Bacteria are divided primarily into monoderms (with one cell membrane, and usually Gram-positive, due to a thick peptidoglycan layer) and diderms (with two cell membranes, and mostly Gram-negative, due to a thin peptidoglycan layer sandwiched between the two membranes). Photosynthetic species are spread among the taxonomic groups, some having type I reaction centers (RCI in monoderm phylum Firmicutes; and diderm phyla Acidobacteria and Chlorobi), others with type II reaction centers (RCII in monoderm phylum Chloroflexi; and diderm taxa Gemmatimonadetes, and alpha-, beta-, and gamma-Proteobacteria), and some containing both (RCI and RCII, only in diderm phylum Cyanobacteria). In most bacterial phylograms, photosystem types and diderm taxa are polyphyletic. A more parsimonious arrangement, which is supported by photosystem evolution, as well as additional sets of molecular characters, suggests that endosymbiotic events resulted in the formation of the diderms. In the model presented, monoderms readily form a monophyletic group, while diderms are produced by at least two endosymbiotic events, followed by additional evolutionary changes.

中文翻译：

---

光合系统暗示了一条通往双鱼的进化途径

细菌主要分为单皮细胞（具有一个细胞膜，通常是革兰氏阳性，由于肽聚糖层较厚）和双皮细胞（具有两个细胞膜，并且大多数革兰氏阴性，由于夹在两个膜之间的薄肽聚糖层）。光合物种分布在分类群中，一些具有 I 型反应中心（在单皮动物门厚壁菌门中的 RCI；以及双皮动物门酸杆菌和绿藻），其他具有 II 型反应中心（在单皮动物门 Chloroflexi 中的 RCII；和双皮动物分类群 Gemmatimonadetes 和 α -、β-和 γ-变形菌），以及一些同时含有（RCI 和 RCII，仅在双皮门蓝细菌中）。在大多数细菌系统谱中，光系统类型和双皮类群是多系的。由光系统进化支持的更简约的安排，以及额外的分子特征集表明，内共生事件导致了真皮的形成。在提出的模型中，单皮动物很容易形成一个单系群，而双皮动物是由至少两个内共生事件产生的，然后是额外的进化变化。