title: Gene transfers from diverse bacteria compensate for reductive genome evolution in the chromatophore of Paulinella chromatophora.
abstract: Plastids, the photosynthetic organelles, originated >1 billion y ago via the endosymbiosis of a cyanobacterium. The resulting proliferation of primary producers fundamentally changed global ecology. Endosymbiotic gene transfer (EGT) from the intracellular cyanobacterium to the nucleus is widely recognized as a critical factor in the evolution of photosynthetic eukaryotes. The contribution of horizontal gene transfers (HGTs) from other bacteria to plastid establishment remains more controversial. A novel perspective on this issue is provided by the amoeba Paulinella chromatophora, which contains photosynthetic organelles (chromatophores) that are only 60-200 million years old. Chromatophore genome reduction entailed the loss of many biosynthetic pathways including those for numerous amino acids and cofactors. How the host cell compensates for these losses remains unknown, because the presence of bacteria in all available P. chromatophora cultures excluded elucidation of the full metabolic capacity and occurrence of HGT in this species. Here we generated a high-quality transcriptome and draft genome assembly from the first bacteria-free P. chromatophora culture to deduce rules that govern organelle integration into cellular metabolism. Our analyses revealed that nuclear and chromatophore gene inventories provide highly complementary functions. At least 229 nuclear genes were acquired via HGT from various bacteria, of which only 25% putatively arose through EGT from the chromatophore genome. Many HGT-derived bacterial genes encode proteins that fill gaps in critical chromatophore pathways/processes. Our results demonstrate a dominant role for HGT in compensating for organelle genome reduction and suggest that phagotrophy may be a major driver of HGT.
authors: Department of Plant Biology, Carnegie Institution for Science, Stanford, CA 94305; Department of Biology, Heinrich-Heine-Universität Düsseldorf, 40225 Dusseldorf, Germany; E.Nowack@uni-duesseldorf.de.
authors: Eva C M
authors: EC
authors: Nowack
authors: Department of Plant Biology and Pathology, Rutgers, The State University of New Jersey, New Brunswick, NJ 08901.
authors: Dana C
authors: DC
authors: Price
authors: Department of Ecology, Evolution and Natural Resources, Rutgers, The State University of New Jersey, New Brunswick, NJ 08901.
authors: Debashish
authors: D
authors: Bhattacharya
authors: Department of Biology, Heinrich-Heine-Universität Düsseldorf, 40225 Dusseldorf, Germany.
authors: Anna
authors: A
authors: Singer
authors: Biozentrum, Universität zu Köln, 50674 Koln, Germany.
authors: Michael
authors: M
authors: Melkonian
authors: Department of Plant Biology, Carnegie Institution for Science, Stanford, CA 94305.
authors: Arthur R
authors: AR
authors: Grossman
mesh_terms: D000656:Amoeba / Q000235:genetics* / Q000254:growth & development; D005075:Biological Evolution; D002856:Chromatophores; D000458:Cyanobacteria / Q000235:genetics*; D022761:Gene Transfer, Horizontal / Q000235:genetics*; D016680:Genome, Bacterial / Q000235:genetics; D058977:Molecular Sequence Annotation; D018087:Plastids / Q000235:genetics; D013559:Symbiosis / Q000235:genetics; D059467:Transcriptome / Q000235:genetics
keywords: coevolution; endosymbiosis; genome evolution; horizontal gene transfer; organellogenesis