Abstract
Chloroplasts of plants and algae are currently believed to originate from a cyanobacterial endosymbiont, mainly based on the shared proteins involved in the oxygenic photosynthesis and gene expression system. The phylogenetic relationship between the chloroplast and cyanobacterial genomes was important evidence for the notion that chloroplasts originated from cyanobacterial endosymbiosis. However, studies in the post-genomic era revealed that various substances (glycolipids, peptidoglycan, etc.) shared by cyanobacteria and chloroplasts are synthesized by different pathways or phylogenetically unrelated enzymes. Membranes and genomes are essential components of a cell (or an organelle), but the origins of these turned out to be different. Besides, phylogenetic trees of chloroplast-encoded genes suggest an alternative possibility that chloroplast genes could be acquired from at least three different lineages of cyanobacteria. We have to seriously examine that the chloroplast genome might be chimeric due to various independent gene flows from cyanobacteria. Chloroplast formation could be more complex than a single event of cyanobacterial endosymbiosis. I present the “host-directed chloroplast formation” hypothesis, in which the eukaryotic host cell that had acquired glycolipid synthesis genes as an adaptation to phosphate limitation facilitated chloroplast formation by providing glycolipid-based membranes (pre-adaptation). The origins of the membranes and the genome could be different, and the origin of the genome could be complex.
Highlights
Cyanobacteria are a distinct group of bacteria that perform oxygenic photosynthesis.They have specialized internal membranes called thylakoid membranes in which photosynthetic systems reside
Essentially all components of primary reactions of photosynthesis and gene expression systems of chloroplast undoubtedly originated from cyanobacteria
The currently accepted view of chloroplast origin assumes a single event of cyanobacterial endosymbiosis, probably a cyanobacterium related to Gloeomargarita [54], which is assumed as the common ancestor of the chloroplasts of Archaeplastida
Summary
Cyanobacteria are a distinct group of bacteria that perform oxygenic photosynthesis. They have specialized internal membranes called thylakoid membranes in which photosynthetic systems reside. The accumulation of supporting data continued until the early 2000s when the genomic sequences of major model organisms became available Under these situations, the organellar genes and nuclear-encoded organelle proteins were used for the phylogenetic analysis that confirmed the close relationships, chloroplasts/cyanobacteria, and mitochondria/α-proteobacteria. It is worth trying to re-examine all available evidence for the endosymbiotic theory and to re-organize the history of the formation of chloroplasts and mitochondria This is a discussion or hypothesis article with a limited length on the historical and biological questions on the endosymbiotic origins of organelles, but I focus mainly on the relationship between cyanobacteria and chloroplasts.
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