XVI International Botanical Congess

The Serial Endosymbiosis Theory (SET) is generally accepted as the best current description of how mitochondria originated within the eukaryotic cell. Central to this theory is the proposition of a host cell that provided the nucleus and that formed an endosymbiotic relationship with a eubacterium that became the mitochondrion. Although gene and genome data have amply confirmed the direct descent of mitochondria from a common ancestor shared specifically with the rickettsial subdivision of the -Proteobacteria, the precise nature of the symbiotic event that gave rise to mitochondria, and whether this event was coincident with or subsequent to the origin of the nuclear genome, is being increasingly questioned. At the same time, mitochondrial genome sequencing projects have sampled the diversity of mitochondrial gene content and organization, have supplied strong evidence for a single origin of mitochondria, and have provided new insights into the probable nature of the ancestral, proto-mitochondrial genome. These data are also beginning to give us a picture of gene flow from the mitochondrial to the nuclear genome, emphasizing that particular genes have been lost from the mitochondrial genome on a number of occasions in different eukaryotic lines.