What is mtDNA?

Mitochondria are the organelles which are the "powerhouse" of the cell. They take in glucose, and through a long and complicated process, produce large amounts of ATP, a molecule which is critical to all reactions in the cell. In order to produce ATP, the mitochondrion needs to be able to produce proteins on its own, a process for which DNA is necessary. Mitochondria have their own ribosomes, tRNA, and DNA, enabling them to produce the proteins and other molecules that are necessary for maintaining the organelle and creating ATP.

Mitochondrial DNA (mtDNA for short) is the DNA which is found inside mitochondria. What makes mtDNA so valuable to researchers is the fact that the code is passed virtually intact through the maternal line. This means that each child receives only his or her mother's mtDNA, as opposed to a mixture of both parents'. This fact has been used by researchers to prove the relation of two members of a royal family by confirming a shared great-grandmother, to accurately identify the bones of Jesse James, and (in our field of research) to help understand the evolutionary history of many species.

A typical mtDNA ring (Wikipedia commons)

Questions:

1. Why would mtDNA only come from the mother? (Matrilineal inheritance)

2. Why would mtDNA undergo less change than nuclear DNA as it moves from generation to generation?

3. Why would the previous two facts be convenient for evolutionary studies?

Answers:

1. Sperm, being much smaller than eggs, contain far fewer mitochondria. In addition, the sperm mitochondria are marked with ubiquitin, which means that they will be destroyed later during development and the egg will provide the mitochondria that the offspring will inherit.

2. During meiosis I, the chromosomes undergo crossing over, which results in genes from one homologous chromosome switching places with another. This process greatly increases genetic diversity and mixes the paternal line with the maternal line. By contrast, mtDNA never undergoes anything more complicated than DNA synthesis, which produces mutations, but not recombination.

3. The almost exact preservation of the maternal line makes mtDNA extremely useful for evolutionary studies because the mutations that occur are much more noticeable, and the line of inheritance can be traced easily. The ease of noticing mutations allows researchers to focus on specific points as markers of change in a species. The ease of tracing inheritance helps researchers to determine the relationships of various species and subspecies.

An example of matrilineal inheritance. The blue figures inherit the original mitochondria. (Wikipedia Commons)

How is mtDNA used in evolutionary studies?

The same attributes of mtDNA which make it so useful in researching family trees can be extended to aid evolutionary studies. In the search for a common ancestor, mtDNA is among the most powerful tools we have. In addition to the convenience matrilineal inheritance provides for researchers, mtDNA is also much easier to extract from ancient skeletons than nuclear DNA. The reason for this is that there are 2-10 mitochondrial "plasmids" per mitochondrion, and many mitochondria per cell, which means that there is a much higher quantity of DNA to be salvaged. The relative ease of extracting mtDNA from skeletons makes it an attractive tool for scientists whose studies range back thousands of years.

Another feature of mtDNA that is useful to evolutionary studies is the D-loop region. The D-loop is the location where mtDNA replication starts. It is a rare non-protein coding region in the mitochondrial genome and has attracted the attention of geneticists because it contains highly diverse “hypervariable” regions that evolve quickly.

In dog genetics research, mtDNA has been used to try to ascertain the location where dogs were first domesticated from wolves. A recent study (Pang et al. 2009) discovered that there are 10 major haplogroups in the shared dog gene pool, and one region of the world, (SE Asia) contains the full complement of haplogroups. In Europe, by contrast, only 4 of the haplogroups were present. The concentration of diversity in SE Asia would suggest that that region is an origin of the species, although local admixture and introgression with Asian wolves might also lead to increased dog mtDNA diversity in this region.