Data from: Phylogenomic analysis supports the ancestral presence of LPS-outer membranes in the Firmicutes

One of the major unanswered questions in evolutionary biology is when and how the transition between diderm (two membranes) and monoderm (one membrane) cell envelopes occurred in Bacteria. The Negativicutes and the Halanaerobiales belong to the classically monoderm Firmicutes, but possess outer membranes with lipopolysaccharide (LPS-OM). Here, we show that they form two phylogenetically distinct lineages, each close to different monoderm relatives. In contrast, their core LPS biosynthesis enzymes were inherited vertically, as in the majority of bacterial phyla. Finally, annotation of key OM systems in the Halanaerobiales and the Negativicutes shows a puzzling combination of monoderm and diderm features. Together, these results support the hypothesis that the LPS-OMs of Negativicutes and Halanaerobiales are remnants of an ancient diderm cell envelope that was present in the ancestor of the Firmicutes, and that the monoderm phenotype in this phylum is a derived character that arose multiple times independently through OM loss.进化生物学领域亟待解决的核心问题之一，便是细菌界中双膜（diderm，两层细胞膜）与单膜（monoderm，单层细胞膜）细胞被膜之间的演化过渡究竟发生于何时、以何种方式完成。负杆菌纲（Negativicutes）与嗜盐厌氧菌目（Halanaerobiales）隶属于经典的单膜型厚壁菌门（Firmicutes），但它们却拥有带有脂多糖的外膜（LPS-OM）。本研究证实，这两类类群可划分为两个系统发育独立的演化支，各自与不同的单膜型细菌类群亲缘关系相近。与之形成鲜明对比的是，它们的核心脂多糖（lipopolysaccharide, LPS）生物合成酶与多数细菌门类一致，均通过垂直遗传方式传承。最后，针对嗜盐厌氧菌目与负杆菌纲中关键外膜（outer membrane, OM）系统的注释分析显示，其兼具单膜型与双膜型细菌的特征，这一组合颇为费解。综上，上述研究结果支持如下假说：负杆菌纲与嗜盐厌氧菌目所携带的脂多糖外膜（LPS-OM），实为厚壁菌门祖先所拥有的古老双膜型细胞被膜的残留；而该门中的单膜型表型则是一种衍生性状，曾多次通过外膜丢失的路径独立演化产生。