It sounds like a nasty fairy tale, but evidence has been found of a “mother’s curse” gene in fruit flies – DNA that harms sons but not daughters. Although such genes have been proposed before, this is the strongest evidence yet seen for their existence in animals.
Genes are transmitted when the organism of either sex breeds. Things can get complicated, however, for mitochondrial genes. A paper in eLife reports on the discovery of a mitochondrial mutation that damages the fertility of Drosophila melanogaster, but may exist far more widely in other species.
Mitochondria were once independent microbes that formed a synergistic relationship with our ancestors, eventually becoming incorporated and becoming the energy sources for our cells.
Our cells contain a nucleus with most of our DNA, and mitochondria with their own much smaller genome. National Institutes of Health
They’ve been with us ever since, but where the nuclear genome is inherited equally from the mother and father, our mitochondria is entirely from our mother. This has made analysis of mitochondrial DNA highly valuable for understanding our maternal line, and led to the first babies with three genetic parents, children whose mitochondria comes from a different mother than their nuclear DNA.
Like any genetic code, mitochondrial DNA can contain damaging errors; three-parent babies are created where a mother’s mitochondrial DNA contains harmful mutations, but the rest of her genes are fine. The purely maternal transmission means these errors are passed on differently from mutations in nuclear DNA. Since males don’t pass on their mitochondrial DNA to their children, a selfish mitochondrial gene is only invested in the survival of females.
This observation led to the “mother’s curse” hypothesis, where mitochondrial DNA might harbor mutations that damage sons but are harmless to daughters. "In the 20 years since this possibility was recognized, a few mitochondrial mutants have been reported that have deleterious effects on male offspring," said first author Dr Maulik Patel of Vanderbilt University in a statement. "But none of them convincingly showed that the mutants did not have any negative effects on the females.”
Patel searched specifically for mitochondrial DNA that damages males while having no effect on females, finding one such gene.
Mutations that are primarily harmful to males can exist in the nucleus as well. Any damaging but recessive form of a gene carried on the X chromosome will have this feature, with hemophilia and color-blindness being famous examples. Nevertheless, where a female inherits two versions of the mutation, she is similarly affected.
Mitochondrial DNA is a step beyond this. In what geneticists have called the “Red Queen hypothesis”, mitochondria evolve to adapt to changes in the nucleus of the cells in which they operate, and accumulate mutations in the process.
Plants have larger mitochondrial genomes than animals, and in consequence have developed many mutations that affect male offspring. Geneticists have expected similar, though rarer, examples in animals, but Patel may be the first to identify an example.
After breeding 35 generations of flies, Patel found that a single amino-acid change in the enzyme cytochrome C oxidase caused male sperm production and quality to fall prematurely, and become completely sterile at temperatures above 29°C (84°F). A mutation causing this effect had become common in his sample because it had no detrimental effect on female flies.
Patel considers the discovery as support for the theory that low fertility in men is often a result of mitochondrial mutations. Curiously, however, some wild male fruit fly carry DNA in their nuclear genome that Patel found restores the fertility of those carrying the mitochondrial mutation, opening up many exciting avenues for further study.
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