The energy-producing structures found in each of our cells are usually inherited exclusively from our mother. But doctors in the US have identified more than a dozen individuals in three different families who have inherited mitochondria from both parents.
It seems that these individuals are rare exceptions to the usual rule, probably because these families harbor mutations that disrupt the mechanism that normally prevents the father's mitochondria from being passed on to their children.
Mitochondria produce the energy cells that need to function, and all human cells, including sperm and eggs, contain many of them. But although a father's mitochondria enter the egg, in humans they have a chemical mark that marks them for destruction, so that usually all mitochondria come from the mother.
However, in 2002, it was discovered that a man's cells contained a mixture of mitochondria from the father and mother. But with no other cases being reported since then, some have questioned whether the 2002 finding was correct.
Now, a team at the Cincinnati Children's Hospital Medical Center in the United States claims to have "unequivocal" evidence after identifying 17 such people with a father's inheritance.
More and more cases
The first individual was identified as suffering from muscle fatigue and pain, suspected to be caused by mitochondrial mutations. It turns out that he inherited mitochondria from both parents, and a new mutation emerged in the paternal mitochondria.
During the investigations, the team found that other family members also had a mixture of maternal and paternal mitochondria in each of their cells. They then studied a few other patients with symptoms of mitochondrial diseases, and found two other families as well.
"The surprise is that we really do not see this anymore," says Nick Lane of University College London, UK, author of a book on mitochondria. His team predicted last year that the "paternal leak" should be relatively common in all organisms with mitochondria.
Because? Because there are two conflicting evolutionary forces at work. In the short term, mixing mitochondria may be beneficial to individuals, because the father's mitochondria, for example, can compensate for a damaging mutation in the mother's mitochondria. But in the long run, this may undermine the ability of evolution to eliminate bad mutations, since they are hidden.
Lane thinks that's why organisms have a surprisingly wide variety of mechanisms to ensure that mitochondria are inherited only from the mother. During the course of evolution, species have repeatedly evolved such mechanisms, lost them and then evolved similar mechanisms, his team proposed.
Because mitochondrial DNA is the most common type of DNA in cells – since each cell can contain hundreds of copies – it has been widely used in genetic studies, for example, to study our evolutionary history. If the paternal inheritance of mitochondria was very common, it would undermine some of the conclusions of these studies, but it is probably still so rare that it makes little difference.
Journal Reference: PNAS, DOI: 101073 / pnas.1810946115
More about these topics: