Re: How did mitochondirial genes migrate into the nuclear genome?

I had often wondered the same thing and hypothesized a similar complementary mechanism; alas, no. Which mitochondrial genes are coded in the nucleus varies from phylum to phylum, while the nuclear versions of genes from one phylum show remarkable homology to mitochondrial versions from other phyla, suggesting a common mitochondrial ancestor to all. The migration of mitochondrial genes into eukaryotic nuclear genomes is an example of the broader process of horizontal gene transfer. One of the best characterized examples of mitochondrial gene migration is found in the thale cress (Arabidopsis thaliana). Mitochondrial to nuclear gene transfer occured at least twice in the line of plants leading to Arabidopsis, and these two independent events suggest independent mechanisms of transfer. In one case, a single gene, the mitochondrial S10 ribosomal protein (Rps10), migrated onto Chromosome 3 of Arabidopsis, during which it lost its mitochondrial introns, suggesting an RNA directed gene transfer. Some time during or after the migration, a novel, mitochondrial-targeting domain and an intervening intron were appended in front of Rps10, allowing it to maintain its function from its new home. Elsewhere, on Chromosome 2, there is an almost complete (80% intact) mitochondrial genome, introns and all, with high homology to the existing mitochondria in the same cells. This would suggest direct chromosomal recombination, between the mitochondrial and nuclear genomes.

Along with evolutionarily recent migrations to the nuclear genome, plant mitochondrial genomes are full of non-coding DNA, such they are many times larger than animal mitochondrial genomes. In nuclear genomics, the expansion of non-coding material has been linked to increased recombinant activity, especially of transposable elements (including retroviral-like elements), and it has been suggested that the continuing nuclear migration of mitochondrial genes in plants and the expansion of their mitochondrial genomes may be linked. Indeed, animal mitochondrial genomes are remarkably small and uniform, and their nuclear genomic members appear to have migrated very early.

For more information on the subjects of mitochondrial evolution and horizontal gene transfer, here are the sources I used:

Michael W. Gray, Gertraud Burger, and B. Franz Lang (1999) Mitochondrial Evolution Science 283(5407): 1476-1481.

Christine Wischmann and Wolfgang Schuster (1995) Transfer of rps10 from the mitochondrion to the nucleus in Arabidopsis thaliana: evidence for RNA-mediated transfer and exon shuffling at the integration site FEBS Letters 374(2): 152-156.

Robert M. Stupar, Jason W. Lilly, Christopher D. Town, Zhukuan Cheng, Samir Kaul, C. Robin Buell, and Jiming Jiang (2001) Complex mtDNA constitutes an approximate 620-kb insertion on Arabidopsis thaliana chromosome 2: Implication of potential sequencing errors caused by large-unit repeats Proceedings of the National Academy of Sciences USA 98(9): 5099– 5103.

Focus on Horizontal Gene Transfer Nature Reviews Microbiology September, 2005: Vol 3, No 9

Meike Hoffmeister and William Martin (2003) Interspecific evolution: microbial symbiosis, endosymbiosis and gene transfer Environmental Microbiology 5(8): 641-649.