A MAN who suffers from muscle weakness has dealt a powerful blow to two long-held dogmas in human genetics. He is living proof that processes which hundreds of scientific papers assume never happen do in fact take place.
Doctors found that this man鈥檚 muscles tire easily because they contain mutant mitochondria, the energy-producing structures inside cells. What astonished them, however, is that most of his muscle mitochondria come from his father, shattering the notion that mitochondria are always inherited from the mother. Sperm are packed with mitochondria (pictured), but until now it was thought these are always destroyed after fertilisation.
And that鈥檚 not all. Mitochondria, thought to have evolved from symbiotic bacteria, possess their own DNA encoding a few dozen genes. It was thought that human mitochondria do not swap large segments of DNA in the way that the chromosomes in a cell nucleus do during sexual reproduction. But in 0.7 per cent of the man鈥檚 muscle mitochondria, the mitochondrial DNA (mtDNA) contained a mixture of sequences from his father鈥檚 and his mother鈥檚 mitochondria (Science, vol 304, p 981). This means different mitochondria must have swapped portions of their DNA. Such recombination occurs in yeast and a few animals, but claims that it happens in humans have been controversial.
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The question now is whether paternal inheritance of mtDNA, and recombination between paternal and maternal mitochondria, occurs frequently enough to undermine the many studies that assume these processes do not occur. Perhaps the most famous example is an analysis backing the 鈥渕itochondrial Eve鈥� theory, which suggests all human mitochondria derive from a woman who lived 140,000 years ago. This date is based on the assumption that mitochondria acquire mutations at a fairly constant rate rather than undergoing wholesale changes.
鈥淚 am sure scholars of population genetics will keep our results in mind,鈥� says Konstantin Khrapko of Harvard Medical School, one of the study鈥檚 authors. However, the researchers point out that even in the case they describe, the man鈥檚 mitochondria are unlikely to end up in his children.
Others are now keen to establish whether the man is just a very rare exception. 鈥淧eople have looked for this before, but then we didn鈥檛 know that human mitochondrial recombination was even biologically possible,鈥� says Christopher Wills, who studies human evolution at the University of California, San Diego. 鈥淭his result will certainly spur researchers to look again.鈥�