Are Mitochondria Polyphyletic or Monophyletic?
Did the endosymbiosis that produced the mitochodrion occur one time or multiple times? All the phylogenetic SSU rRNA and protein evidence support a single origin of mitochondria. For example, all of the genes found in all known mt DNAs are a subset of those found in R. americana mt DNA. This would be hard to reconcile with several independent reductions of larger eubacterial genomes. Furthermore all mt genes (both in the mt DNA and the nuclear DNA) tree with the alpha-proteobacteria. From Andersson et al. (1998), Fig. 6.
