Organelle Genetics Evolution of Organelle Genomes and Gene Expressio
Mitochondria and chloroplasts are eukaryotic organelles that evolved from bacterial ancestors and harbor their own genomes. The gene products of these genomes work in concert with those of the nuclear genome to ensure proper organelle metabolism and bioge
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Mitochondrial Origins Toni Gabaldo´n
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Introduction
Mitochondria are cellular organelles surrounded by a double membrane. Mitochondria, or evolutionary-related organelles such as hydrogenomes or mitosomes (see below), have been identified in every eukaryotic organism that has been carefully examined to date. This indicates that the origin of these organelles preceded the diversification of all known groups of eukaryotes, estimated to date back 1.5–2 billion years (Brocks et al. 1999). Regarding this origin, there is now a widespread consensus in that mitochondria originated from an alpha-proteobacterial ancestor (the so-called proto-mitochondrion) by means of an endosymbiotic process. Extant representatives of alpha-proteobacteria constitute a large and highly diversified group, in which a large variety of metabolic capacities and lifestyles can be observed. Most probably, none of the currently existing alpha-proteobacterial species can be regarded as an accurate model for the original proto-mitochondrion, since they thrive in environments that are likely very different from the one that governed the establishment of the initial endosymbiosis. Even modern alpha-proteobacteria that have intra-cellular lifestyles such as the insect endosymbiont Wolbachia, or the intracellular pathogens of the genus Ricketsia, represent parallel adaptations to intra-cellular life and should be considered different scenarios since, contrary to the protomitochondrion, they inhabit full-fledged eukaryotic host cells that already possess mitochondria. Moreover, phylogenetic analyses of mitochondrial genes cannot identify a particular group within the alpha-proteobacteria as the ancestor of mitochondria (Esser et al. 2004). Comparative genomics and phylogenomics have served to circumvent the problem of a lack of an extant model by identifying eukaryotic genes with a clear alpha-proteobacterial ancestry (Gabaldo´n and Huynen
T. Gabaldo´n (*) Bioinformatics and Genomics Programme, Centre for Genomic Regulation (CRG), and Universitat Pompeu Fabra, Doctor Aiguader, 88, 08003 Barcelona, Spain e-mail: [email protected] C.E. Bullerwell (ed.), Organelle Genetics, DOI 10.1007/978-3-642-22380-8_1, # Springer-Verlag Berlin Heidelberg 2012
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2003, 2007b). This has enabled a partial reconstruction of the proto-mitochondrial metabolism, which has shed light on the possible metabolic scenarios that favored the initial symbiosis. In contrast, there is much debate on what was the probable nature of the host cell. The classical view considers a rather developed eukaryotic host, presenting a cell nucleus and a cytoskeleton, and with the ability to perform phagocytosis, which would have enabled the engulfing of the alpha-proteobacterial endosymbiont (de Duve 2007). Alternatively, other authors envision a prokaryotic host, with the mitochondrial endosymbiosis event itself giving rise to the formation of the eukaryotic cell (Martin and M€ uller 1998), by means of selective pressures that favored the creation of the cell nucleus and othe
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