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.

进化生物学领域的重大未解之谜之一，便是细菌域中双膜（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。脱铁杆菌纲（Negativicutes）与盐厌氧菌目（Halanaerobiales）隶属于经典的单膜类群厚壁菌门（Firmicutes），却拥有携带脂多糖（lipopolysaccharide, LPS）的外膜（LPS-OM）。本研究显示，这两个类群构成系统发育上相互独立的两个支系，各自与不同的单膜类群亲缘关系相近。与之形成鲜明对比的是，它们的核心脂多糖生物合成酶与大多数细菌门类一样，均通过垂直遗传方式获得。最后，对盐厌氧菌目与脱铁杆菌纲的关键外膜系统进行注释后发现，其兼具单膜与双膜类群的特征，这一特征组合令人颇感困惑。综合上述结果，本研究支持如下假说：脱铁杆菌纲与盐厌氧菌目所携带的脂多糖外膜，乃是厚壁菌门祖先所拥有的古老双膜细胞被膜的遗留物；而该门中的单膜表型则是通过多次独立的外膜丢失事件演化而来的衍生性状。