The human mitochondrial genome is a small circular genome of 16,570 ± 50 bp (Rubino et al., 2012). It contains only a few genes but it is still invading the nuclear genome. The average human genome contains about 600 fragments of mitochondrial DNA ranging in size from 30 bp to almost the full size of the mitochondrial genome (Simone et al. 2011). They are called NumtS or nuclear mitochondrial sequences. 1
Some of the genome inserts are 100% identical in sequence to the standard mitochondrial genome sequence indicating a recent colonization event. Others are as little as 63% identical, the cut-off similarity. The total amount of mitochondria-derived DNA in one individual was 627,410 bp amounting to only 0.02% of the genome (Simone et al., 2011).
About 550 NumtS in the human genome are also found in the chimpanzee genome (Calabrese et al., 2012). These represent insertions that pre-date the common ancestor of humans and chimpanzees. There are only 53 human-specific NumtS in the reference genome representing insertions that occurred along the lineage leading to modern humans (Lang et al., 2012). Some (14) of the human-specific insertions are polymorphic in the human population and these are presumed to be the most recent insertions.
Only a small number of these 14 polymorphic NumtS were found in the Neanderthal and Denisovan genomes. That suggests that most of the insertions happened since the split between modern humans and our ancient ancestors. However, Lang et al. (2012) point out that the quality of the Neanderthal and Denisovan genomes is not good enough to make any definitive statements.
Three or four insertions are confined to non-African populations implying that they took place in populations that had already migrated out-of-Africa. That probably means insertions events that occurred less than 100,000 years ago.
Invasions of mitochondiral DNA have to occur in germline cells in order to show up in the databases in more than one individual. The vast majority of these insertions are in junk DNA regions of the genome so they are neutral alleles. They become fixed in the population by random genetic drift but that's a slow and improbable mechanism of evolution. The fact that there are 53 human-specific NumtS suggests that there are about 0.3 insertions every 1000 generations. That may not seem significant but keep in mind that there are billions of people so that means millions of newborn babies have new insertions every generation.
If you were to look at every cell in your body, chances are high that your mitochondria have invaded your genome in at least one cell.
Image Credit: Moran, L.A., Horton, H.R., Scrimgeour, K.G., and Perry, M.D. (2012) Principles of Biochemistry 5th ed., Pearson Education Inc. page 175 [Pearson: Principles of Biochemistry 5/E]
1. I'm told that this is pronounced "new-mites."
Calabrese, F.M., Simone, D., and Attimonelli, M. (2012) Primates and mouse NumtS in the UCSC Genome Browser. BMC bioinformatics, 13(Suppl 4), S15. [doi:10.1186/1471-2105-13-S4-S15]
Lang, M., Sazzini, M., Calabrese, F.M., Simone, D., Boattini, A., Romeo, A., Luiselli, D., Attimonelli, M. and Gasparre, G. (2012) Polymorphic NumtS trace human population relationships. Human Genetics 131: 757-771. [doi: 10.1007/s00439-011-1125-3]
Rubino, F., Piredda, R., Calabrese, F.M., Simone, D., Lang, M., Calabrese, C., Petruzella, V., Tommaseo-Ponzetta, M., Gasparre, C., and Attimonelli, M. (2012) HmtDB, a genomic resource for mitochondrion-based human variability studies. Nucleic acids research, 40: D1150-D1159. [doi: 10.1093/nar/gkr1086]
Simone, D., Calabrese, F.M., Lang, M., Gasparre, G., and Attimonelli, M. (2011) The reference human nuclear mitochondrial sequences compilation validated and implemented on the UCSC genome browser. BMC genomics, 12:517-527. [doi:10.1186/1471-2164-12-517]