Photosynthetic Systems Suggest an Evolutionary Pathway to Diderms
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Photosynthetic Systems Suggest an Evolutionary Pathway to Diderms Scott O. Rogers1 Received: 15 May 2020 / Accepted: 23 November 2020 © The Author(s) 2020
Abstract 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. Keywords Monoderms · Diderms · Gram-positive · Gram-negative · Photosynthesis · Endosymbiotic
1 Introduction The search for a universal tree of life, especially one that resolves the earlier events in biological evolution, has been confounded by a number of factors, including the antiquities of branch points, the absence of sequence data from extinct organisms, rapid mutation rates (especially in the early stages), and horizontal (or lateral) gene transfers (HGTs). These factors lead to discordant placements and timings for branch * Scott O. Rogers [email protected] 1
Department of Biological Sciences, Bowling Green State University, Bowling Green, OH 43403, USA
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points on the trees, resulting in confusion regarding the evolutionary events that produced the myriad taxa. HGTs are common and widespread within and among all domains of life (Archibald and Keeling 2002; Bowler et al. 2009; Dagan et al. 2008; Gao and Gupta 2012; Jain et al. 2002; Keeling 2010; Ku et al. 2015; Lake and Rivera 1994; Lang and Gray 1999; Lang et al. 2012; Martin and Muller 1998; Poole et al. 1999; Raymond et al. 2002; Sousy et al. 2015; Syvanen and Kado 1998). They can be caused by transfer of DNA from one organism to another, importation of DNA (within a food organism or freely floating), transfer by viruses, or by endosymbiotic events. In particular, endosymbiosis can transfer large numbers of genes in a single event, and have led to acquisition of complex functions (e.g., oxidative phosphorylation in eukaryotes and photosynthesis in plants). Endosymbioses led to the origin of eukaryotes, which joined at least one bacterium with an archaeal cell (Eme et al. 2017; Sagan 1967), and have resulted in the evolu
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