Mitochondrial transfers: Nuclear copies of mitochondrial fragments or numt pseudogenes have been identified in a variety of animals and plants but there does not seem to be any consistency among their distributions across taxa. Numt size and abundance do not correlate with genome size, gene density or the abundance of mitochondrial transcripts. It is not known how or why these fragments integrate into the nuclear genome but their transfer may be mediated by chromosomal repair mechanisms. Numts are generally thought to be non-functional upon insertion but can insert within coding regions and have been associated with human disease. There has been a growing interest in numt pseudogenes and what they can reveal about the evolutionary history of their host. They can be used to compare rates of mitochondrial and nuclear evolution, as neutral molecular markers or to estimate divergence dates. Numt insertion sites can also be examined to assess whether numts preferentially integrate into certain regions of the genome.
Arvicoline rodents: Rodents are an appealing animal model for the study of numts because their genomes are evolving rapidly at both the sequence level and the chromosomal level. More specifically, arvicoline rodents are especially appealing because their karyotype is exceedingly plastic; diploid numbers within the vole genus Microtus range from 2n=17-64. Additionally, the rate of evolution within Microtus is faster than most rodents; over 60 species have developed in less than two million years. The accelerated speciation rate within Microtus has been attributed to chromosomal rearrangements and if numt integration is facilitated by chromosomal repair mechanisms, these rearrangements would provide numerous opportunities for nuclear integrations.
Rates of evolution and mitochondrial transfers within Microtus: To first assess whether the rapid rate of evolution within microtine voles was also found within its mitochondrial genome, I sequenced the mitochondrial genome of a European vole, M. rossiaemeridionalis. After comparing rates of mitochondrial evolution across mammalian taxa, I found that rates were the highest within Microtus. Next, I identified multiple numt transfers within its nuclear genome including one transfer that spanned over 25% of its mitochondrial genome. I characterized one of the mitochondrial pseudogenes in 21 additional arvicoline species and dated the transfer at ~4 MYA, predating the origins of most arvicolines. After comparing numt integration within Microtus to that of Mus and Rattus, I found that numt integration appears to be more extensive in Microtus than in other rodents.
Microtus rossiaemeridionalis
