Scientists have argued whether or not the often-studied mitochondrial DNA molecule is clonally inherited. It is with assuming clonal inheritance this type of DNA has been used to track the origin of modern human as well as to draw pictures of genetic relationships among other animals and plants. The conflict has now been solved by two evolutionary geneticists from Uppsala University in Sweden, who present the new evidence in this week's issue of Nature. Their results show that mitochondrial DNA is stably transmitted from mothers to their offspring only. This clonal inheritance indeed makes mitochondrial DNA suitable for use in evolutionary studies.
The genetic material in humans and other animals is arranged in chromosomes that are inherited both from the mother and the father. A small fraction of the genetic material is also assembled in the mitochondria and has been assumed to only be inherited from mothers to offspring. A belief that often has been challenged. If there is stable maternal inheritance of mitochondrial DNA, distinct maternal lineages would be created and these can be used for evolutionary and phylogenetic studies. Examples are the origin of modern humans and the link between humans and apes.
Recent findings suggest that mitochondrial DNA is not entirely inherited from mother to offspring; instead paternal mitochondrial DNA occasionally slips through with sperm. Also, genetic changes may occur in mothers mitochondrial DNA, similar to what happen with our chromosomes when egg and sperm are formed. There is therefore an ongoing debate whether or not mitochondrial DNA can be used for phylogenetic analyses.
To test the hypotheses that mitochondrial DNA is entirely transmitted through females one would have to follow its inheritance for hundreds or thousands of generations. In practice, this proves impossible. A new idea posits to compare the inheritance of mitochondria with some other genetic material that is transmitted through females only. If the pattern of inheritance is identical, this evidence stable maternal inheritance of the mitochondrial DNA.
In humans there is no genetic material that is only transferred through females. Hans Ellegren and Sofia Berlin therefore chose to use birds as model organisms to unravel the issue. Female birds have a unique chromosome, the W-chromosome, that is inherited from mothers to daughters in stable maternal lineages.
To be able to do the comparison between mitochondrial DNA and the W-chromosome the scientist found a fitting W-chromosome genetic type in peregrine falcons. The genetic relationships proved to be spot on identical! This shows that mitochondrial DNA must have been inherited in the same way as the W-chromosome, that is only through females.
Through calculations based on the genetic relationships, Hans Ellegren and Sofia Berlin could approximate a stable pattern of inheritance of mitochondrial DNA for at least 20.000 falcon generations, or 200.000 years. This shows that the inheritance of mitochondrial DNA really is stable even on an evolutionary time scale.
The results are important since there is evidence that parts of the genetic material are unique for females, which give rise to maternal lineages. If that had not been the case one would have had to re-evaluate many earlier findings, from the origin of humans to the relationships among extant animal and plant species. The study is also important since it for the first time compares the inheritance of a chromosome and the mitochondrial DNA.
For further information, contact Professor Hans Ellegren, Evolutionary Biology Centre, Uppsala University, tel. 46-18 471-64-60, mobile 070-425-06-37. E-mail: Ellegren@ebc.uu.se