Epulopiscium sp. type B (Lachnospiraceae) is an exceptionally large, highly polyploid, intestinal symbiont of the coral reef dwelling surgeonfish Naso tonganus. These obligate anaerobes do not form mature endospores and reproduce solely through the production of multiple intracellular offspring. This likely makes them dependent on immediate transfer to a receptive host for dispersal. During reproduction, only a small proportion of Epulopiscium mother-cell DNA is inherited. To explore the impact of this unusual viviparous lifestyle on symbiont population dynamics, we investigated Epulopiscium sp. type B and their fish hosts collected over the course of two decades, at island and reef habitats near Lizard Island, Australia. Using multi-locus sequence analysis, we found that recombination plays an important role in maintaining diversity of these symbionts and yet populations exhibit linkage disequilibrium (LD). Symbiont populations showed spatial but not temporal partitioning. Surgeonfish are long-lived and capable of traveling long distances, yet the population structures of Epulopiscium suggest that adult fish tend to not roam beyond a limited locale. Codiversification analyses and traits of this partnership suggest that while symbionts are obligately dependent on their host, the host has a facultative association with Epulopiscium. We suggest that congression of unlinked markers contributes to LD estimates in this and other recombinant populations of bacteria. The findings here inform our understanding of evolutionary processes within intestinal Lachnospiraceae populations.

Epulopiscium sp. B 型（Lachnospiraceae）是珊瑚礁刺猬Naso tonganus的一种异常大型、高度多倍体的肠道共生体。这些专性厌氧菌不形成成熟的内生孢子，仅通过产生多个细胞内后代来繁殖。这可能使它们依赖于立即转移到接受宿主来进行传播。在繁殖过程中，只有一小部分Epulopiscium母细胞 DNA 被遗传。为了探索这种不寻常的胎生生活方式对共生体种群动态的影响，我们研究了Epulopiscium sp。 B 型及其鱼类宿主是二十年来在澳大利亚蜥蜴岛附近的岛屿和珊瑚礁栖息地收集的。通过多位点序列分析，我们发现重组在维持这些共生体的多样性方面发挥着重要作用，但群体表现出连锁不平衡（LD）。共生体种群表现出空间划分，但没有时间划分。刺尾鱼寿命很长，能够长距离游动，但鳖鱼的种群结构表明，成年鱼往往不会超出有限的区域。这种伙伴关系的共同多样性分析和特征表明，虽然共生体必然依赖于它们的宿主，但宿主与Epulopiscium具有兼性关联。我们认为，未连锁标记的聚集有助于该细菌和其他重组细菌群体的 LD 估计。这里的发现有助于我们了解肠道毛螺菌科种群的进化过程。