discussions. This study was supported by National Institutes of Health Research Grant GM-35087 to J.D.P., U.S. Department of Agriculture Plant Biotechnology Fellowship 95-38420-2214 to K.L.A., and National Institutes of Health Postdoctoral Fellowships GM-17923 and GM-19225 to Y.L.Q. and C.L.P., respectively.
Adams, K. L., Clements, M. J. & Vaughn, J. C. ( 1998) The Peperomia mitochondrial coxI group I intron: Timing of horizontal transfer and subsequent evolution of the intron. J. Mol. Evol. 46, 689–696.
Adams, K. L., Song, K., Roessler, P.G., Nugent, J. M., Doyle, J. L., Doyle, J. J. & Palmer, J. D. ( 1999) Intracellular gene transfer in action: Dual transcription and multiple silencings of nuclear and mitochondrial cox2 genes in legumes. Proc. Natl. Acad. Sci. USA 96, 13863–13868.
Allen, J. F. ( 1993) Control of gene expression by redox potential and the requirement for chloroplast and mitochondrial genomes. J. Theor. Biol. 165, 609–431.
Allen, J. F. & Raven, J. A. ( 1996) Free-radical-induced mutation vs redox regulation: Costs and benefits of genes in organelles. J. Mol. Evol. 42, 482–492.
Bittner-Eddy, P., Monroy, A. F. & Brambl, R. ( 1994) Expression of mitochondrial genes in the germinating conidia of Neurospora crassa. J. Mol. Biol. 235, 881–897.
Blanchard, J. & Lynch, M. ( 2000) Organellar genes. Why do they end up in the nucleus? Trends Genet. 16, 315–320.
Brown, T. A., Ray, J. A., Waring, R. B., Scazzocchio, C. & Davies, R. W. ( 1984) A mitochondrial reading frame which may code for a second form of ATPase subunit 9 in Aspergillus nidulans. Curr. Genet. 8, 489–492.
Cho, Y., Qiu, Y.-L., Kuhlman, P. & Palmer, J. D. ( 1998) Explosive invasion of plant mitochondria by a group I intron. Proc. Natl. Acad. Sci. USA 95, 14244–14249.
Cho, Y. & Palmer, J. D. ( 1999) Multiple acquisitions via horizontal transfer of a group I intron in the mitochondrial cox1 gene during evolution of the Araceae family Mol. Biol. Evol. 16, 1155–1165.
Claros, M. G., Perea, J., Shu, Y., Samatey, F. A., Popot, J.-L. & Jacq, C. ( 1995) Limitations to in vivo import of hydrophobic proteins into yeast mitochondria. The case of a cytoplasmically synthesized apocytochrome b. Eur. J. Biochem. 228, 762–771.
Covello, P. S. & Gray, M. W. ( 1992) Silent mitochondrial and active nuclear genes for subunit 2 of cytochorme c oxidase (cox2) in soybean: evidence for RNA-mediated gene transfer EMBO J. 22, 3815–3820.
Doolittle, W. F. ( 1998) You are what you eat: a gene transfer ratchet could account for bacterial genes in eukaryotic nuclear genomes. Trends Genet. 14, 307–311.
Eyre-Walker, A. & Gaut, B. S. ( 1997) Correlated rates of synonymous site evolution across plant genomes Mol. Biol. Evol. 14, 455–460.
Fauron, C. M.-R., Moore, B. & Casper M. ( 1995) Maize as a model of higher plant mitochondrial genome plasticity. Plant Science 112, 11–32.
Figueroa, P., Gomez, I., Holuigue, L., Araya, A. & Jordana, X. ( 1999a) Transfer of rps14 from the mitochondrion to the nucleus in maize implied integration within a gene encoding the iron-sulphur subunit of succinate dehydrogenase and expression by alternative splicing Plant J. 18, 601–609.
Figueroa, P., Gomez, I., Carmona, R., Holuigue, L., Araya, A. & Jordana, X. ( 1999b) The gene for mitochondrial ribosomal protein S14 has been transferred to the nucleus in Arabidopsis thaliana. Mol. Gen. Genet. 262, 139–144.
Gray, M. W. ( 1999) Evolution of organellar genomes. Curr. Opin. Genet. Dev. 9, 678–687.