in-depth phylogenetic analyses, metatranscriptomics, and fluorescence in situ hybridisation microscopy. We obtained five metagenome-assembled-genomes (MAGs) from different sponge species that, together with a previously published MAG (AqS2), comprise two families within a new gammaproteobacterial order that we named ^UTethybacterales. Members of this order share a heterotrophic lifestyle but vary in their predicted ability to use various carbon, nitrogen and sulfur sources, including taurine, spermidine and dimethylsulfoniopropionate. The deep branching of the ^UTethybacterales within the Gammaproteobacteria and their almost exclusive presence in sponges suggests they have entered a symbiosis with their host relatively early in evolutionary time and have subsequently functionally radiated. This is reflected in quite distinct lifestyles of various species of ^UTethybacterales, most notably their diverse morphologies, predicted substrate preferences, and localisation within the sponge tissue. This study provides new insight into the evolution of metazoan–bacteria symbiosis.

对于任何现存的后生动物谱系来说，细菌和海绵之间的共生关系可以说是最长的进化历史，但人们对这一过程中的细菌进化或适应知之甚少。海绵中通常占主导地位和广泛分布的细菌共生体的一个例子是未培养和未表征的变形菌的进化枝。在这里，我们开始使用宏基因组学、深入的系统发育分析、宏转录组学和荧光原位杂交显微镜来表征这一群体。^{我们从不同的海绵物种中获得了五个宏基因组组装基因组 (MAGs)，它们与先前发表的 MAG (AqS2) 一起，在我们命名为U}的新 gammaproteobacterial 目中包含两个家族泰氏杆菌目。该命令的成员共享异养生活方式，但他们预测使用各种碳、氮和硫源的能力不同，包括牛磺酸、亚精胺和二甲基磺基丙酸盐。^U Tethybacterales 在 Gammaproteobacteria 内的深层分支以及它们在海绵中几乎排他性的存在表明它们在进化时间相对较早地与宿主进入了共生关系，并随后进行了功能辐射。这反映在^U Tethybacterales 的各种物种的完全不同的生活方式中，最显着的是它们的不同形态、预测的底物偏好和海绵组织内的定位。这项研究为后生动物-细菌共生的进化提供了新的见解。