by Jane Harris Zsovan
Mitochondrial DNA is often touted in both academic journals and popular media as the historical record as our birth as a species.
Some media stories even claim that MtDNA traces our species through migrations and catastrophic near extinctions.
Spencer Wells, National Geographic explorer in residence and author of The Journey of Man: A Genetic Odyssey (2002), told CNN in a 2008 interview that "Studies using Mitochondrial DNA, which is passed down through mothers, have traced humans to a single 'mitochondrial Eve' who lived in Africa."
Wells, who thinks our common mother dates from about 200,000 years ago, has studied the mitochondrial DNA of the Khoi and San Peoples of Africa, though he is better known for another theory that captured public attention, "Y-chromosome Adam."
Mitochondrial Eve and Y-chromosome Adam
All people carry MtDNA in their bodies, but only males have a Y chromosome. In The Journey of Man, Wells writes about a potential male counterpart for mitochondrial Eve, "Y-chromosome Adam," who lived much later, approximately 60,000 years ago:
Every piece of DNA in our bodies can be traced back to an African source. The Y-chromosome traces back to eastern or southern Africa, around 60,000 years ago. The present-day inhabitants of Ethiopia, Sudan and southern Africa carry the clearest signals of our earliest ancestry, signals that have been lost in the rest of us. So they give us a glimpse of our 60,000 year-old Adam. Adam would have been fully modern, both in terms of his appearance and his brain function. It is speculation, of course, but perhaps the San Bushmen of the Kalahari - who in many ways are a composite model of facial features from people all over the world - give us a portrait of Adam and his fellow early humans.
While researching Y-chromosome Adam may also help trace human prehistory, it is mitochondrial Eve, the common mother of the human race, that has really caught fire among media and researchers. Central to the idea is that a child's mitochondrial DNA is inherited from mothers, not fathers. THus, a single Eve as the mother of all living humans may be possible.
Mammalian sperm also pass on MtDNA
While popular media assume that MtDNA comes only from mothers, as we noted in Part One, paternal MtDNA inheritance has been demonstrated in mammals. For example, a study Paternal Inheritance of mitochondrial DNA in the Sheep authored by Zhao Xingbo1 , Chu Mingxing2, Li Ning3 and Wu Chang demonstrates paternal MtDNA inheritance in sheep.
Misconceptions about Mitochondria and Mammalian Fertilization: Implication for Theories on Human Evolution, by Friderun Ankel-Simons of the Duke University Primate Center and Jim M. Cummins, states that "In the majority of mammals-- including humans--the midpiece mitochondria can be identified in the embryo even though their ultimate fate is unknown."
Ankel-Simons and Cummins also cast doubt on rodent studies used to bolster the idea of strict maternal inheritance of MtDNA. For example, Kaneda et. al. found elimination of paternal MtDNA in intraspecies hybrids of Mus musculus ("house" mouse) but not in interspecies hybrids of musculus and Mus spretus, the Algerian mouse.
In addition, they note that rodent inheritance patterns differ from human inheritance. For example, the rodent centrosome is inherited from the mother (maternally) but in humans and other vertebrates it is inherited from the father (paternally).*
Does the maternal MtDNA inheritance predominate in the offspring?
The egg likely does contribute much more MtDNA to offspring than the sperm. For example, the mid-piece of the sperm tail contains 50-75 mitochondria, much fewer than 100,000 to 100,000,000 in the egg. That may well be a good thing. MtDNA damage is implicated in mental illness, endocrine and immune disorders, as well as premature aging.
The smaller genome contained in MtDNA, coupled with a tendency to inherit MtDNA primarily from the mother, may be help protect protect offspring from MtDNA damage.
In this connection, Ankel-Simons and Cummins note something else as well: Maternal contribution to the MtDNA is also restricted to a small pool:
Intriguingly, the oocyte's MtDNA derives from a very small pool (perhaps as few as five) precursor mitochondrial genomes during oogenesis. This selective pressure may serve as a selective genetic filter against defective mitochondrial genomes, just as it may explain why some intergenerational changes may occur so rapidly.
So we know that the paternal MtDNA is there, but we cannot be certain how much or how little influence it has -or anyway, not as certain as pop culture would probably like.
*Note: The Chinese hamster demonstrates different patterns of inheritance from other mammals. According to Ankel-Simons and Cummins, "Tail and midpiece structures can be traced for several division cycles. The only known exception to this is the Chinese hamster, C. Griseus."
Resource: Misconceptions abut mitochondria and mammalian fertilization: Implications for theories on human evolution. This paper can be read without charge.
Citation: Proceedings of the National Academy of Sciences of the United States of America, Vol. 93, No. 24 (Nov. 26, 1996), pp. 13859-13863
Abstract:
In vertebrates, inheritance of mitochondria is thought to be predominantly maternal, and mitochondrial DNA analysis has become a standard taxonomic tool. In accordance with the prevailing view of strict maternal inheritance, many sources assert that during fertilization, the sperm tail, with its mitochondria, gets excluded from the embryo. This is incorrect. In the majority of mammals--including humans--the midpiece mitochondria can be identified in the embryo even though their ultimate fate is unknown. The ``missing mitochondria'' story seems to have survived--and proliferated--unchallenged in a time of contention between hypotheses of human origins, because it supports the ``African Eve'' model of recent radiation of Homo sapiens out of Africa. We will discuss the infiltration of this mistake into concepts of mitochondrial inheritance and human evolution.
Next: Part Three: African Eve - when pop culture falls in love with science
Back to Part One: Our Mitochondria: A piece in the puzzle of our origins?