Fig. 4.

Convergent evolution of gene content in mitochondria and chloroplasts. The ancestors of both organelles were prokaryotes with genomes encoding around 5,000 genes. During the course of endosymbiosis, genes are transferred from each organelle to the hosts’ nuclear genome, and the corresponding gene products are imported back to the organelles. The initial genome size of around 5,000 genes decreased to 3–67 genes in mitochondria and 23–200 genes in chloroplasts. The color coding within compartments in the lower part of the figure illustrates the convergent evolution of genes retained in the two bioenergetic organelles: genes for components of oxidative phosphorylation, photosynthesis, and proteins of 50S and 30S ribosomal subunits. Organellar-encoded genes are colored brown for mitochondria and green for plastids. TIC/TOC, protein translocator of the inner/outer chloroplast membrane; TIM/TOM, protein translocator of the inner/outer mitochondrial membrane. Schemes for oxidative phosphorylation and photosynthesis are adapted from ref. 4. Reproduced from ref. 90.