The origin of mitochondria in light of a fluid prokaryotic chromosome model

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Abstract
Biologists agree that the ancestor of mitochondria was an α-proteobacterium. But there is no consensus as to what constitutes an α-proteobacterial gene. Is it a gene found in all or several α-proteobacteria, or in only one? Here, we examine the proportion of α-proteobacterial genes in α-proteobacterial genomes by means of sequence comparisons. We find that each α-proteobacterium harbours a particular collection of genes and that, depending upon the lineage examined, between 97 and 33% are α-proteobacterial by the nearest-neighbour criterion. Our findings bear upon attempts to reconstruct the mitochondrial ancestor and upon inferences concerning the collection of genes that the mitochondrial ancestor possessed at the time that it became an endosymbiont.